An embryo transfer study of birth weight in Finnish Landrace and Targee sheep.

Sources of variation in Finnish Landrace and Targhee birth weights from pregnancies following natural matings and intra-breed, inter-breed and 2-breed embryo transfers were studied. Birth weights were recorded on 94 lambs from naturally mated ewes and 117 transfer lambs. Two analyses were done, one on observed weights, and one on deviations of observed weights from weights predicted from litter size and donor and recipient mature weights. The prediction equation removed effects of lit ter size on birth weight. Ewe lambs born following natural matings were lighter at birth than ram lambs; however, sex was not a significant source of variation in birth weights of lambs following embryo transfer. Targhee lambs born to Finn cross ewes were smaller (P<.05) than Targhee lambs born to Targhee ewes (4.5 vs 5.7 kg for singles and 3.0 vs 4.2 kg for twins). Finn lambs had similar birth weights whether born to Targhee ewes or Finn cross ewes. Birth weights of transferred Targhee lambs were related (P<.01) to donor ewe weight. Gestation length was a significant source of variation in observed birth weight in all groups except naturally mated Finns. Targhee lambs weighed more at birth than predicted on the basis of mature weight, either in naturally mated ewes, or following transfer to Targhee ewes. Finn lambs weighed less than predicted on the basis of mature size, regardless of recipient breed. These differences were reduced by adjustment for gestation period. Targhee Iambs carried by Finn cross ewes