Precision animal nutrition: A tool for economic and eco-friendly animal production in ruminants

D V Reddy and N Krishna* 

Rajiv Gandhi College of Veterinary and Animal Sciences, Kurumbapet, Puducherry 605009, India

dv.duvvuru@gmail.com

^*College of Veterinary Science, S V Veterinary University, Rajendranagar, Hyderabad-500 030, India

Abstract  

The primary objective of raising livestock is to produce good quality animal products in the form of milk and meat to the consumers as a profitable enterprise with least contribution to climate change. Methane is the second major contributor to global warming being 23 times that of carbon dioxide. It has been estimated that methane production by ruminants is about 15% of total atmospheric methane emissions and amounts to loss of 6% of gross energy intake. Precision animal nutrition (PAN) is the effective utilization of available feed resources with the aim of maximizing the animals’ response to nutrients. The PAN appears to be an ideal approach to improve the productivity of animals in developing countries in view of inadequate feed resources. The term ‘PAN’ is defined as providing the animal with the feed that precisely meet its nutritional requirements for optimum productive efficiency to produce better quality animal products and to contribute cleaner environment and thereby ensure profitability.

The tools to achieve PAN include improved feed processing techniques, precise ration formulation, implementing phase feeding and the use of feed additives. It is generally agreed that the more feed an animal consumes each day, the greater will be the opportunity for increasing its daily production, which is dependent upon improving the nutrient digestibility. Towards this, urea-ammoniation treatment of straws has been promising. Strategic supplementation of nutrients enhance rumen fermentative digestibility, which stimulates intake of feed. Further, it also balances the end products of rumen fermentation in lowering enteric methane production from ruminants. Efficiency of ruminal fermentation and digestibility of the nutrients are key factors in improving the efficiency of feed use.

Nutrient partitioning is the major component of productive efficiency that differs among individual cows with increase in productive efficiency achieved with exogenous bGH (recombinantly derived growth hormone). Use of restricted protein levels and supplementation of rumen-protected amino acids with matching ruminal energy and monitoring the milk urea nitrogen help assess efficiency of nitrogen use. Feed additives play a pivotal role in achieving increased efficiency and reduced environmental load per unit of the animal product. The role of supplementation of unsaturated fatty acids, ionophore antibiotics, organic acids, plant secondary metabolites, essential oils, probiotics and fibrolytic enzymes has been discussed to achieve precision animal nutrition.

Key words: Feed additives, methane emission, plant secondary metabolites, precision animal nutrition, productive efficiency, strategic supplementation, tree foliage

Introduction

One of the main objectives of raising livestock and poultry is to produce unadulterated wholesome food in the form of milk, meat and eggs to the consumers as a profitable enterprise with least contribution to climate change. Towards meeting this, they need to be fed with wholesome feed. Nutrition involves various chemical reactions and physiological processes that transform foods into body tissues and activities. It involves the ingestion, digestion and absorption of the various nutrients, their transport to all body cells, and the removal of unusable elements and waste products of metabolism. With this objective in view, an earnest attempt has been made to define ‘precision animal nutrition’ and to review the published literature on the tools to realize it. This review is the full-length article of the Abstract (Reddy 2007) presented in the Animal Nutrition Conference held at Karnal, India.

Precision animal nutrition (PAN) is defined as providing the animal with the feed that precisely meet its nutritional requirements for optimum productive efficiency to produce better quality animal products (milk, meat and eggs) and to contribute cleaner environment and thereby ensure profitability. Cleaner environment means reducing the enteric emission of methane, excretion of nitrogen (ammonia), phosphorus and other compounds into the environment. It is aimed at supplying the nutrients to the animals matching their requirements to improve not only the animal physiology and health but also the enrichment of their products for the well being of the consumer.

Voices of concern

Objections have been raised over the use of hormones and antibiotics in food animal production since they leave their residues in meat, milk and eggs. There is some concern that feeding of low concentration of antibiotics may favour the proliferation of antibiotic-resistant microorganisms, which could have serious consequences for disease control in humans and domestic animals.

Proliferation of bovine spongiform encephalopthy (BSE) in the British cattle (in 1986) is thought to have emerged originally from initial interspecies transmission of the scrapie-agent into cattle by the feeding of scrapie-infected sheep and goat meal, plus bone meal products to cattle. In view of the possible link between BSE and human’s Creutzfeldt-Jacob disease, meat and bone meal (MBM) was banned from using in ruminant feed while some countries most notably UK, have a complete MBM ban for all farm animal feed.

Dioxins are unintentional byproducts of many industrial processes and the byproducts such as fish meal, fish oil, recovered vegetable oil, and byproducts of food industry have found their way into animal feeds as energy sources. Thereby the dioxins get concentrated in animal products and are highly toxic posing a real health hazard.

Challenges of modern animal nutrition   

Food safety has become a major issue in light of global disasters of mad cow disease / BSE, dioxin contamination of feeds (in Belgium in May 1999), and lesser mishaps (drug, mycotoxin, pesticide, hormone residues in food products) and the tide of consumer trust in food safety is at its low ebb. The concept of ‘quality in, quality out’ has to be remembered and so quality and safe food can only be produced from quality and safe feed. The challenge for modern animal nutrition is to produce animal products that are acceptable to the consumer, to develop rations without antibiotics, meat and bone meal, or other objectionable feeds and to formulate rations that do not cause environmental pollution, that maintain animal health and remain economically viable.

Tools to achieve precision animal nutrition 

The tools to achieve PAN include more precise ration formulation based on nutritional value of each batch of ingredient, proper weighing and mixing of ingredients, use of nutricines / feed additives and improved feed processing techniques, implementing phase feeding and split-sex feeding. PAN is the only way to improve the productivity of animals in developing countries by effective utilization of available feed resources with the aim of maximizing the animals’ response to nutrients. It is generally agreed that the more feed an animal consumes each day, the greater will be the opportunity for increasing its daily production, which is dependent upon improving the digestibility

Increased productivity through strategic supplementation

Straw treatment-Ammoniation

Cereal crop residues (lignocellulosic materials) comprise the main feed for livestock in India and other developing countries. Dr R A Leng challenged the description of crop residues as being of low quality and prefers to relate to them as imbalanced forages (Leng 1990). To unlock the energy of cellulosic material, whose gross energy equals to that of starch in cereal grains, delignification and simultaneous reduction of cellulose crystallinity is imperative. Of all chemicals including oxidizing agent hydrogen peroxide (Reddy et al 1989), ammonia treatment through urea hydrolysis is a promising method because of simple technology and low cost involved. Ammonia treatment of wheat straw significantly enhanced the soluble phenolics by 52% and decreased the total cell wall phenolics by about 12% (Reddy and Singh 1992). The CP content was enhanced to 10.37% from 2.59 while ME content was enhanced to 1.99 Mcal/kg DM from 1.62.

