Genetic improvement of mammalian fertility: A review of opportunities

Abstract

The possibility of genetic improvement in the reproductive performance of domestic mammals is introduced. The paper is principally concerned with biological variation and change, but the need to consider economic factors when relating change to improvement is illustrated from time to time. Both male traits, such as semen quality, and female traits, such as litter size, are considered. There is considerable variation among existing populations and the importance of the choice of the most suitable of available populations whether pure or crossbred is emphasized. Variation in litter size is greatest among breeds of sheep (approx. 300%) and least among cattle (only 4%). The possibility that differences in performance may be specific to particular environments and the implications of such an interaction between genotype and environment for the choice of populations are discussed. Among cattle, variation in the incidence of calving difficulties may be marked; it may increase three-fold in cows mated to bulls of a large breed relative to cows mated to bulls of a small breed. Cross-breeding may improve reproductivity by 20%. Once the best population has been chosen, further improvement is dependent on selection within that population. Selection within populations has been rare due to low predicted rates of response. Low selection differentials have probably contributed more to slow responses than have low heritabilities, and recent marked changes achieved by group breeding schemes illustrate what can be done with intense selection. A principal reason for small selection differentials is the inability to measure all reproductive traits in both sexes; one sex has to be chosen at random. The common physiology of reproduction in males and females indicates common genetic control, and the possibility of indirect selection on, say, testis growth to alter ovarian activity is considered along with the use of other physiological selection criteria, including ovulation rate and hormone levels, to increase the rate of response to selection. Finally, the implementation of improvement schemes is discussed. The improvement of reproduction has to be balanced against improvement in other traits such as growth; as the benefits of selection for other traits decline, reproduction would receive greater emphasis. International collaboration to develop strains of extremely high merit for subsequent use by crossbreeding is advocated.