Performance and carcass characteristics of hair sheep lambs finished on tropical pasture or rangeland and supplemented with maize

S A Weiss, R W Godfrey, R Ben-Avraham and R C Ketring

University of the U.S. Virgin Islands Agricultural Experiment Station, RR1 Box 10,000, Kingshill, St. Croix, U.S. Virgin Islands 00850 USA
sweiss@live.uvi.edu

Abstract

In the tropics and other warm season climates, improved cultivated pastures (IP) provide alternative grazing management compared to grazing traditional native pasture (NP) for hair sheep production. Post-weaning hair sheep feeder lambs grazed on IP with energy supplementation can provide an alternate forage-based pasture finishing option compared to traditional grain-based finishing methods that rely upon a high-cost concentrate diet. To evaluate alternative pasture finishing methods, Dorper X St. Croix White hair (DSTX) lambs were managed under two types of post-weaning alternative pasture finishing systems and live animal performance and carcass characteristics were measured during the fall of 2006 and 2007. For eight months after weaning (120 days after birth), all lambs grazed NP consisting of guinea grass (Panicum maximum Jacq.) and hurricane grass (Boithrocloa pertusa L. A. Camus). Following preconditioning, lambs were stratified by weight (average 27 kg) and sex into two treatments (NP or IP) with three replications. All NP was of similar forage composition containing a mixture of guinea grass and hurricane grass. Improved pasture consisted of a mixture of seeded tropical legumes including cow pea (Vigna unguiculata L. Walp., cv ‘Iron and clay’), blue pea (Clitoria ternetea L., cv. Tehuana), lablab (Lablab purpureus L. Sweet, cv. Rongai), volunteer desmanthus (Desmanthus vergatus L. Wild.), and volunteer guinea grass. All lambs were supplemented with maize (Zea mays L.) daily at a rate of one percent of their body weight for 100 days and slaughtered at approximately 365 days of age.

Improved pasture lambs had greater total weight gain than NP lambs (11.1 vs. 7.6 kg, respectively). In addition, IP lambs had higher average daily gain (ADG) than NP lambs (114 vs. 73.3 g/day, respectively). IP lambs compared to NP lambs were heavier at slaughter (39 vs. 35 kg, respectively), had heavier chilled carcass weights (19 vs. 16 kg, respectively), and greater dressing percent (50 vs. 47.8 percent, respectively). Further, IP lambs had greater leg circumference (43.9 vs. 41.6 cm), increased body wall thickness (14.4 vs. 10.8 mm), and larger loin eye area (12 vs. 10.1 cm²) compared to NP lambs. Back fat thickness was greater for IP compared to NP lambs at 3.33 and 2.66 mm, respectively. The IP lambs also had greater  kidney/pelvic fat than NP lambs (696 vs. 530 g, respectively). Results of this study indicate that the adoption of alternative pasture finishing practices utilizing improved pasture with cultivated forage legumes and maize supplementation can lead to improvements in animal performance, carcass muscularity, and beneficial fat accumulation of crossbred hair sheep lambs under tropical conditions.

Keywords: forage legumes, poly-culture, St. Croix White hair sheep, tropical pasture

Introduction

Pasture finishing systems for ruminant livestock production can provide forage based alternatives to conventional livestock finishing operations that rely heavily on grain-based diets. Commercial grain and soybean production, for use as a total mixed ration in confinement livestock feeding systems, depend heavily upon non-renewable fossil fuel inputs (fertilizers, machinery, pesticides, and other inputs; Pimentel et al 2008). In the United States of America (USA), confinement livestock feeding systems are “incredibly energy intensive” with a combination of grain and forage fed requiring 40 kcal to produce 1 kcal of beef protein compared to one-half (20 kcal) as much energy to produce 1 kcal of beef protein on pasture (Pimentel et al 2008). Maize (Zea mays L.) accounts for more than 90 percent of total feed grain value and production and is by far the most widely produced feed grain in the USA (USDA 2010). Grain and soybean commodity prices have rapidly increased over the past years with maize prices reaching an all-time record at U.S. $0.327 per kg in June, 2011 (McFerron 2011). In contrast, low-external-input pasture finishing systems produce animal protein from grassland productivity attained through native rangeland, improved or cultivated pasture, improved forage varieties, and the use of grass-legume mixtures. Forage-based livestock production systems are becoming more attractive economically as high-external-input costs increase.