Voluntary intake of feed may be increased not only by physico-chemical treatment (chopping and ammoniation) but also through enhanced rumen fermentative digestibility by supplementation of critical nutrients (Reddy 1989), which stimulates intake of feed.

Complete diet was formulated with ammoniated wheat straw (72.8%), molasses (15%), wheat bran (10%), fish meal (1%), and mineral mixture and salt (1.2%) fortified with vitamins A and D₂ (Reddy 1989) because complete diet system reduces diurnal variation and improves efficiency of microbial protein synthesis in the rumen. Knowledge of quantitative nutrition has clearly shown that substantial increases in productivity of ruminants on forage-based diets can be obtained through the balanced nutrient approach that considers the efficiency of the rumen ecosystem and the availability of dietary nutrients postruminally (Leng 1993). Hence strategic supplementation of poultry droppings at 5.2% level in complete diet enhanced rumen degradation kinetics (Reddy et al 1990) and nutrient digestibility and feed intake (Reddy and Singh 1991) while 3 kg green berseem per day to buffalo bulls fed on ammoniated wheat straw diets provided optimum conditions for improved rumen degradation of straw (Reddy et al 1991) and daily feed intake (Reddy et al 1992).

Rice straw

Ammoniation of straws through urea hydrolysis, though simple, could not become popular among the farmers. The most practical approach would be to use rice straw and supplement it with several strategic supplements to ensure efficient rumen function and balance the fermentation end products with bypass nutrients postruminally (Reddy 1995). Supplements such as caged poultry droppings (CPD), sugarcane molasses, deoiled rice bran, drought-tolerant green forages, legume straws, tree foliages are locally available and smallholder livestock raising on such feed resources in integrated farming systems’ approach has the potential to become the environmentally sustainable alternative to industrial livestock production.  Strategic supplementation increases the efficiency of ruminant productivity on straw-based diets (Leng 1991) and thus pave the way for developing ‘environment-friendly’ livestock production system.   

Supplementation of rice straw with 30g mineral mixture and 30g salt daily (Purushotham Reddy et al 1996) to buffaloes enhanced the nutritive value of rice straw to 0.81% DCP and 49.9% TDN (Table 1).

Further supplementation of 500g caged poultry droppings (sundried and ground) and 250g molasses significantly (P<0.01) increased the rice straw intake. However, the maximum CP was only 5.0% which is less than the 6.9 to 8.8% CP on dry matter basis considered necessary to optimize intake of forages (Blaxter and Wilson 1963). Supplementation of 1 kg deoiled rice bran (Reddy 1996) maximized (P<0.05) the utilization of rice straw. The ratio of N / digestible organic matter (N / DOM) for this diet was 0.021, while the optimum should be 0.032 g N per g DOM (ARC 1980). Further addition of protein meals non-significantly increased rice straw and significantly (P<0.10) increased the total DM intake (Prakash et al 1996). Among the protein meals, animals fed fish meal diet had lower urinary N excretion and higher (P<0.01) N retention. The increase in dry matter intake in fish meal-rice straw fed buffalo bulls may be attributed to the rumen undegraded protein content of fish meal, which has been reported previously (Egan 1977; Lee et al 1985).

All legume straw supplemented diets apparently provided rumen degradable N to satisfy the N requirements of the rumen microbes. Though statistically insignificant, rice straw intakes were depressed by the legume straw supplements (Reddy 1997), while blackgram straw increased (P<0.10) the protein digestibility and N retention. Green forage supplementation did not influence the total intake of the animals, although they decreased (P<0.01) the rice straw intake compared to that of control diet (Reddy 1998). Legume pasture or tree foliage were better supplements.

Productive efficiency

The goal of farming is the improvement of economic and productive efficiency of farm animals for example dairy cows in case of dairy farming. Hence livestock are fed for production, and generally not for maintenance. Maintenance of an animal is an important ‘overhead’ of the livestock enterprise. A high milk-producing animal uses less percent of its total ME requirement for maintenance. For example, a dry cow needs 365 g protein, 8.45 Mcal ME, 20.4 g calcium and 14.1 g phosphorus per day (NRC 1989) for its body maintenance. Nutrient requirements per litre of milk in a cow producing 5, 10 and 15 litres per day, respectively, are calculated to be 151, 114 and 102 g protein and 2.37, 1.53 and 1.24 Mcal ME.

How is it possible? An animal’s maintenance requirement must be met before there is any production. The nutrient requirements for production basically increase on a linear basis when production increases. As a result, there is a definite increase in nutrient efficiency as production increases, in general. The increases in productive efficiency that occur in genetically superior cows are principally due to a dilution of maintenance requirement.

Review of research in applied dairy cattle nutrition during 1981 – 2005 revealed that milk yield per cow continues to increase with a slower rate of increase in DM intake. Hence   efficiency of ruminal fermentation and digestibility of the dietary components are key factors in improving the efficiency of feed use (Eastridge 2006).

Bauman et al (1985) reviewed the sources of variation and prospects for improvement of productive efficiency in the dairy cow. Feed constitutes a major component of farm expenditure. Numerous studies have demonstrated that nutrition has a major impact on milk production and thus on farm profitability. Hence, productive efficiency, simply, is the yield of milk obtained in ratio to the nutritional costs associated with maintenance, milk synthesis and loss of body condition during lactation because the cow has to return to the level of body condition that existed before the onset of lactation.

Optimum productive efficiency

An understanding of the several biological processes is vital for improving efficiency by systematic manipulation of metabolism. The biological processes are divided into digestion and nutrient absorption, maintenance requirement, utilization of metabolizable energy for production and nutrient partitioning. Improvements in efficiency could occur as a result of changes in digestion and absorption of nutrients, maintenance requirement, utilization of ME for production or nutrient partitioning.

