The maternal influence on size in sheep

Abstract

The influence of the maternal environment on the size of lambs from birth to 8 months has been studied by reciprocally crossing the large Border Leicester and the small Welsh Mountain breeds of sheep and by transferring fertilized eggs between ewes of these breeds.Ewes which lambed later in the spring gained more weight during the last 6 weeks of pregnancy than did the earlier-lambing ewes. The birth weight of the lambs was not, however, affected by either the date of lambing or the ewes' gain in weight during the last six weeks of pregnancy.Significant breed differences in gestation length were recorded; these were 144·7 days for the Border Leicester and 147·1 days for the Welsh Mountain. Border Leicester ewes carrying pure-bred Welsh lambs had an average gestation of 149·6 days, while Welsh Mountain ewes carrying pure-bred Border Leicester lambs had an average gestation of 146·0 days. From the births of ‘transferred’ lambs, it appeared that the genetically smaller lamb was a cause of a longer gestation period. On the other hand, within groups, both live weight and length of ‘cannon bone’ of the lambs at birth increased when the gestation period was longer.The maternal environment affected foetal growth when the genotype for size of the foetus was different from that of the mother. The mean live weight and ‘cannon-bone’ length of cross-bred lambs born to Border Leicester ewes were 1·08 lb. and 0·37 cm. greater at birth than those of cross-bred lambs born to Welsh ewes. Comparable figures for the maternal influence from the results of transferring eggs from one breed to the other were 1·96 lb. and 0·33 cm. On the other hand, within breeds, there was no effect on the size of the lambs at birth of variations in the size of the ewes. The length of the ‘cannon bone’, which grows relatively rapidly during the early stages of growth (an early-maturing part of the body), was less influenced by the maternal environment than the late-maturing live weight.Mature ewes produced larger lambs at birth than young ewes. The mean differences at birth in weights and in lengths of ‘cannon bone’ of the lambs born to young and mature ewes were 1·38 lb. and 0·31 cm. respectively.From these and other experiments, the conclusions were drawn that: (i) the maternal influence on the size of young at birth is greater, in the larger species which have longer gestation periods, than the effect of the genotype of the young; (ii) the maternal organism competes with the foetus for nutrients, thus limiting the size of the young at birth; and (iii) the maternal organism may also influence pre-natal foetal growth by means of some internal secretion or metabolic substance.The amount of milk consumed by the lambs had a marked effect on their live weight gains during the first half but not during the second half of lactation. Variations in milk consumption by the lambs did not affect the growth in length of their ‘cannon bones’. The influence of the post-natal maternal environment is largely exerted through the milk yield of the ewe and causes of variation in this milk yield were reflected in the post-natal maternal influence estimated in these experiments.The total milk yield of ewes was affected by their date of lambing, that of late-lambing ewes being less than the yields of ewes which lambed earlier in the season.