AUTUMN-GRAZED ORCHARDGRASS-WHITE CLOVER PASTURE - NUTRITIVE-VALUE OF HERBAGE AND LAMB PERFORMANCE

Performance of livestock grazing pasture is inherently variable. Pastu res must be managed to optimize quantity and quality of herbage to mee t the nutrient requirements of a specific class of livestock involved in a defined production goal. Consumer desires for healthier meat prod ucts have shifted the emphasis to lean, trim carcasses from livestock production systems using forage crops. The purpose of this study was t o characterize herbage nutritive value changes in grass-legume pasture s grazed during autumn and to compare growing lamb (Ovis aries) perfor mance and carcass quality when grazing autumn pasture or fed concentra te in feedlot. Less crude protein (CP) from legume was available for u se by grazers in 1992 than in 1991. Frequent clipping of autumn pastur e resulted in herbage with greater nutritive value lower neutral deter gent fiber [NDF] and higher in vitro organic matter disappearance [IVO MD]) than more mature stockpiled herbage. A computer model of energy d etermined from acid detergent fiber (ADF) nutritive value and herbage mass data predicted greater potential for lamb growth on intensively m anaged (clipped) vs. stockpiled herbage. Lambs fed grain had greater ( P < 0.05) cumulative weight gain, average daily gain (ADC), slaughter weights, chilled carcass weights, dressing percentages, leg conformati on scores, ribeye area (REA), fat over rib, quality grade, and yield g rades than lambs grazed on autumn pasture. However, carcasses from lam bs grazing pasture in autumn bad 14% less fat and about 8% more protei n than lambs fed grain. While total fat content of carcasses was lower for pasture-grazed than grain-fed lambs, saturated fatty acid composi tion 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 pro duced from botanically complex autumn pasture.