Nutrient partitioning and partition of nutrients

The term ‘nutrient partitioning’ refers to the processes by which available nutrients are channelled, in varying proportions, to different metabolic functions. Specifically in dairy cow it refers to the partition of nutrients between milk output and body reserves. This partition is a homeostatic mechanism in response to change in nutritional environment.

Nutrient partitioning may be seen as a mechanism for accommodating discrepancies between the composition of the feed and the composition of products formed while the  ‘overflow’ being diverted to body reserves, i.e. partition as a homeostatic mechanism in response to environmental changes.

Once feed is ingested the digestible nutrients are used for maintenance of the animal, used for growth / work / milk / wool production, or reproduction. The partition of nutrients is an exceptionally complex process controlled by the genotype of the animal, the stage of development of the animal, the quantity and quality of feed available, and environment.

 Little variation is observed among cows in their digestion and nutrient absorption, maintenance requirement and utilization of ME for production and hence, such factors do not respond to selection for increased milk yield. However, individual cows differ substantially in feed intake and in the partitioning of absorbed nutrients among body tissues (Swan 1976; Bauman and Currie 1980; Hart 1983).

It is well documented that partitioning of nutrients changes with stage of lactation, and lipolysis and lipogenesis are up or down regulated at different stages of the reproductive cycle. The net result of such changes is that nutrients are channelled to differing extents to different organs and life functions such as growth, milk yield, body lipid reserves, reproduction, immune function etc. Nutrient partitioning that takes place with changes in stage of lactation / reproductive cycle is not as a function of changing nutritional environment but rather as a function of (physiological) time. The onset of lactation provides the classic example of this homeorhetic, or teleophoretic mechanism with the uncoupling of growth hormone (GH) and insulin-like growth factor (IGF) and the resulting channelling of nutrients to the mammary gland (Bauman 2000). This emphasizes that there is an aspect of nutrient partitioning that is genetically driven and the animal has genetic drives.

Analyzing data published by Nielsen et al (2003) of 400 cow lactations, Friggens and Newbold (2007) reported that the cows were in substantial negative energy balance at 14 days after calving and feed intake on that day was only 80% of the maximum intake attained. It was concluded that the observed body lipid mobilization in early lactation was largely genetically driven.

Despite consuming equal quantities of the same diet, two heifers at their first calving (Table 2) exhibited marked differences in nutrient partitioning during the first 67 days of lactation. Cow 1 yielded 12.3 kg milk and gained 39.1 kg of body weight while cow 2 yielded 26.3 kg milk and lost 51.8 kg body weight. Similar differences in partitioning of absorbed nutrients were observed in the comparison of genetically diverse groups of cows.

High milk yields in dairy cows are related to the ability to mobilize body energy reserves. Animals of high genetic merit produce more milk, have greater voluntary intakes and use more of their body reserves in early lactation than those of low merit.

Nutrient partitioning is the major component of productive efficiency that differs among cows. A sustained, high level of milk yield is dependent on the adaptation of many tissues of the body. A coordinated approach of metabolism of body tissues in support of lactation is important. One adaptation of major importance is the use of body fat reserves in the first portion of lactation. Marked mobilization of body reserves during early lactation, and replacement of these reserves in late lactation, is an important component of the increased productive efficiency that genetically superior cows achieve by dilution of maintenance. Control of nutrient partitioning is a multigene trait.

How to improve productive efficiency?

Digestibility of the diet is markedly influenced by many factors related to animal, plant and feed preparation and these include type of diet, level of intake and physical form of the diet. Improvements are possible to enhance productive efficiency by manipulating the nutrient pattern presented to tissues (Baldwin et al 1980).

Response of animal to nutrient supply 

It is well known that optimum utilization of nutrients occur when the individual nutrients are present in balanced proportion. Otherwise the animal’s productivity will be dictated by the lack of limiting nutrient, even if all nutrients are adequate with the exception of say, one amino acid. The same concept applies to the supply of essential vitamins and minerals. Interaction among nutrients, for example, an excess of the amino acid leucine can result in an apparent deficiency of isoleucine and valine, although the valine and isoleucine content of the diet might appear to be adequate. Similarly, the interaction among copper, molybdenum and sulphur is an example in minerals. Such interactions have greater effects on productivity at low planes of nutrition.     

Tremendous potential exists if we can understand the digestive processes for the entire gastrointestinal tract such that dietary manipulation can be used to deliver a quantity and pattern of nutrients that precisely meet the requirements of the animal’s tissues.

As noted from data presented in Table 2, substantial variation exists among cows for nutrient partitioning, and selection for milk yield alters nutrient partitioning.

Improvements in milk production can also be achieved by the administration of agents that alter nutrient partitioning and (or) metabolism of various tissues. Administration of exogenous bGH (recombinantly derived growth hormone) to dairy cows produces increases in milk yield (Table 3) by specifically altering nutrient partitioning. Digestibility of dietary energy and protein, maintenance requirement and the partial efficiency of milk synthesis are not affected. Therefore, the increase in productive efficiency achieved with exogenous bGH is the same as that achieved by selection (for a higher level of production) that results in the dilution of maintenance of costs. Growth hormone is a homeorhetic control causing a coordinated response in which nutrients are preferentially used for milk synthesis. The increase in milk production is not acute, but rather progressive over the first few days of administration and apparently persists as long as exogenous GH is given. Voluntary intake is gradually increased to accommodate nutrient requirements for the increased production of milk, without any evidence of stress or health effects. 

Environment, milk yield and metabolic heat output

Nutritional amelioration

The average body weight of dairy cows has increased over time as cows have been selected to produce more milk. Larger cows have larger gastrointestinal tracts that allow them to consume and digest more feed. This in turn provides more substrates for milk synthesis.

The lactating cow uses ME for milk production at an average efficiency (q) of 65% (Moe et al 1970). Heat increment during milk synthesis is also dependent on the quality and quantity of feed that the animal consumes. On roughage diets rumen fermentation yields a large molar excess of acetate compared with propionate and if the latter is insufficient to provide enough NADPH₂ then acetate may need to be converted, by some form of futile cycle, to heat in order to prevent a metabolic excess (MacRae and Lobley 1982). On a concentrate diet the molar proportion of propionate increases while that of acetate decreases and so supply of NADPH₂ is unlikely to prove limiting.

