The effects of pregnancy nutrition and shearing on lamb birth weight in highly fecund Booroola-merino cross sheep

The effects of ewe nutrition, live-weight change and shearing during pregnancy on lamb birth weight were studied using highly fecund Booroola-Merino cross Coopworth ewes. They were allocated to a high (H) or low (L) early pregnancy (0-49 days) nutrition treatment aimed at offering sufficient feed to increase their average live weight by 3 and 0kg respectively. Each of the 2 ewe groups was subsequently subdivided and allocated to one of 2 mid-pregnancy (50-98 days) nutrition treatments (H,L) in which target body-weight changes were the same as for the early pregnancy period. One hundred and nineteen days after mating, half of the ewes in each of the 4 nutritional subgroups were shorn and fed to the estimated metabolizable energy (ME) requirement of unshorn ewes. Two hundred and twenty-one lambs born to 102 ewes conceiving at a synchronized oestrus and 152 lambs from 58 ewes which conceived at the next oestrus were weighed within 12 hours of birth. The high level of nutrition of ewes in early pregnancy was associated with higher mean birth weight of lambs. Change in lamb birth weight per unit change in ewe live weight during this period was 46gkg-1 (±17) and 76gkg-1 (±34) for early- and late- lambing groups respectively (p0.05). Shearing of ewes at 119 days post-mating increased lamb birth weight (p0.05) in the early-lambing group. It is concluded that in highly fecund sheep, lamb birth weight is affected by ewe nutrition in early pregnancy and that nutrition levels which cause a loss of ewe live weight immediately after mating are detrimental to lamb birth weight