Plane of nutrition during the rearing phase for replacement ewes of four genotypes: II - effects on performance during first pregnancy and to weaning, and of their progeny.

Plane of nutrition (PN) offered to ewe replacements during the rearing phase (8 to 17 months) and first pregnancy may affect reproductive and progeny performance when replacements are joined at ~19 months. The effects of PN offered to spring-born ewe replacements during their first winter (winter_1), second summer (summer_2) and subsequent winter (mid and late pregnancy; winter_2) were evaluated, over 3 consecutive years, using 287 ewe lambs of four genotypes: Charmoise×Scottish Blackface (C×SBF), Belclare×SBF (Bel×SBF), Belclare (Bel) and Belclare×SBF (heterozygous for a gene, either FecGH or FecXG mutations, that increases ovulation rate: BelMG×SBF). Ewe lambs were offered, daily, a deferred-grazed herbage allowance (HA) of either 0.75 or 1.75 kg dry matter during winter_1. During summer_2 the replacements were set-stocked to maintain sward heights of 4 or 6 cm. Ewes were housed during mid and late pregnancy (winter_2) and offered either medium or high-feed-value grass silage, thus yielding a 2×2×2 factorial design. Increasing HA during winter_1 and residual sward height during summer_2 increased ewe BW post joining (P0.05) on number of lambs reared. Increasing HA during winter_1 increased lamb BW at birth (P<0.05) and reduced carcass fat score (P<0.05). There was a significant interaction between PN offered during winter_1 and summer_2 for average daily gain (ADG) from birth to 5 weeks (P<0.01) and from 5 to 10 weeks (P<0.05): progeny of replacements that experienced either a high or low PN throughout the rearing phase had lower ADG than lambs born to the replacements that experienced a low PN only during winter_1 or summer_2. Increasing PN during winter_2 increased ewe BW (P<0.001), lamb BW at birth (P<0.001) and weaning (P<0.05) and reduced the proportion of ewes that failed to lamb (P<0.01). Ewe genotype had a significant (P<0.001) effect on litter size (1.48 to 2.45), number of lambs reared, lamb BW at birth and weaning, lamb ADG and age at slaughter. It is concluded that, among the nutrition treatments examined, increasing PN during mid and late pregnancy had the greatest effect on ewe and progeny performance. Although increasing PN offered during winter_1 increased lamb birth BW, PN offered during summer_2 had no effect on lamb performance. There were no significant interactions between PN and ewe genotype.