Kadzere et al (2002) calculated the average heat increment (HI) between 1940 and 1995 in the USA at four efficiencies of milk production of q = 50, 60, 65 and 70% to estimate the impact that changes in efficiencies would have on HI. The results indicated that increased milk production is related to elevated HI. Total milk production and HI have increased over time, although the rate of increase of HI has been slower than of milk production.

The thermal environment is a major factor that can negatively affects milk production of dairy cows, especially in animals of high genetic merit. ‘Metabolism and productivity run parallel’ (Brody 1945). The biological mechanism by which heat stress impacts production and reproduction is partly explained by reduced feed intake, but also includes altered endocrine status, reduction in rumination and nutrient absorption, and increased maintenance requirements (Collier et al 2005) resulting in a net decrease in nutrient / energy availability for production. Reductions in energy intake during heat stress result in a majority of lactating cows entering into negative energy balance. Such cows largely depend on nonesterified fatty acids (NEFA) for energy. Despite having a much greater energy content, oxidizing fatty acids generates more metabolic heat (about 2 kcal / g or 13% on an energetic basis) compared to glucose. Hence precursors for gluconeogenesis such as amino acids, propionate are in greater need.

Hot weather reduces milk production in cows with high genetic merit for milk production. As average milk production per cow has doubled, the metabolic heat output per animal has increased substantially rendering animals more susceptible to heat stress (Collier et al 2006). This, in turn, has altered cooling and housing requirements for dairy animals. Successful cooling strategies for lactating dairy animals are based on maximizing available routes of heat exchange, convection, conduction, radiation, and evaporation. Provision of fans, wetting the animal’s body surface, high pressure mist to cool the air in the animal shed, and facilities designed to minimize the transfer of solar radiation (use of sprinklers, thatch material over the roof of the shed) are used for heat abatement. 

During periods of heat stress protein excesses aggravate the situation because protein itself is calorigenic, and nitrogen excretion requires energy. Hence diet needs to be balanced for amino acids to prevent the feeding of excess crude protein.  Recent advances in animal nutrition, including the feeding of ruminally-protected fats and protected proteins are among other endeavours to reduce metabolic heat production and supply the correct profile of nutrients to high producing cows in early lactation. An adequate supply of nutrients must also include well-balanced mixture of dietary minerals, especially of Na, K, Cl and SO^2-₄.  These play a pivotal role in the thermal physiology of the cow.

Brumby et al (1978) demonstrated that milk was produced most efficiently on diets in which fat provided 16 to 20% of total ME at 5 to 6% level in the total diet. However, the upper level of incorporation of fat in the forage-based diets has been reported to be 2 to 3% (Jenkins and Palmquist 1984), since it affects fibre digestibility, microbial protein synthesis, volatile fatty acid production and feed intake. Thus supplemental fat, as rumen-inert fat, is used as an effective source of energy with low heat increment on straw-based diets in tropical environment.

Rumen protected protein and fat produced from oilseeds provide a practical feeding strategy to regulate nutrient and energy partitioning and augment essential / bioactive fatty acids (linoleic and alpha-linolenic acids) and conjugated linoleic acid content of meat and milk (Gulati et al 2005). Feeding of 1 kg of formaldehyde-treated oilseed protein meal supplement increased milk yield by 10% compared with untreated protein meal supplements in cows and buffaloes of India, which were fed on straw based diets (Garg et al 2002; 2003).  Improving productivity in dairy animals exposed to adverse environmental conditions may be achieved by improving the environment around the animals and improving their nutritional management.

How do you precisely meet nutritional requirements?

Nutritionists formulate cattle rations as per the nutritional recommendations which often contained significant ‘safety factors’ because nutritional requirements and availabilities for all types of cattle, feeds and environmental or management conditions are at variance. The extra nutrients contained in these safety factors to ensure that nutrient requirements were met often increased nutrient excretion and contributed to adverse effects on water and air quality. Thus, an accurate assessment of both animal requirements and dietary nutrient supply is economically and environmentally important. Hence Cornell Net Carbohydrate and Protein System (CNCPS) has been developed to evaluate diet and animal performance. The CNCPS is a mathematical model developed from basic principles of rumen function, microbial growth, feed digestion and passage and animal physiology.

More precise diet formulation

Chemical analysis will continue to be the benchmark to validate other values obtained by rapid procedures. Such procedures are available by utilizing technologies such as near infrared analysis (NIRA), X-ray fluorescence spectroscopy, laser technology and in vitro assays. Analytical information obtained from such quick assays can be used in certain prediction equations to provide further nutrient values to help in achieving more realistic diet formulation.

Environmental pollution 

Ruminants are important producers of greenhouse gases (GHG) such as ammonia and methane (CH₄). Milk production responses to increased nutrient intake usually show a curvilinear pattern that follows the law of diminishing returns. Dietary intake or nutrient density at which efficiency of production (output divided by input) is maximised is by and large different from that which maximises financial profits (VandeHaar and St-Pierre 2006). As a consequence, surpluses of nutrients that may negatively affect the environment have increased dramatically with increased production levels. Nutrients causing environmental concern are in particular those containing excessive nitrogen (N), phosphorus (P) and potassium (K) (Tamminga 2003). Nitrogen surpluses are associated with losses from the farm system through ammonia volatilisation, nitrate leaching, and dissipation as N₂, N₂O, NO and NO₂, which negatively affect the quality of surface and groundwater and air. P wastes from dairy farms are associated with eutrophication in streams and lakes. This abnormal increase in population of plankton resulted in an increased BOD (biological demand). Another major environmental concern is global warming. Enteric fermentation in ruminants gives rise to methane excretion that is a potent greenhouse gas. Thus nutrition has a large impact on profitability as well as on environmental sustainability, and a proper balance is sought by farmers and governments.

Animal manure should be treated as a commodity rather than a waste in wholefarm systems (Tamminga 2003). The environmental impact of N excreted in faeces and urine depends on a number of factors, including the manure N and OM composition, in turn largely affected by diet composition and intake level. Increasing the level of dietary CP not only increases total excretion of N in faeces and urine, but generally gives a proportionally larger rise in urinary N excretion (Castillo et al 2000), which has been associated with increased ammonia volatilisation. Urea in faeces is rapidly converted into ammonia, which is potentially and immediately available for plant uptake. Undigested feed N in faeces is relatively recalcitrant in soils and mineralises slowly, and is therefore unavailable to plants over the short term. Faecal organic N arising from endogenous secretions in the gastrointestinal tract and from undigested microbial protein is intermediate in terms of its availability to plants compared to that from ammonium-N and undigested feed N.

