Fat lamb studies in Victoria. II. The interaction between pre- and post-natal planes of nutrition on the production of wool and lambs by crossbred sheep
The interactions between two planes of pre- and post-natal nutrition of grazing ewes and the level of fertility on: ( a ) birth weight of the lamb, ( b ) its subsequent growth rate, and ( c ) wool growth, were determined in 140 crossbred ewes (52 Romney Marsh X Merino, 88 Border Leicester X Merino). The effectiveness of pregnant mare serum gonadotrophin (PMS) , injected on the 12th to 14th day of the oestrous cycle, for raising fertility ' was also investigated. Romney cross ewes had commenced their breeding season before January 21, a t least 3 weeks earlier than Border Leicester cross ewes. They lambed correspondingly earlier. The overall lambing percentage was very high (139 per cent.). PMS had no significant effect. There was no breed difference. Low- and high-plane ewes gained an average of 20 and 54 lb respectively during pregnancy. The mean weight of the conceptus was 34 lb (twins 39 Ib, singles 25 lb). Hence low-plane ewes lost an average of 14 lb body tissue as compared with a gain of 20 lb by high-plane ewes. Mean birth weights of lambs - single and multiple pregnancies - from the two nutritional groups were indistinguishable. Lamb losses and lambing troubles were fewer in the low--plane group. There was no interaction of pre- and post-natal nutrition on subsequent lamb growth rate. Post-natal plane of nutrition, sex of the lamb, and number reared all had highly significant effects on lamb growth rate. There were also significant interactions between plane of nutrition of the ewe and number of lambs reared and sex of lambs. Wether lambs needed the best possible nutritional environment fully to exhibit their inherent superior growth rate over ewe lambs. Restricted feed intake either in pregnancy or in lactation reduced wool growth to approximately the same degree. Although birth and weaning weights were affected by the number of lambs born and reared, wool production of the dam was not affected.