Abstract:
Proc. Aust. Soc. Anim. Prod. (1978) 12: 195 PRODUCTION FROM SHEEP SELECTED FOR FERTILITY, WEANING WEIGHT AND AT RANDOM G.E. ROBARDS* At pasture, lambs in a Merino flock selected for increased net reproductive rate with some emphasis on weaning weight and hogget fleece weight (Fertility) are up to 19% heavier by weaning at 12 weeks and 23% heavier by 16 months of age (Atkins, unpublished data) than lambs in an unselected flock (Random). Faster growth rate and higher efficiency of conversion of feed to live weight were shown to contribute to post-weaning differences favouring Fertility lambs (Atkins and Robards 1976). . To determine the reasons for superior pre-weaning growth of Fertility lambs a study was conducted of the milk production and growth of single lambs from 23 Fertility and 9 Random two-year-old ewes, and 9 ewes of the same age from a flock selected for weaning weight alone (Weight Plus). Lucerne hay was fed at individual levels estimated to maintain maternal live weight for 6 weeks before and 8 weeks after lambing. Milk production was measured over 4 hours at 3, 6 and 8 weeks after lambing. Fertility ewes were 12,s heavier (P<O.O5) than Random ewes, but being fed in proportion to live weight they had a similar rate of liveweight change during lactation. Despite higher actual intakes the milk production of Fertility ewes was only slightly higher than that of Random ewes. In contrast, Weight Plus ewes which were 9% heavier than Random ewes produced 17% more milk and their.lambs grew 19% faster. .There were no differences between the three groups in the efficiency of conversion of food consumed by the-ewe to either milk production or lamb liveweight gain. ' Similarly, there were no significant differences between groups or sampling times in either the fat or protein content of the ewe's milk.' With restricted intake in an animal house Fertility ewes did not express the superiority over Random ewes which is observed at pasture, or which was achieved by Weight Plus ewes. It remains to be determine'd whether the production of Fertility ewes subjected to limited nutrition under field conditions is similarly restricted, and whether with unlimited nutrition Fertility ewes achieve higher production than Randoms by consuming more or expressing a greater efficiency of conversion of feed to milk. ATKINS, R-D., and ROBARDS, G.E. 16 : 315. (1976). Aust. J. exp. A+Tric. Anim. Husb. * Drought Research Unit, Veterinary Research Station, Glenfield, N`SW 2167. 195.