Growth and efficiency of post-weaning gain in lambs from Merino flocks selected for high and low weaning weight.

Abstract

GROWTH AND EFFICIENCY OF POST-WEANING GAIN IN LAMBS FROM MERINO FLOCKS SELECTED FOR HIGH AND LOW WEANING WEIGHT W. A. PATTIE* and A. J. WILLIAMS* Summary An examination has been made of the growth and post-weaning growth efficiency of lambs from three flocks, one selected for high weaning weight (Weight Plus), one for low weaning weight (Weight Minus), and a control flock (Random). Weight Plus lambs were 20% heavier at birth than Weight Minus lambs and grew faster to weaning. Although there were significant differences between the flocks in skeletal size, feed intake after weaning, and weight of carcase components, they were proportional to the differences in body weight and there were no differences in efficiency of conversion of feed to meat. I. INTRODUCTION Body weight changes in three Trangie selection flocks have been described by Pattie ( 1965a, 1965b). These flocks were selected for high weaning weight (Weight Plus), low weaning weight (Weight Minus) and at random for a control flock (Random). This paper describes the growth to weaning, the efficiency of post-weaning gain, and the carcase composition of six-months old lambs from each flock. II. MATERIALS AND METHODS The selection history, management and environment of the three flocks have been described by Pattie ( 1965a). The lambs used were born to mature ewes (three to six years old) in August 1964. Samples of ten ram and ten ewe lambs, each consisting of six twins and four singles, were chosen from each flock. Commencing at three weeks of age, the lambs were weighed and measurements made every two to three weeks of leg length (elbow to coronet), cannon bone length, chest depth and width of hips. The lambs were weaned on October 21, but regular measurements continued until November 12, 1964 ( 101 days old). Age variations in each measurement were corrected by interpolation. On November 14, 1964 a further sample of seven single-born ram lambs was chosen from each flock, and after one month on a pre-treatment ration, were fed lucerne pellets ad libitum for 72 days. The lambs were weighed each week and the weights corrected for wool growth. Data from the last 21 days were discarded following the introduction of a new batch of pellets which caused scouring and loss of appetite among the lambs. After slaughter on March 4, 1965 the carcases were separated into bone, meat and fat. III. RESULTS Mean body weights for each flock from birth to 101 days (Figure 1) and mean growth rates were significantly different (P < 0.05). There were significant 'Department of Agriculture, Agricultural Research Station, Trangie, N.S.W. 305 Fig. l.-Mean body weight for each flock from birth to 101 days of age. differences between the flocks in body measurements made at the same age, but these were eliminated when the means were adjusted for body weight. In Figure 2 body measurements are plotted against age and body weight. Mean body weight, feed intakes and body weight changes of ram lambs during their 51 day feeding perio$d TABLE 1 306 Fig. 4.-Mean weights of separable bone, meat and fat plotted against carcase weight. Mean body weights, feed intakes, and growth efficiencies during the feeding period are given in Table 1. Only the differences in body weight were significant. Figure 3 shows feed intake plotted against body weight. There were no significant differences between the flocks in the regression coefficients, or in mean intakes adjusted for body weight. 308 TABLE 2 Mean weights of separable b)one, meat cllnd fat in the carcases of seven ram lambs frolm each selection group The mean weights of against total carcase weight the flocks in the weight of carcase weight. separable bone, meat and fat (Table 2) are plotted in Figure 4. There were significant differences between each component at the same age, but not at the same 0 IV. DISCUSSION Selection for weaning weight has increased body weight at a given age without changing the animals' relative shape or carcase composition. Feed intake has changed in proportion to body weight so there has been no change in the efficiency of conversion of feed to meat. This is in contrast to selection for fleece weight, where increased production per head comes mainly from increased efficiency of conversion of feed to wool (Williams 1966). From these results, selection for weaning weight in Merinos would be of no value as a means of increasing meat production per acre. V. ACKNOWLEDGMENTS We would like to thank Mr. L. Fahey for his technical assistance. VI. REFERENCES PATTIE, W. A. ( 1965~). Selection for weaning weight in Merino sheep. I. Direct response to selection. Australian Journal of Experimental Agriculture and Animal Husbandry 5: 353. PATTIE, W. A. ( 1965b). Selection for weaning weight in Merino sheep. 2. Correlated responses in other production characters. Australian Journal of Experimental Agriculture an Animal Husbandry 5: 361. W ILLIAMS , A. J. ( 1966). The efficiency of conversion of feed to wool during limited and unlimited feeding of flocks selected on clean fleece weight. Australian Journal of Experi- mental Agriculture and Animal Husbandry 6: 90. 309