Meat from pasture finished livestock is gaining renewed market/consumer acceptance and provides a means for livestock producers to obtain economic premiums for their products. Forage-fed beef sales from pasture-based production systems have been growing at a rate of approximately 20% per year and are on the increase (Mathews and Johnson 2010). In converting pasture and forages into marketable products, sheep are 26 times more efficient than cattle (Outhouse et al 2007). The U.S. Virgin Islands (USVI) imports 100% of the concentrated feed used for animal production and the added transportation increases the cost of concentrate feed by 2 to 3 times that of the cost of feed sold in the continental USA. Alternative pasture finishing systems that utilize legumes may provide viable options to livestock producers. 

Legume improved pastures

Improved pastures that contain a mixture of both grasses and a variety of legumes offer grazing livestock a choice of different forages compared to monocultures dominated by a single graminaceous species. In a review by Rutter (2006), cattle and sheep did not graze at random, but consumed mixed diets of grass and legumes with a partial preference of approximately 70% for legumes. Various theories discussed by Rutter (2006) and Dewhurst et al (2009), detail the importance of a mixed grass and legume diet for ruminant production that include carbon/nitrogen balance, maintaining rumen function, and the basic need to maximize the nutrient benefit achieved per unit energy expended grazing. 

Forage legumes, when incorporated into small ruminant diets, have been shown to increase ruminant performance due to an increased intake potential and a higher crude protein (CP) concentration compared to perennial grass based grazing systems. Speijers et al (2004), found that lambs grazing legumes had higher daily growth rates and required fewer days to finish than lambs grazing perennial rye grass pasture. In a similar study Fraser et al (2004), finished lambs on two different types of legume improved pasture (LIP) compared to perennial ryegrass (Lolium perenne L.) pasture. Results in that study indicate that the legume forages had higher CP concentration when compared to the ryegrass and lambs grazing legumes had increased dry matter (DM) intake than lambs grazing ryegrass pasture. Lamb performance improved by increased live weight gain and decreased time to slaughter for lambs grazing legume improved pasture compared to lambs grazing perennial ryegrass pasture. Fat-tailed sheep grazed on tropical native grass pasture improved with the leguminous shrub Leucaena leucocephala had nearly twice the growth rate as animals grazed on native rangeland (Nguluve and Muir 1999). Goats grazing pasture improved with legumes have also demonstrated greater average daily gain (ADG) compared to goats grazing native pasture with greater forage CP levels associated with the legume improved pasture (Muir and Massaette 1996; Goodwin et al 2004; Muir and Weiss 2006).

Ruminant pasture finishing systems, rely upon a forage diet that are often limiting in energy. Energy supplementation is utilized to increase digestible energy intake to improve animal performance. The use of a high energy, rapidly degraded starch, such as maize fed to supplement ruminants on pasture is not a novel practice, and has been correlated to decreases in forage intake, but increases in total dry matter intake (DMI) and forage digestibility (Jochims et al 2010; Islam et al 2000; Kawas et al 1999). Guinea grass (Panicum maximum, Jacq) is typically characterized as a low nitrogen (N) tropical grass, low in digestibility, and high lignin content. In metabolic trials, Kozloski et al (2006; 2007) showed that nutrient supply to lambs fed a low N tropical grass based diet decreased when a low N, high starch source was supplemented, but was improved through supplementation that included high starch and high degradable N sources. Amaral et al (2010) through similar metabolic trials of lambs evaluated different combinations of high/low starch and degradable/undegradable N supplements on intake, digestion and nutrient supply. Results concluded that supplementation of ryegrass (Lolium multiflorum Lam.) diets should include both starch and true protein sources. Therefore, the development of a complete dietary finishing system for livestock should include a mix of grass and legume forage species that includes an energy source fed on a supplemental level.

