Reproductive efficiency in farm livestock can be measured as a variety of different tra its . These include numbers of offspring born and reared, conception rate, age at puberty, length of breeding season and semen quality. Most traits may be further broken down into components: for example, l i t te r size at birth is the product of ovulation rate and embryonic survival. The term prolificacy conventionally describes numbers of offspring, fe r t ility describes- conception rate, and fecundity describes the combination of the two.

Compared with production traits such as growth rate, carcase composition or milk yield, reproduction has received l i t t le emphasis in past selection programmes. The main reason has been the relatively lower heritability of reproduction, resulting from its closer association with fitness. In addition., since reproduction is sex-limited and expressed later in l i f e , accurate prediction of genetic merit on large numbers of animals has been more d iffic u lt, with a relatively longer generation interval. Where selection has concentrated on production tra its , there is now concern as to whether adverse correlated changes in reproduction might result. So far, the genetic relationships between many aspects of reproduction and production are relatively unknown.

This Symposium brings together the latest studies on a wide variety of factors affecting the efficiency of selection for reproduction, and on genetic relationships between reproduction and production. Some of the conclusions and questions which they raise are reviewed here from the papers available in advance.

A. J Webb

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 7. Symposia (1), , 465-473, 1982
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