Feeding dairy cows to minimize N excretion

In ruminants N losses from the rumen are reduced by using a reduced dietary N level, a reduced rumen degradable N (RDN) or a more efficient capture of RDN by rumen microbes, by using a complete diet, by increasing the frequency of feeding, etc.

Ruminants make efficient use of diets that are poor in protein both in quantity and quality because of rumen microbial capacity to capture recycled urea nitrogen and synthesize microbial (animal) protein. Further, dairy cows utilize feed crude protein (CP) much more efficiently than other ruminant livestock but still excrete about 2 to 3 times more nitrogen in manure than in milk. This inefficient N utilization necessitates feeding large amounts of supplemental protein, which contributes to increased costs of milk production and to environmental N pollution.

Effects of increasing CP intake on N excretion as well as production

CP content is the major factor dictating N utilization and excretion

The function of dietary CP is to supply the cow with metabolizable protein (MP) as absorbed amino acids (to meet requirements for maintenance and production) but any extra dietary CP that does not contribute to absorbed amino acids (that are used in production) will be largely lost in the urine. Urinary N is the most polluting form of excretory N because much of it is lost as atmospheric ammonia or into surface and ground water. The effects of increasing dietary CP at about 15.1, 16.7, and 18.4% of DM at each NDF level of 36, 32, and 28% (Broderick 2003) arrived at by reducing forage from 75, to 62, and 50% of dietary DM while energy density increased was studied. There was no interaction between energy density and CP level; that means that the cows responded to CP the same way at all 3 energy levels. Milk and protein yield both increased with the first CP increment, but there was no difference between production at 16.7 and 18.4% CP. There was a linear increase in N excretion with increased CP in the diet and most of the extra manure N was found in the urine. Virtually the entire incremental urinary N was excreted as urea, the form that can be quickly broken down and lost as volatile ammonia.

Optimum level of CP

To know the optimum level of CP, stepwise increases of 1.5 percentage units, from 13.5 to 19.4 % CP, were added to 50% forage ration (Olmos Colmenero and Broderick 2003). The production was highest on the 16.5% CP diet. As expected, milk urea N (MUN), urinary urea, and milk N: N-intake reflected the linear decline in N efficiency with increasing CP. Over-feeding protein actually appeared to suppress production. There is a cost of about 7 kcal of net energy per g of N converted to urea (NRC 2001). Similar findings of no increase (Groff and Wu 2003), or even reduced milk yield with more than about 16.5% dietary CP have been reported from a number of trials. Studies of Wu and Satter (2000) suggested phase feeding of higher CP of 17.4% during the first 16-weeks after calving followed by 16% CP during the remaining 28 weeks of lactation to obtain optimum yield of fat-corrected milk. Indeed optimizing biological efficiency of N transfer from feed to milk decreases the cost of production to compete on the ‘global market’ and reduces the nutrients in the excreta.

Rumen undegradable protein (RUP) and protected amino acids

The approach of testing various CP levels in conjunction with substituting different sources of rumen undegradable protein (RUP) is the preferred one to know the optimum level. Olmos Colmenero and Broderick (2004) tested whether dietary CP level could be reduced below 16.6% by feeding a protected SBM and found reduction in milk yield when dietary CP was 15.6% even though the diet was supplemented with a SBM high in RUP. Methionine and lysine are the two amino acids most often cited as limiting for lactating dairy cows (e.g., Schwab 1996). Responses to ruminally-protected methionine have been more consistent than to protected lysine (Armentano et al 1997) and this has reduced commercial interest in supplying protected lysine products. The potential value of this strategy of supplementing ruminally protected methionine (RP-Met) has been exploited (Krober et al 2000) where supplementing RP-Met to a 14.7% CP diet resulted in milk protein secretion equal to that of 17.5% CP diet, but at 31 versus 27% conversion of dietary N to milk N. Rumen-protected amino acids improve the digestion and absorption and thus could reduce the N content in the diet and faeces simultaneously.

What is the practical way of finding out the optimum CP level?

Frequent sampling and analysis of feed ingredients is very important for tracking the CP contents of the actual diet fed. Monitoring MUN can also be used to assess both dietary CP and urinary N excretion in lactating cows and thus is a very useful technique to assess the adequacy of protein feeding. Urea is the primary form of excretory N in mammals and blood urea equilibrates rapidly throughout body fluids, including milk and MUN concentrations reflect blood urea (Rook and Thomas 1985) and equilibrium between blood and milk occurs within 1 to 2 hours (Gustafsson and Palmquist 1993). Therefore, MUN serves as a useful index of inefficient N utilization in the lactating dairy cow (Kohn et al 2002). Broderick and Clayton (1997) reported a strong relationship between dietary CP concentration, expressed either on DM or energy basis, and MUN. Urea in body fluids, including milk, results not only from excess protein degradation in the rumen but also from N inefficiency caused by excess supply of protein to the tissues. 

Matching fermentable energy with RDP

NRC (2001) model prioritize to meet the RDP requirement first, since microbial protein accounts for most of the dairy cow’s MP, followed by supplying matching ruminal fermentative energy. Matching ruminal energy fermentation with RDP will be effective for improving N efficiency, regardless of dietary protein degradability. The effective way to reduce ammonia release from ruminants is to use restricted protein levels in rations supplemented with energy from easily digested carbohydrates (Frank et al 2002).

Methane emission

Methane is the second major contributor to global warming being 23 times that of carbon dioxide (IPCC 2001). It has been estimated that methane production by ruminants is about 15% of total atmospheric methane emissions (Takahashi et al 2005). Given the environmental concerns, Schils et al (2005) developed the whole-farm model with five subsystems (animal, feed, manure, soil, crop) to estimate methane emissions. In the animal submodel, methane production is a linear function of milk production assuming a fixed conversion factor of 0.01 kg methane produced per cow per year per kg milk. Equally, the Intergovernmental Panel on Climate Change (IPCC) recommends calculation of methane emissions based on a fixed conversion value of gross energy (GE) to methane (for dairy cattle: 6% of GE intake, except for diets with a large quantity of grain; (IPCC 1997)).