Hair sheep production

Hair sheep breeds originate from tropical/subtropical environments, can be traced back to African seed stock, and are the primary class of sheep raised in the Caribbean for meat production due to their environmental adaptability (Shelton 1991; Godfrey and Weis 2005).  Carcass weight of breeds of hair sheep lambs raised under a variety of conditions in the tropics has been reported to range from 4.6 kg to 18.1 kg (Martinez et al 1991; Wildeus and Fugle 1991; Hammond and Wildeus 1993; Godfrey and Collins 1999). Attempts at the University of the Virgin Islands, Agricultural Experiment Station to produce heavier carcasses from hair sheep have primarily focused on crossbreeding. Godfrey and Collins (1999) reported that Suffolk X St. Croix White lambs had higher ADG and carcass weights than St. Croix White lambs, but the lower feed efficiency and higher cost of gain of the crossbreds, coupled with the high cost of imported concentrate feed eliminated any economic advantage of the growth and size of the Suffolk sired lambs. In a subsequent study, Godfrey and Weis (2005) reported that Dorper X St. Croix White lambs had a lower cost of gain and higher ADG than St. Croix White lambs fed a concentrate diet. This led to a net value for the crossbred carcasses of $12 compared to $0.67 for the St Croix White lamb carcasses. The high cost of imported concentrate feed for small ruminant production systems in the USVI has led to greater interest in utilizing native and improved pastures to provide nutrients during the finishing phase of production. The aim of this research is to compare the effect of grazing high N containing tropical legumes (true protein source) and native grass with maize supplementation to grazing native pasture (grass only) with maize supplementation on lamb growth performance and carcass characteristics.

Materials and methods

Location

The experiment took place in St. Croix, U.S. Virgin Islands (USVI) at the University of the Virgin Islands, Agricultural Experiment Station in the fall of 2006 and 2007. St. Croix lies in the eastern Caribbean at 17º 43’ N latitude and 64º 48’ W longitude. St. Croix is characterized as having a tropical/subtropical environment with a bimodal rainfall climate with 20-yr annual rainfall averages of 1100 mm and mean monthly temperature ranges from 22°C to 32.8°C. The annual mean temperature on St. Croix is 28.3°C with a mean high temperature of 32.8°C (Godfrey and Hansen 1996). The average annual relative humidity is 77.8% and average monthly relative humidity ranges from 74% in March to 81% in September. 

Grazing system treatments and animals

Treatments consisted of two independent pasture-based lamb finishing systems comprised of either native pasture (NP) or improved pasture (IP). The NP treatment served as the traditional grazing system (control) and contained guinea grass (Panicum maximum Jacq.) and hurricane grass (Boithrocloa pertusa L. A. Camus). The IP treatment was cultivated and planted with a mixture of forage legumes and improved guinea grass. Prior to planting, the IP was plowed and lightly disked; seeds were broadcast onto the surface, and packed into the soil using a culti-packer roller in August of 2006 and 2007.

Cultivated species included in the IP mix included: Vigna unguculata L. Walp. cv. ‘Iron and Clay’, Clitoria ternetea L. cv. Tehuana, Lablab purpureus L. Sweet cv. Rongai, and Panicum maximum Jacq. cv. Mombasa at 25 percent of their individual recommended seeding rates (50, 10, 20, and 3 kg/ha, respectively). Volunteer desmanthus (Desmanthus vergatus L. Wild.) and hurricane grass were also present. Urbana laboratories‚ “cowpea type” rhizobium inoculant was added directly to all legume seeds prior to planting. No irrigation, fertilizers, or herbicides were applied for the duration of the experiment. The IP and NP were adjacent to one another and therefore, received similar precipitation in 2006 and 2007. In 2006 and 2007, there was a season-long rainfall total of 1,074 and 1,098 mm, respectively, compared to the 24-year mean of 932 mm for the same location (Figure 1).

