Immunisation has implications for genetic improvement programmes by influencing the intensity of selection, the amount and nature of genetic variation able to be exploited by selection, the accuracy of the selection process or by adding flexibility to animal husbandry that might otherwise be the objective of dynamic crossbreeding strategies. Since immunisation changes the reproductive rate of a flock there are consequential effects on the intensity of selection that is possible. Improving the effective lambing percentage from 120% to 140% can increase the intensity of selection of female replacements by about 19%. The improvement is less on the male side, the combined effect on genetic improvement in a ram-breeding flock of 400 ewes being about 6%. Alteration to the balance of physiological events may even serve to display genetic variation not otherwise available for selection. Good evidence is available that natural ovulation rate is a useful selection criterion for improvement in lamb production, but there is no direct evidence on the value of selection following immunisation of ewes to steroid hormones controlling reproduction. Indirect evidence for sheep that litter size responses to immunisation are essentially additive with respect to the effects of age of ewe, flushing treatment, breed and strain differences, suggests no major antagonisms with natural ovulation rate. Indirect evidence from mice is not so comforting - Land and Falconer's selection experiment for natural and PMS-induced ovulation rate gave an estimate of only 0.33 for the genetic correlation between these two traits, suggesting that they were largely independent genetically. Regardless of the true genetic situation for sheep it seems important that recording programmes recognise and adjust for the different distributions of ovulation rate and litter size under natural v immunised circumstances.
Proceedings of the New Zealand Society of Animal Production, Volume 45, , 184-187, 1985
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