Mitigation strategies to reduce methane emission

Hydrogen and formate, formed in the rumen during the course of fermentative digestion of feeds, are principally utilized for methanogenesis. Hence production of hydrogen and formate need to be reduced. But these are associated with acetate production, which is the end product of fibre fermentation. It is desirable to utilize fibre maximally for improved ruminant production and hence, this cannot be compromised because lignocellulosic crop residues are the primary feeds in the developing countries. Alternative pathways are to be found to utilize the hydrogen. Otherwise, accumulated hydrogen would suppress rumen digestion (Wolin et al 1997).            

Feed additives

To make animal production highly efficient the nutrient losses are to be minimized, which means more units of output per unit of input. Feed additives play a pivotal role in achieving increased efficiency and reduced environmental load per unit of the animal product. Better quality animal products have higher lean muscle mass and lesser fat laying which is of higher efficiency since protein deposition requires about six times less energy input than fat deposition. Feed additives are used to increase the health status, fertility and performance of animals, and they improve the feed efficiency (feed required to gain unit weight or to produce unit milk) mainly by increasing digestibility of nutrients. These include antibiotics, organic acids, direct-fed microbials (probiotics), prebiotics, enzymes and plant extracts. The use of antibiotics as feed additives, such as ionophore (monensin and lasalocid) antibiotics, has proved to be a useful tool to reduce methane energy and nitrogen (in the form of ammonia) losses from the diet. Review of literature on beneficial effects of ionophores (Tedeschi et al 2003) revealed that monensin might decrease protein degradation in the rumen and increased feed protein utilization by 3.5 percentage units. Ionophores could decrease methane production by 25% and decreased feed intake by 4% without affecting animal performance.  

Halogenated methane analogues, cyclodextrin-combinations, sarsaponin

Compounds such as halogenated methane analogues e.g., bromochloromethane, mevastatin and lavastatin are directly toxic to methanogens and stop methane production. But the hydrogen gets accumulated in the rumen, which could suppress the activity of rumen fermentation (Wolin et al 1997). Addition of α-cyclodextrin-horseradish oil complex decreased methanogenesis and nitrogen content of faeces and urine in steers (Mohammed et al 2004a). Cyclodextrin-iodopropane complex partially inhibited in vivo ruminal methanogenesis without adverse effects on digestion of nutrients (Mohammed et al 2004b) while sarsaponin (Lila et al 2005) decreased methanogenesis by 13%, improved ruminal fermentation but fibre digestibility was decreased.

Organic acids

It has been suggested that the addition of organic acids: fumarate and malate (Castillo et al 2004), the intermediates of carbohydrate degradation in the rumen, would stimulate the production of propionic acid in the rumen and could reduce methane losses by acting as a hydrogen sink. Newbold et al (2005) tested 15 potential precursors of propionate, including pyruvate, lactate, fumarate, acrylate, malate and citrate, in short-term batch cultures. Sodium acrylate and sodium fumarate produced the most consistent effect decreasing methane production by between 8 and 17%. Free acids rather than salts were more effective in reducing methane, but also decrease pH with possible negative effects on fibre degradation.

Dietary means to decrease methane emissions

Strategic supplementation of nutrients to balance the end products of rumen fermentation is an important approach to help ameliorate the greenhouse effect that is, lowering of enteric methane production per unit of feed intake or per unit of milk or meat from ruminants (Leng 1993). Altering forage quality could decrease methane production in the range of 20-40% (Leng 1993). Feeding forage legumes like Lucerne or red clover tended to decrease methane losses compared to grass. This reduction can be further enhanced by legumes that contain condensed tannins such as sulla (Hedysarum coronarium). Higher proportions of concentrates in the feed decrease methane emissions (Yan et al 2000). Due to the presence of non-structural carbohydrates (starch and sugars), concentrates normally ferment faster than forage, giving rise to elevated levels of propionic acid. Methane production can be lowered by almost 40% when a forage rich diet is replaced by a concentrate rich diet. With regard to the ingredient composition of concentrates, selecting carefully defined carbohydrate fractions, such as more starch of a higher rumen resistance and less soluble sugars could significantly contribute to a reduction in methane emission.

Sunflower oil (400 g/d, approximately 5% of dry matter intake) supplementation decreased methane emission by 22 % whereas monensin decreased it by 9 %, but fibre digestibility is impaired with oil supplementation (McGinn et al 2004). Supplementation of unsaturated fatty acids: lauric and myristic acids (Giger-Reverdin et al 2003) should be considered to trap the hydrogen produced during the rumen fermentation. Potential reductions in methane losses are estimated at 10% at maximum. However, added fat should not disturb the microbial ecosystem. These dietary means reduce methane emissions either by directly inhibiting methanogens and protozoa, or by diverting hydrogen ions away from methanogens.

Plant secondary metabolites

Intensified efforts are made to use plant extracts as potential natural alternatives for enhancing livestock productivity. Plants produce a variety of secondary compounds to protect themselves against microbial and insect attack. Some of them are also toxic to animals, but others are not. As a side effect, in some instances inhibiting effects on methane have been observed, most likely mediated through an effect on rumen protozoa, since elimination of ciliates from the rumen can reduce methane emission from the rumen. These include condensed tannins, essential oils, saponins.

Condensed tannins

Sulla, lotus, sericea lespedeza, chicory are some of the condensed tannin-rich forages. Woodward et al (2002) investigated the feeding of sulla on methane emission and milk yield in Freiesian and Jersy dairy cows and reported that cows grazing sulla produced lesser methane per kg dry matter intake (19.5 vs 24.6 g) than those grazing perennial ryegrass pasture. Stall-feeding of sheep with different forages (lucerne, sulla, red clover, cichory and lotus), resulted in decreased methane losses (g CH₄ / kg dry matter intake) by between 20 and 55% as compared to animals pastured on ryegrass/white clover mixtures (Ramirez-Restropo and Barry 2005). By feeding of goats on sericea lespedeza (Lespedeza cuneata) forage, Puchala et al (2005) observed a reduction in methane loss of over 30% compared with goats fed with a mixture of crabgrass (Digitaria ischaemum) and tall fescue (Festuca arundinacea). Similar observations have been made in dairy cows in New Zealand grazing sulla (Hedysarum coronarium) or lotus (Lotus corniculatus) instead of perennial ryegrass (Ramirez-Restrepo and Barry 2005). Cichory (Chichorium intybus) is considered a promising forage to reduce methane losses in ruminants. A possible reduction of methane is at least partly counteracted by an increased loss of CO₂ from the soil and tractor fuel used for tillage, chicory being an annual plant.