Figure 1. Monthly mean rainfall levels (mm) during the 2006 and 2007 pasture finishing trial conducted on St. Croix, United Stated Vigin Islands

Dorper X St. Croix White (DSTX) crossbred lambs were selected from the University of the Virgin Islands research flock and were 9 to10 months of age with a mean starting weight of 27 kg. Seven days prior to the start of the experiment, wether and ewe lambs were stratified by weight and gender and randomly assigned to either the IP or NP in groups of six animals with three replications per treatment (n=36) within each year. Herbage availability was monitored during the grazing trial for both IP and NP lambs and herbage availability was determined to provide forage levels that exceeded animal intake, thereby allowing selective grazing by lambs. At no point in the 100 day experiment did forage availability for either the IP or NP fall below the recommended daily DM intake level of 3.3 kg forage/lamb/day based on the highest level of intake requirements recommended by Gibb and Orr (1997) for lactating ewes on pasture. Mean herbage availability for the 100 day grazing trial for the NP and IP measured 11 and 7 kg forage/lamb/day, respectively. Forage availability for both the NP and the IP exceeded recommended DM intake levels and thus indicate that forage DM availability did not limit lamb intake.

All lambs received maize (11.5% CP and 88% TDN) supplement daily at 1% body weight for the duration of the 100 day pasture finishing period while grazing either IP or NP. All lambs were weighed on a weekly basis to determine average daily gain (ADG) and the amount of supplement was adjusted weekly throughout the trial to account for changes in lamb body weight. Maize supplement was fed in troughs (152 cm long x 15 cm wide and 10 cm from the ground) with at least 0.25 m of linear feed trough space per lamb to minimize feeding competition. Lambs had free choice access to salt and water at all times throughout the trial.  

Total gain was determined as the difference between body weight at the start and end of the 100 day pasture finishing period. Average daily gain was calculated for each lamb as total weight gain divided by the number of pasture finishing days ending at the final live weight prior to slaughter. At the termination of the 100 day pasture finishing trial, all lambs were slaughtered by treatment; and at approximately 24 hours postmortem, carcasses were evaluated for chilled weight, back fat thickness (mm) over the 12^th rib, body wall thickness (mm) below the 12th rib, kidney/pelvic fat (KPF; g), leg circumference at the base of the tail (cm), and loin eye area (cm²) measured between the 12^th and 13^th rib (LEA). Dressing percent yield was calculated as hot carcass weight / live weight x 100.

Forage sampling

Forage above-ground biomass yield, chemical composition, and grass/legume/forb pasture composition were determined each year at 30, 60, and 90 days after the onset of the 100 day trial by clipping forage at 3 cm above the ground, separated by forage class (grass, legume, or forb, corrected for DM, and reported on a per hectare basis. Forage samples were harvested from three randomly selected points within each sub-paddock for the three sampling periods previously described using a 0.25 m² quadrat. Sub-samples of each forage class from each quadrat were weighed at harvest, dried in a forced-air oven at 55° C until weight loss ceased and weighed to determine percent DM. Dried samples were ground in a Wiley mill to pass a 1-mm screen and analyzed for percent acid detergent fiber (ADF), acid detergent lignin (ADL; Van Soest and Robertson 1980), and total nitrogen (N; AOAC 1990). Forage N concentrations were analyzed using an aluminum block digester (Gallaher et al 1975). The digest used was 5 g of 33:1:1 K₂SO₄:CuSO₄TiO₂, and the solution was digested for two hours at 400°C using 17 ml of H₂SO₄. Mineral concentration of the digestate was determined by semi-automated colorimetry (Hambleton 1977) with a Technicon Autoanalyzer II (Technicon Industrial Systems, Tarrytown, New York). Plant nitrogen concentration is used to estimate the total nitrogen available in above ground herbage (percent plant nitrogen x herbage biomass) and is reported as kg N/ha. Plant nitrogen concentration is also used to determine crude protein (CP) that is estimated as 6.25 X N (Van Soest 1994). Crude protein combined with ADF concentrations can be used to predict forage nutritive value in cattle (Lippke and Herd 1990) while ADL concentration, as an indigestible cell wall component, is a key predictor of cell wall degradation (Hatfield et al 1999).

