The effect of early-pregnancy nutrition on lamb birth weight in fecund Booroola-Merino cross sheep

Abstract

The effects of ewe nutrition and live weight change during early-pregnancy on lamb birth weight were studied. 218 Booroola cross Coopworth ewes carrying at least one copy of the F gene were stratified according to live weight, breed and age, and randomly allocated to a high(H), medium (M) or low (L) early-pregnancy (0-42 days) nutritional treatment. Target live weight gains over the 6 week period were +3.0kg, 0kg and -3.0kg for the H, M and L treatments respectively. The live weight gains achieved were +1.8 (H), +0.4 (M) and -1.1 kg (L). Ewe nutrition and live weight change during early-pregnancy did not significantly alter lamb birth weight (p>0.05), although a trend of increasing lamb birth weight with increased early-pregnancy nutrition was evident (3.14 ± 0.08 kg (H) vs 3.02 ± 0.08 (M) vs 3.00 ± 0.08 (L)). It is possible that substantial ewe live weight loss during mid-pregnancy of 3.8 kg, 4.2 kg and 3.0 kg for the H, M and L ewes respectively masked differences in lamb birth weight due to early-pregnancy nutrition. Non-nutritional factors that influence lamb birth weight include lamb birth rank, with lambs born of larger litters having lower birth weights (p<0.01), and lamb sex, with male lambs heavier at birth than their female counterparts (p<0.0l) (3.18 ± 0.06 vs 2.93 ± 0.07). Initial ewe live weight and birth date also significantly affect lamb birth weight by 30 g/kg (p<0.01) and 100 g/day (p<0.01) respectively.