Autumn-Grazed Orchardgrass-White Clover Pasture: Nutritive Value of Herbage and Lamb Performance

Abstract

Performance of livestock grazing pasture is inherently variable. Pastures must be managed to optimize quantity and quality of herbage to meet the nutrient requirements of a specific class of livestock involved in a defined production goal. Consumer desires for healthier meat products have shifted the emphasis to lean, trim carcasses from livestock production systems using forage crops. The purpose of this study was to characterize herbage nutritive value changes in grass-legume pastures grazed during autumn and to compare growing lamb (Ovis aries) performance and carcass quality when grazing autumn pasture or fed concentrate in feedlot. Less crude protein (CP) from legume was available for use by grazers in 1992 than in 1991. Frequent clipping of autumn pasture resulted in herbage with greater nutritive value (lower neutral detergent fiber [NDF] and higher in vitro organic matter disappearance [IVOMD]) than more mature stockpiled herbage. A computer model of energy determined from acid detergent fiber (ADF) nutritive value and herbage mass data predicted greater potential for lamb growth on intensively managed (clipped) vs. stockpiled herbage. Lambs fed grain had greater (P ≤ 0.05) cumulative weight gain, average daily gain (ADG), slaughter weights, chilled carcass weights, dressing percentages, leg conformation scores, ribeye area (REA), fat over rib, quality grade, and yield grades than lambs grazed on autumn pasture. However, carcasses from lambs grazing pasture in autumn had 14% less fat and about 8% more protein than lambs fed grain. While total fat content of carcasses was lower for pasturegrazed than grain-fed lambs, saturated fatty acid composition and cholesterol concentrations in lean tissue were not different. Percentage legume in the sward may have influenced the proportion of individual fatty acids in lean and fat. Market weight lambs may be produced from botanically complex autumn pasture. Research Question Livestock performance varies on pasture probably as a result of fluctuations in forage plant nutritive value, herbage productivity, and botanical composition. Canopy management, as well as livestock genetics, can influence the variability. Our study characterized herbage nutritive value changes of grass-legume pastures resulting from grazing during the autumn growth interval. In addition, performance and carcass quality were compared for lambs that grazed autumn pasture or were fed concentrate in feedlot. Literature Summary Extensive knowledge of the dynamics of grass-legume swards exists for the cool-temperate, continental climatic zone of the eastern USA. Most of what is known about autumn management of cool-season forages has been obtained from studies of stockpiled, grass-dominated canopies with relatively limited information obtained under actual grazing conditions. Finishing lambs on forage rather than concentrate leads to relatively lean carcasses. Consumer desires for healthier meat products (e.g., less fat) have shifted the emphasis to leaner, trimmer carcasses from livestock production systems. Study Description An established mixed sward of orchardgrass and white clover growing in southem West Virginia was used. Growing lambs grazed either pasture or were fed concentrate in feedlot. Herbage nutritive value in pasture was monitored for 3 yr during the autumn grazing interval and lamb performance and carcass quality were determined in each of 2 yr. Applied Questions Did autumn grazing management of grass-legume pastures influence herbage nutritive value? Less crude protein was available for use by grazers in 1992 than in 1991 resulting primarily from less legume in the sward in 1992. Herbage of higher nutritive value (lower neutral detergent fiber and greater digestibility) was available from intensively managed (frequent clipping) canopies compared with that of stockpiled canopies. Did autumn pasture or concentrate feed influence lamb performance and carcass quality? Lambs fed concentrate had greater cumulative weight gain, average daily gain, slaughter weights, chilled carcass weights, dressing percentages, leg conformation scores, ribeye area, fat over rib, quality grade, and yield grade than lambs grazed on autumn pasture. Total fat content of carcasses was lower for pasturegrazed than grain-fed lambs, but saturated fatty acid composition and cholesterol levels were greater. Recommendation We are suggesting that market weight animals may be produced from mixed-plant-species autumn pasture. Results depend upon animal genetics and the subsequent potential for rapid and efficient gain on pasture, as well as carcass weight and composition desired by the market at any given time.