Statistical analysis

The data analyses for this study were generated using SAS/STAT software, Version 9.3 of the SAS System for Windows. Copyright © [2011] SAS Institute Inc. SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS Institute Inc., Cary, NC, USA. Analysis of variance was conducted using the generalized linear mixed model procedure (PROC GLIMMIX) and pairwise comparisons of least square means were conducted by invoking the DIFF option. The level of significance was 5% (LSD_0.05). Animal performance data (dependent variables) consisting of ADG, total gain, and carcass measurements was analyzed using treatment, sex, year, replication, and the appropriate interactions in the model. Lamb performance means were calculated using data collected from each animal sampled (six replicated experimental animal units) within each treatment replication. When no differences in means were detected between replication and year, data was pooled across years for final analysis. Forage DM yield, pasture composition, nitrogen availability, and forage chemical composition data were analyzed by treatment, year, and forage class (where applicable).

Results and discussion

A significant treatment x year and treatment x sex interaction was detected for ADG and a significant treatment x sex interaction was present for total weight gain. The simple effects of treatment and sex for both ADG and total weight gain were significant and are reported followed by their relative interactions in Table 1. Overall, IP lambs out performed NP lambs. Increased lamb performance was observed for total weight gain and ADG (Table 1) when pasture finishing lambs on IP compared to NP treatments. Lambs in the IP treatment gained on average 4 kg/head (32%) more than lambs from the NP treatment. Average daily gain followed a similar trend with IP lambs gaining 36% (41 g/day) more than NP lambs. Wether lambs had 27% greater total weight gain and increased ADG by 28% compared to ewe lambs. Analysis of the treatment x sex interaction indicated that growth performance was greatest for IP wethers that exhibited increased ADG rates of 34, 42, and 51% compared to IP ewes, NP wethers, and NP ewes, respectively. The combined effect of higher nutrition and increased physiological gain potential of castrated male lambs over female lambs influenced these results. Ewes grazing IP gained 22% more than ewes grazing NP, but were not different than wethers grazing IP. The increase in animal performance of IP lambs resulted in IP lambs to be 12% heavier at slaughter compared to NP lambs (Table 2).

Improved growth performance of the IP lambs resulted in increased carcass quality postmortem. Carcass characteristics of IP and NP lambs are shown in Table 2. Improved pasture lambs had heavier chilled carcass weights and dressing percent increased by 4.4% compared to NP lambs. Increased muscle mass was observed in the IP lambs compared to the NP lambs with IP lambs exhibiting greater leg circumference, body wall thickness, and loin eye area. Improved pasture lambs had 20% greater back fat thickness and 24% more kidney/pelvic fat than NP lambs.

Herbage characteristics for overall treatment means were determined for the NP and by totaling IP forage class values to balance the analysis. No interactions were detected between treatment and year. Total forage biomass was 37% greater in the NP than the IP. However, herbage from the IP had 31% more available N than the NP. Herbage in the IP had 58% higher CP and 17% lower ADF than NP herbage (Table 3). When analyzed by forage class, treatment and year; herbage characteristics had numerous 2-way and 3-way interactions. Therefore, herbage means are reported for treatment and forage class separately for 2006 and 2007 (Table 3). A shift in IP plant composition was recorded from Year 1 to 2. In 2006, IP cultivated legumes were 82% of the total pasture composition and in 2007 legumes represented only 10% of the total pasture composition. This represents a 79% decline in legume biomass. During the same period, grass and volunteer forb biomass in the IP increased by 94 and 58%, respectively (Table 3).

