Genetic variation in ovulation rate, both within and between breeds, is recognised as a major source of variation in litter size, while breed differences in embryonic survival are generally assumed to be unimportant. Results reported here for several trials, years and locations and involving large numbers of naturally multiple ovulating ewes show significant and consistent genetic differences in the proportion of ewes producing 2 lambs following conception to twin ovulations (uterine efficiency). Compared to contemporary Romneys (uterine efficiency = 0.59), 4 to 34% more ewes of 5 other genotypes produced twin lambs following conception to twin ovulations. At the extreme, Border Leicester x Romneys ranged from 24 to 52% superior in 3 comparisons. At ovulation rates higher than 2, inter-ovarian ovulation pattern was also found to influence litter size. Increasing disproportionality of ovulation rate between ovaries resulted in lower litter size. This may be due to uneven distribution of implantation sites and increased losses due to crowding.
Proceedings of the New Zealand Society of Animal Production, Volume 43, , 193-196, 1983
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