Predicting ovulation rate from liveweight in ewes

Abstract

The relationship between ovulation rate (OR), liveweight at an autumn mating (LW) and changes in LW (DW) prior to mating was examined in 393 mature Corriedale ewes, in several sets of published data involving Merino and Romney Marsh ewes and their crosses with Border Leicesters, as well as Perendales and Scottish Blackfaces. Between groups OR was strongly associated with LW, but in only one study with DW. Within groups of Corriedales there was a slight association of OR with LW. Responses of OR to LW, over all breeds and crosses except Scottish Blackface, were parallel, being an increase of approximately 2% ovulation rate for each kilogramme of mean LW increase. The fact that OR was much more closely related to LW than DW, over a wide range of genotypes, indicates the ovarian response to the current plane of nutrition to be less than to the body reserve status of the ewe. This indicates that the value of ‘flushing’ ewes over a brief period before joining is unlikely greatly to improve lambing percentages, unless ewes are initially in poor condition. The incorporation of these results in models of sheep production will require information on the general level of ovulation in the ewes and the adequacy of the diet, especially the protein content, at the time of joining.