Feed intake, rumen fermentation, microbial protein synthesis and nitrogen retention in growing cattle given maize or molasses with two levels of crude protein as supplements to a basal diet of rice straw and grass

Doan Huu Luc, Nguyen Van Thu* and T R Preston**

An Giang University, Vietnam
* Cantho University, Vietnam
dhluc@agu.edu.vn
**TOSOLY, Socorro, AA48, Colombia

Abstract

In an experiment with growing cattle fed a basal diet of rice straw and Para grass (Brachiaria decumbens), the 4 treatments arranged in a 2*2 factorial design within a 4*4 Latin Square were: two crude protein levels of 150 and 200 g/100kg LW/day (achieved by giving a supplement of  Sesbania grandiflora foliage and urea); and two energy sources (0.15 kg of molasses or 0.12 kg of maize per 100 kg live weight per day).

The relative contribution of the protein supplements to total dietary crude protein intake was 3-6% for the 150CP treatment and 16-20% for the 200CP treatment. The higher level of dietary crude protein led to increases in DM intake, rumen ammonia concentration, N retention, live weight gain and rate of urinary excretion of purine derivatives. The supplementary energy source, which represented about 5% of the total DM intake, did not affect DM intake, rumen ammonia concentration and N retention. However, maize appeared to support more efficient synthesis of rumen microbial protein than molasses, as the rates of excretion of purine derivatives and of live weight gain were higher for maize. .

Key words: growth rate, purine derivatives, Sesbania grandiflora, urea

Introduction

Traditionally, local cattle have been kept by small-holders in Southeast Asia to provide meat consumption and draught power for land preparation and transportation in production of rice and other crops. In recent years, beef  production has become important in the Mekong delta of Vietnam due to increasing demands from the human population. Laisind cattle (Red Sindhi x local Yellow) were therefore promoted for improving body size and milk production.

Roughages play a major role as feed for ruminants in the tropics. Efficient utilization of roughages depends on the availability of nutrients needed by both rumen microbes and the animal with the ultimate aim of maximizing feed intake and performance (Preston and Leng 1987). During the dry season, most ruminants in Vietnam are fed crop residues and byproducts, such as rice straw, maize stover and sugar cane tops. These feeds are low in protein and are of low digestibility. Appropriate forms of supplementation are therefore key to improving productivity.

Provided that protein and mineral levels in the diet are adequate, the rumen microbial protein production will be a function of the availability in the diet of digestible organic matter (ARC 1984). Positive effects of synchronizing energy and nitrogen release on microbial protein synthesis (MPS) have been reported by Dewhurst et al (1999, 2000). These authors concluded that the degree of synchrony in ruminal release of energy and nitrogen is likely to influence MPS only in diets containing adequate concentrations of readily fermentable carbohydrate.

The aim of this study was therefore to study the potential benefits on feed  utilization, rumen fermentation,  microbial protein synthesis and nitrogen retention in  growing cattle, of combining two contrasting sources of rumen fermentable carbohydrate (Maize grain and molasses) with two levels of crude protein given as supplements to a basal diet of rice straw and grass.
 

Materials and methods 

Treatments and design

The experiment was carried out at the experimental farm of Cantho University.

Four growing Laisind (Red Sindhi*Local Yellow) cattle with average live weight of 173 kg were fed a basal diet of rice straw and Para grass (Brachiaria decumbens) and allocated to 4 treatments arranged as a 2*2 factorial within a 4*4 Latin Square. The factors were:  

Level of crude protein

This was fixed at 150 and 200 g/100 kg live weight and achieved by supplying foliage of Sesbania grandiflora and urea (ratio of 2:1 as N) 

Source of energy

This was molasses or ground maize grain (0.15 kg of molasses or 0.12 kg of maize per 100 kg live weight per day).

Each experimental period was 14 days including 7 days for adaptation and 7 days for collection of feces and urine.

Feeds and management

Para grass was offered at 1% of live weight (DM basis); rice straw was offered ad libitum.  . The supplements were fed at 06:30h and 14:00h, followed  by the Para grass and rice straw. The urea was mixed with the energy supplement; Sesbania was fed as fresh foliage.  Feed offered and residues were measured every morning. Feces and urine were collected daily during the last 7 days of each period. Rumen fluid was collected for determination of pH, total volatile fatty acids (VFA) and ammonia. Samples were taken  3 h after the morning feed  on the last day of each period using a stomach tube.  

Chemical  analysis

Feed samples and feces were analyzed for DM, ash and crude protein according to the standard methods of AOAC (1990). Neutral detergent fiber (NDF) was determined by the method of Van Soest et al (1991). Rumen ammonia concentration was determined by distillation and titration with sulphuric acid (http://mekarn.org/labman/Amoniac.htm). Rumen volatile fatty acids (VFA) were determined by the procedure of Barnett and Reid (1957).

Digestibility and nitrogen balance 

Apparent digestibility coefficients for DM, OM and NDF, and nitrogen balance, were determined by the methods described by McDonald et al (1998).

Purine derivatives, microbial nitrogen

Allantoin was determined by the method of Young and Conway (1942), while uric acid was analysed according to the method of Fujihara et al (1978). The daily production of rumen microbial N was estimated using the formulae in the paper by Liang et al (1999).

Live weight change

The animals were weighed  on two consecutive days, in the morning prior to feeding, at the beginning and end of each experimental period.

