Reciprocal crosses of Shire and Shetland ponies and transferring embryos between small and large breeds of horses and pigs have shown that maternal size, ratherthan foetal genotype, is the primary determinant of birth size. In the present study, 40 Cheviot (C) ewes and 40 Suffolk (S) ewes were split into two groups. Each group was mated to either two Suffolk rams or two Cheviot rams to generate CC, CS, SC and SS lambs. Birth weights differed significantly (P<0.01) between the four lamb genotypes (CC 4.12 ± 0.21; CS 4.43 ± 0.23; SC 5.08 ± 0.19; SS 5.17 ± 0.19, kg). Furthermore, lower birth weight was significantly (P<0.01) associated with lower maternal circulating plasma placental lactogen (oPL) concentrations in the Cheviot dams at days 90 (C 25.8 ± 10.0 vs. S 71.8 ± 11.6, ng/ml) and 110 (C 84.5 ± 16.8 vs. S 175.9 ± 19.0, ng/ml) but not at day 130 of gestation. These results indicate that the size of the ewe affects the intrauterine development of her lamb. Moreover, the lower concentrations of oPL in the Cheviot dams may have contributed to the constraint observed in foetal growth.
Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 187-191, 2007
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