Essential oils

Essential oils are the volatile components responsible for the characteristic aroma of spices. The use of plant extracts, because of their antimicrobial properties, appears as one of the most natural alternatives to the antibiotic use in animal nutrition.

Cinnamaldehyde and garlic oil

Several in vitro experiments were conducted to evaluate the effects of cinnamaldehyde and garlic oil on rumen microbial fermentation (Busquet et al 2005a, 2005b). Cinnamaldehyde (main active compound of cinnamon oil) reduced the ammonia N concentration and increased the molar proportion of propionate compared to control. Garlic oil increased the molar proportions of propionate and butyrate and reduced the molar proportion of acetate compared with the control in in vitro trials. These results suggest that cinnamaldehyde and garlic oil may improve the efficiency of energy and N utilization in the rumen. The decrease in methane production observed in garlic oil and its compounds confirms their ability to inhibit methanogenesis.

Menthol

Menthol (L-menthol), which is the main essential oil of peppermint, has an antimicrobial function and antiprotozoal function much like that of saponin. Wallace et al (2002) reported that rates of ammonia N production from amino acids in rumen fluid were decreased by dietary essential oil. Studies conducted in Holstein-steers revealed that peppermint (200 g of sun-dried / steer / day) feeding caused a depression of ammonia N concentration or protozoal number (Ando et al 2003). Later studies in lactating dairy cows (Hosoda et al 2005) showed reduction in nutrient digestibility and methanogenesis and changes in energy metabolism upon peppermint feeding. 

The application of a blend of essential oil compounds (major components thymol, guajacol, limonene) to the rumen of mature sheep (110 mg per day) were small (Newbold et al 2004) and seemed restricted to a reduced deamination. The addition of a similar extract of essential oils (1g per day) to diets of beef cattle in Canada had no measurable effect on methane emissions (Beauchemin and McGinn 2006).

Saponins

Saponons are naturally occurring secondary compounds present in many plants. Examples include Yucca schidigera, Enterolobium cyclocarpum, Sesbania sesban and fruits of Sapindus saponaria, Sapindus rarak. Yucca saponins decreased gas, increased microbial protein and increased true digestibility, suggesting that the saponins affected partitioning of degraded nutrients such that higher microbial mass was produced at the cost of gas, and /or short chain fatty acids production  (Makkar 2008). These saponins increased efficiency of microbial mass synthesis. Saponins are considered to have detrimental effects on protozoa through their binding with sterols present on the protozoal surface (Francis et al 2002).

Saponins decreased the rumen ammonia concentration, which may be due to an indirect result of decreased protozoa. Saponins also form complexes with proteins and could decrease protein degradability. They could also decrease rumen proteolytic activity. Recently, Goel et al (2007) found that saponin rich plant materials such as Sesbania (Sesbania sesban) leaves and seeds of Fenugreek (Trigonella foenum-graecum L) increase the partitioning of nutrients to microbial mass and decrease marginally the methane production per unit of feed degraded. Because of their antiprotozoal activity, saponins might have potential to reduce methane and could be beneficial for improved ruminant productivity depending on the diets and the saponins involved. Some of them have shown positive effects in vitro (Pen et al 2006), but it has also been shown that microbial adaptation to saponins may occur (Wallace 2004). Diaz et al (1994) reported that supplementation of pericarp of the fruit of Sapindus saponaria in sheep inhibited rumen protozoa and stimulated bacterial and fungal counts, and dry matter degradation. No long term in vivo studies with cattle have been reported.

It has been observed that some plant products lose their effects on continuous ingestion of the plants by animals. Thus, a lack of persistency of effects because of microbial adaptation under practical in vivo conditions seems to limit prospects in this area. Perhaps carefully designed alternating feeding strategies could circumvent the problem of adaptation.

Nutritional interventions to reduce CH4 losses

Dairy stock is separated into early lactation cows, mid lactation cows, late lactation and dry cows, and young stock. It is suggested to include more bypass starch in diets for early lactation cows, some extra fat in diets for mid lactation cows and high tannin forage legumes in diets for late lactation and dry cows (Tamminga et al 2007). It is also suggested to further investigate the applicability in practice of feeding young stock more intensive, to accelerate their growth and have them ready for the first calving 1 to 2 months earlier than normal, leading to less methane from young stock

Tree foliage as sustainable feed resource 

Trees of several species could provide palatable and nutritious fodder during drought and scarcity periods by lopping their branches (Reddy 2006). There are many advantages of the forages from multi-purpose tree crops (Devendra 1992). The leaf fodder of some trees is almost as nutritious as that of the leguminous fodder crops. However, presence of antinutritional factors (ANF), especially tannins limits their use as animal feed.

Based on their structures and properties, tannins are distributed into two major classes-hydrolysable tannins (HT) and condensed tannins. Condensed tannins (CT) are hydrolytically cleaved to anthocyanidins and related compounds and are more correctly called proanthocyanidins or proflavanoids. Tannins are hydrosoluble polymers that form complexes with proteins, starch, cellulose and several minerals (Makkar 2003). These complexes are broken under conditions of high acidity (pH < 3.5) or high alkalinity (pH > 7.5) (Jones and Mangan 1977). Hence condensed tannins may be used as organic protectant of protein from rumen degradation making feed protein available post-ruminally for production purposes.

Bhatta et al (1999) indicated the formation of tannin-protein complex even at room temperature without processing, which helped its protection from rumen degradation. Studies of Dey et al (2006) revealed that supplementation of condensed tannins at 1-2% level significantly reduced the in vitro nitrogen degradability of groundnut cake. However, results are to be viewed with caution in view of the observations of Vitti et al (2004) casting a doubt on the generalization that small amounts of CT (2-4%) produce beneficial effects or that high levels (>5%) are harmful. Rather it appears that tannins in some plants are either particularly beneficial or detrimental

Condensed tannin supplementation significantly decreased ruminal ammonia concentration and urinary N excretion without affecting body nitrogen and energy retention, and significantly reduced methane release by 13% on average (Carulla et al 2005). The results suggest that supplemented Acacia mearnsii tannins can be useful in mitigating methane and potential gaseous nitrogen emissions. Sliwinski et al (2004) also indicated that low urinary N excretion might have potential to decrease ammonia N emission from animal excreta (during manure storage). Lesser urinary N and higher faecal N mean decreased environmental pollution because up to 70 % of urine-nitrogen can be lost to the environment. Condensed tannins suppressed NDF and ADF digestibilities suggesting reduction of cellulose degradation, which could be related to a selective suppression of cellulolytic bacteria (McSweeney et al 2001). The reduced fibre digestion was associated with the expected shift in the VFA profile from acetate to propionate at a constant total VFA concentration in ruminal fluid.

