Ratios as criteria for selection in animal or plant breeding, with particular reference to efficiency of food conversion in sheep

Abstract

Ratios are frequently advocated as criteria for selection, in either animal or plant breeding; for example, production per unit of food eaten is currently receiving attention, particularly with Merino sheep. Clean wool weight per head (W) is already widely used as a selection criterion, but suggestions have been made that wool weight per pound of body weight (B) or per unit skin area (estimated as B0.6, or measured) should be used instead, as estimates of the efficiency of conversion of food into wool. This paper develops formulae for writing the heritability of any ratio, as well as its genetic correlations with its numerator and its denominator, in terms of the relationships between the numerator and the denominator. From these formulae the likely result of selecting for the ratio can be predicted, for a given range of values of the genetic and phenotypic parameters of the numerator and denominator. The formulae can be used for any ratio, provided gene x environment interactions and epistasis can be neglected. In particular, it is shown that the genetic correlation between W and W/B or W/B0.6 is consistently positive for a wide range of values of the parameters of W and B, so that selection for high wool weight will also increase efficiency, as measured by either ratio. The genetic correlation between B and W/B or W/B0.6, on the other hand, is nearly always negative, sometimes very strongly so, and selection for high values of W/B or W/B0.6 will decrease body weight. The theoretical predictions are confirmed by results from experimental selection groups, in which the observed correlated responses are all in the predicted direction, and in most cases agree well in magnitude. It is suggested that more information is required on the importance of body size itself, in relation to factors such as fertility and survival, before the value of selection on W/B or W/B0.6 can be accurately assessed.

Estimates of genetic parameters and selection strategies to improve the economic efficiency of postweaning growth in lambs1

“…Selection for growth rate or live weight would be expected to increase feed intake as well as to improve feed conversion ratio (Turner, 1959;Sutherland, 1965). These estimates agree with previous estimates of genetic correlation for postweaning ADG with feed conversion ratio (feed/gain) of −0.43 for Rambouillet and −1.00 for Romnelet (Vesely et al, 1970) and 0.80 for postweaning ADG with total feed intake for crossbred lambs (Leymaster et al, 2002).…”

Variation in the Sulphur Content of Wool of Merino Sheep Associated with Genetic Differences in Wool-Producing Capacity

“…The ewes were born in 1963 on the CSIRO National Field Station, "Gilruth Plains", Cunnamulla, Qld., in an experiment discussed by Turner (1959). In one group selection is solely for high and in the other solely for low clean wool weight per head at c. 16 Wool samples for sulphur analysis were taken in June, 1965, when the ewes were c. 20 months old and were carrying 4 months' wool, grown while they had been grazing together on natural pasture.…”

Alternative Selection Criteria for the Genetic Improvement of Feed Efficiency in Broiler Chickens

“…Selec-tion for feed efficiency in broilers has met with some success (Guill and Washburn 1974; Pym and Nicholls 1979) and has been found to be heritable across species 1fi2:0.37 for chickens (Chambers and Gavora 1982); h':0.42 for beef (Simm 1983)). However, use of the feed:gain ratio as a measure of feed efficiency has to be carefully considered, specifically: (i) the existence of a part-whole correlation between a ratio and either its numerator or denominator imparts some concerns (Turner 1959;Sutherland 1965) (Belsley et al 1980), the D influence staristic (Cook 1977), residual analysis, and power transformations (Box and Cox 1964) For personal use only. …”