Differences in forage biomass and pasture composition may be attributed to varying rainfall levels and environmental conditions in 2006 and 2007 during the months preceding the grazing trials (including the establishment period for the IP) and during the 100 day pasture finishing trial (Figure 1). The potential negative effects of reduced legume availability on IP lamb growth in 2007 were not realized statistically and could have been diminished by the higher nutritional contribution of the forb and legume biomass and the increase in grass biomass recorded in year 2. Muir and Weiss (2006) found similar results in a study evaluating pasture finished goat kids receiving three levels of energy supplement where shifts in yearly herbage of improved pasture biomass did not affect goat performance and the results indicate that the ADG of goat kids finished on improved pasture was greater than that of goat kids finished on native rangeland.

Since the NP was already well established in dominant native grasses prior to the grazing experiment, forage biodiversity was minimal and limited to two grass species (guinea grass and hurricane grass). However, plant biodiversity was high in the IP polyculture, relative to the NP monoculture. Herbage biodiversity is evident in the pasture composition of the IP which contained multiple species of both seeded and volunteer legumes, grasses, and broad leaf forbs in 2006 and 2007. In comparison only two grass species were present in the NP.

Overall lamb performance in the IP treatments could be attributed to improved rumen function as a result of high forage diversity that includes multiple forage legume species. In a review of the nutritive value of forage legumes by Dewhurst et al (2009), the biological and chemical factors associated with a holistic approach to ruminant physiology and grazing systems that include mixed grass/legume forages are discussed in detail. Key concepts detailed by Dewhurst include: grazed herbage that include legumes result in increased ruminant intake resulting in increased animal performance; legumes have a reduced rate of decline in digestibility with advancing maturity compared to grasses; reticular legume vein structure breaks down more readily [than long grass fibers] resulting in small particle size contributing to increased digestibility and improved rumen function; and a diverse mixed diet of grass and legumes promotes and maintains effective rumen function.  

Legumes have been recognized to contain high nitrogen levels resulting in high crude protein concentration and improved animal performance (Brown and Pitman 1991; Fraser et al 2004; Speijers et al 2004). Crude protein levels from the seeded legumes and volunteer broadleaf forbs sampled in the IP were greater than grass CP levels from either the IP or the NP where grass CP levels did not differ between the IP or NP treatments (Table 3). An increase in total nitrogen available from herbage was seen in the IP and resulted from the higher recorded plant nitrogen levels found in the legumes and broad-leaf forbs.

Greater nitrogen levels in the herbage correspond to increased total nitrogen available (represented by kg nitrogen/ha) for lamb growth and development resulting in increased lamb performance observed in the IP. Overall, the greater quantity of nitrogen available to lambs through grazing the IP compared to a lower quantity of available N in the NP resulted in an improved rate of gain, muscle development, and fat deposition in lambs from the IP finishing system. Forage quality observed over the length of the 100 day pasture finishing period for the IP and the NP indicated that ADF (g/kg herbage) was consistently lower in the IP. When compared by forage class within the IP, grass herbage in the IP had higher ADF compared to either legume or forb herbage. Similar ADF concentration was observed between grass from the NP and IP. However, ADL (g/kg herbage) levels between treatments and within each treatment showed no prominent trends.

Conclusion

• In conditions where forage nutrient resources are limited and the cost of input resources are high, integrated pasture finishing systems can provide economic incentives and overall increased farm sustainability. This is particularly the case in tropical climates where native pasture forages are typically low in feed value and the cost of imported feed and other production based inputs are either unavailable or economically unfeasible. Results of this experiment indicate that lambs finished on poly-culture pastures improved with a diversity of forage legumes and a mix of native forbs and grasses can increase growth performance and improve carcass quality compared to lambs pasture finished on native grass pasture. Tropical forage legume based pasture finishing systems could provide alternative livestock management options to small farmers with limited resources to reduce imported commercial feed dependence and improve lamb carcass quality.

Acknowledgments

This research was supported, in part, by USDA Tropical Sub-tropical Agriculture Research Program grant No. 2005-34135-16036.

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Received 18 October 2015; Accepted 20 October 2015; Published 1 November 2015

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