The Influence of the Sire on the Oestrogen Production of the Bovine Foetus-Placental Unit

Abstract

Ease of parturition in heifers is limited by the size of the calf and by the available space in the birth canal during parturition. Variation in the ease of birth is mainly explained by the foetal genotype. Selection for ease of birth itself is not very promising for the parameter can not be measured easily and objectively. Selection for a lower birth weight is theoretically quite possible but it is not wanted because of the higher economic value of the heavier calf. Studies on the possibilities for selection for a wider birth canal (at parturition) seem therefore to be worthwhile. The space in the birth canal has been considered in the past to be a purely maternal character which can be improved by selection via progeny tests of sires (Menissier et al., 1975). The foetal genotype also influences the ease of birth, however. This is probably mediated by oestrogens produced by the foeto-placental unit. High urinary oestrogens in late gestation coincide with easy births, even in the case of high birth weights. These oestrogens may exert a local preparative effect on the birth canal. This hypothesis requires to be studied and if it proves to be true a selection programme could be initiated for high levels of oestrogen in late gestation. There is a need for experimental models to test this hypothesis. It is also necessary to investigate what part of the birth canal (pelvis, cervix or vagina) is the most limiting in the primiparous cow and how this can be measured objectively.