Nutrition and gastrointestinal parasitism 

Gastrointestinal parasitic infection is one of the causes for economic and production losses in livestock. In general gastrointestinal parasites reduce nutrient availability to the host through both reductions in voluntary feed intake / or reductions in the efficiency of absorbed nutrients. Mehra et al (1999) reported that there was 30% reduction in feed intake in Fasciola gigantica infected buffalo than control animal and this was main reason for reduced feed conversion efficiency. The consequences of gastrointestinal parasitism on protein metabolism of ruminants were reviewed by MacRae (1993). Parasitism induces an increase in the loss of endogenous proteins viz. blood, plasma, mucin and sloughed epithelium into the gut lumen during infection. Some endogenous protein was fermented to ammonia in the large intestine, for absorption and eventual secretion in urine as urea, while some was excreted undigested in faeces, causing a loss of amino acids to the infected host. In addition, there also appears to be a repartitioning of protein away from productive functions such as muscle, bone and wool growth towards repair functions of the gastrointestinal tract, to mucus secretion and / or plasma and blood replacement. Thus parasitic infections reduce feed utilization and leads to environmental pollution. 

Plant-based anthelmintic compounds

Tree foliage contains many polyphenolic compounds that are inhibitory to gastrointestinal nematodes. For example, feeding of Acacia karoo dried leaves at 40% level in the feed of goats, which were infected with Haemonchus contortus significantly decreased the faecal egg counts and worm burdens by 34% and this was attributed to tannins in the Acacia karoo leaves (Kahiya et al 2003). Studies conducted on calves in Bangladesh showed that pine apple (Ananas comosus) leaves (1.6 g/kg body weight) and neem (Azadirachta indica) leaves (1 g/kg body weight) (both leaves on dry matter basis were 200 mg/kg body weight) had anthelmintic effects (Akbar and Ahmed 2006).

Probiotics and prebiotics  

Several studies have shown that live yeast and yeast culture supplementation may increase feed intake and milk production of dairy cows. Average rumen pH was greater (P<0.01) when yeast was supplemented than when no yeast was provided (Bach et al 2007). Feeding of Saccharomyces cerevisiae increased bacterial population, increased degradation of fibre in the rumen leading to increased feed intake and increased flow of microbial protein from rumen resulting in improved animal productivity. Body weight and feed efficiency increased significantly (Panda et al 1995; Singh et al 1998). However, Kamalamma et al (1996) found no impact of yeast feeding in dairy cows in mid lactation.

As most of the nonstarch polysaccharides and oligosacchharides are degraded / digested by the diverse population of microbes in the rumen, these chemical compounds lack the basic criteria of the definition of prebiotic i.e.the compound must be nondigestible in the upper part of the gastrointestinal tract. 

Enzymes

The use of exogenous fibrolytic enzymes holds promise as a means of increasing forage utilization and improving the productive efficiency of ruminants. Beauchemin et al (2003) reviewed the various aspects of fibrolytic enzymes in ruminant feeding and found the average increase in dry matter intake of 1.0 ± 1.3 kg/d, and the average increase in milk yield was 1.1 ± 1.5 kg/d. Though the response was positive, the variability was also high. There is no doubt that these products will play an increasingly important role in the future. Animal responses to exogenous enzymes are expected to be greatest in situations in which fiber digestion is compromised and when energy is the first-limiting nutrient in the diet. High-producing dairy cows and growing cattle require high levels of available energy to meet the demands of milk or meat production. In such cases, the digestibility depression is common due to rapid transit of feed through rumen and potential of the animal is not exploited. Enzymes help bridge the gap between potential and actual performance of the animal.

Conclusions 

• Strategic supplementation of critical nutrients stimulate intake of feed in ruminants and increase the efficiency of ruminant productivity on straw-based diets.
  
  
• Genetically superior cows have greater ability to mobilize body energy reserves in early lactation to support high milk yields. The same feat can be achieved by administration of exogenous bGH (recombinantly derived growth hormone).
  
  
• Rumen protected protein and protected fat provide a practical feeding strategy to regulate nutrient and energy partitioning thereby increase the milk yield and improve reproductive performance. 
  
  
• Phase feeding of higher CP of 17.4% during the first 16 wk after calving followed by 16% CP during the remaining 28 wk of lactation has been suggested to obtain optimum milk yield in cows.
  
  
• Ionophores could decrease methane production by 25% and decreased feed intake by 4% without affecting animal performance. Monensin might decrease protein degradation in the rumen with increased feed protein utilization by 3.5%.
  
  
• Dietary means such as supplementation of unsaturated fatty acids and higher proportion of compound feed reduce methane emissions either by directly inhibiting methanogens and protozoa, or by diverting hydrogen ions away from methanogens.
  
  
• Plant extracts have been conceived as potential natural alternatives for enhancing livestock productivity. Plant secondary metabolites, in some instances, have inhibiting effect on methane, which is most likely mediated through their effect on rumen protozoa, since elimination of rumen ciliates can reduce methane emission and reduce rumen proteolysis. Plant secondary metabolites such as condensed tannins may be used as organic protectant of protein from rumen degradation thus providing more dietary and microbial protein post-ruminally for production purposes.
  
  
• Tree foliage contains many polyphenolic compounds that are inhibitory to gastrointestinal nematodes, whose infection is one of the primary causes for economic and production losses in livestock. 
  
  
• Feeding of Saccharomyces cerevisiae increased bacterial population and fibre degradation in the rumen leading to increased feed intake and flow of microbial protein.
  
  
• Fibrolytic enzymes help bridge the gap between potential and actual performance of the animal.  In high-producing dairy cows and growing cattle digestibility depression is commonly observed due to rapid transit of feed through the rumen with the potential of the animal not being exploited fully.

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Received 3 February 2008; Accepted 1 February 2009; Published 10 March 2009

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