Statistical analysis

The data were subjected to an analysis of variance (ANOVA) using the General Linear Model option in the  Minitab (2000) software. Sources of variation were: animals, periods,  protein level, energy source, interaction crude protein*energy and error.

Results and discussion

Chemical composition of feeds

The values obtained for the chemical composition of the feeds (Table 1) indicated that these were in the normal range according to average values found in the literature (http://www.fao.org/ag/AGA/AGAP/FRG/afris/default.htm).

Feed intake and rumen parameters

The contribution of the energy sources to the overall diet DM was relatively small (about 5%; see Figure 1).

Thus it is not surprising that there were no effects on intake (Table 2)  due to the contrasting sources of the supplementary energy source (sugar in the molasses and starch in the maize).

The overall crude protein of all the diets was below the level considered to be the minimum for satisfactory rumen microbial growth (Van Soest 1994), thus it is to be expected that all the digestible dietary crude protein would be utilized by rumen micro-organisms. The fact that there was no interaction between protein level and energy source implies that, at their low levels in the diet, both energy sources were equally effective in supplying energy for microbial growth.

The relative contribution of the protein supplements to total dietary crude protein intake was 3-6% for the 150CP treatment and 16-20% for the 200CP treatment (Figure 2).  

This difference was sufficient to stimulate DM intake (Table 2) although the increase was relatively small (4.6%).   

The increase after feeding, in rumen ammonia concentrations with increase in dietary protein level (Table 3), confirms that the positive effect on DM intake of the increased supply of crude protein was brought about primarily at the level of the rumen.

The maximum ammonia concentration of 15mg/litre was, however, considerably lower than the level of 20-25 mg/100ml considered to be necessary to maximize intake of straw-based diets (Perdok and Leng 1989), which implies that the 200 g crude protein/100 kg live weight treatment was not providing a sufficient amount of ammonia for the rumen flora. The lack of effect of the energy source, and also of the protein level,  on rumen VFA concentrations can be explained by the relatively low contribution of  the supplementary sources of both energy and protein to the total diet DM (Figure 1).

Apparent digestibility, nitrogen retention and live weight gain

The increase in apparent digestibility of dietary crude protein (Table 4) was almost certainly a consequence of increased crude protein intake as this results in the metabolic fecal N being a smaller proportion of the total fecal N.

Increments recorded in N intake, and in fecal and urinary N and N retention, all follow from the increased supply of dietary crude protein in the CP200 compared with the CP150 treatments (Table 4). The fact that the N retention as a proportion of N digested was the same on both CP treatments (0.504 and 0.505 for CP150 and CP200, respectively), indicates that the increase in crude protein supply had no effect on the overall biological value of the dietary crude protein. Energy source did not affect apparent digestibility coefficients nor the components of the N balance.

Both crude protein level and energy source affected the change in cattle live weight which was higher with increased protein supply and with maize rather than molasses. The positive effect on live weight change with maize compared with molasses, although at variance with the data for N retention (which showed no difference between energy sources), is supported by theoretical considerations. Molasses is composed mainly of soluble sugars and is rapidly fermented in the rumen giving rise to a VFA pattern dominated by butyric and acetic acids and with low levels of propionic acid (Marty and Preston 1970). By contrast, maize is composed principally of starch which is more resistant to rumen microbial attack giving rise to a rumen VFA pattern with high levels of propionate. Moreover, a proportion of the starch may escape the rumen fermentation passing directly to the abomasum and small intestine where it is digested by gastric enzymes to produce glucose (Preston and Leng 1987). For these reasons maize is considered to be more glycogenic than molasses (Preston and Leng 1987). In support of this hypothesis, it was reported that maize was vastly superior to molasses for milk production in Holstein cattle fed a basal diet of Pangola grass in Cuba (Clark et al 1972). Replacing molasses with maize was associated with major changes in the pattern of rumen fermentation with linear increases in propionic acid and decreases in butyric acid. In cattle fattened on ammoniated Star grass (Cynodon nlemfuensis), those supplemented  with maize grain had 20% higher growth rate than those fed molasses at the same level of  20% of diet DM (Royes et al 2001).

Purine derivatives and microbial protein synthesis

Rate of excretion of purine derivatives and of synthesis of rumen microbial protein was higher for the higher level of dietary protein and for the maize compared with the molasses supplement (Table 5).

These results correspond with the positive effects of the supplements on live weight change, which was closely correlated with the rate of microbial N production (Figure 3). 

On the basis of the measurement of purine derivatives excreted in the urine, it would appear that maize was better than molasses as a substrate for rumen microbial growth. Surprisingly this was not reflected in the data for N retention where there was no interaction between protein supply and source of energy, implying that both molasses and maize were equally effective in supporting rumen microbial synthesis.  In future research in this area it would be appropriate to study wider ranges of intake, both of crude protein and of the two energy sources.

Conclusions

• Increasing the level of dietary crude protein (from a combination of urea and Sesbania grandiflora foliage) from 150 to 200 g/100 kg live weight led to increases in DM intake, rumen ammonia concentration, N retention, live weight gain  and rate of urirary excretion of purine derivatives of cattle fed a basal diet of rice straw and Para grass.
  
  
• Maize appeared to support more efficient synthesis of rumen microbial protein than molasses, which was reflected in a higher live weight gain although not in N retention.

References

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Received 14 June 2009; Accepted 20 June 2009; Published 1 July 2009

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