Plane of nutrition, fertility and fecundity

In female animals the primary determinant of fertility (i.e. whether or not the animal conceives) and fecundity (i.e. litter size) is the number of ova shed from the ovaries (the ovulation rate). In the cow the rate is normally 1, in the ewe it is normally 1-3 (but may reach 10) and in the sow it is 15-25. Not all ova are fertilised and survive to birth. For example, a flock of 100 ewes in a single oestrous cycle might produce 220 ova, of which 190 would be fertilised. Of the 190 embryos, 175 might survive for 15 days, to the point at which they become attached to the placenta, and of these 170 might complete the full gestation period of 147 days. Thus, the final lambing percentage (lambs born per 100 ewes mated) would be 170 per cent. 

After Gunn R G, Doney J M and Russel A J F 1969 Journal of Agricultural Science, Cambridge 73 289. 

In mammals, the spermatozoa and ova and the secretions associated with them represent only very small quantities of matter. The average ejaculate of the bull, for example, contains 0.5 g of dry matter. It therefore seems reasonable to suppose that 

Many nutrient deficiencies influence fertility indirectly, through their effects on the general metabolism of the animal. For example, phosphorus deficiency in grazing ruminants, which has often been associated with poor fertihty, appears to affect reproduction because it restricts many metabolic processes, hence food intake and the general plane of nutrition. However, there is also some evidence that phosphorus deficiency has a direct effect on reproduction through suppressing oestrous cycles. 

Vitamin A deficiency must be prolonged if it is to affect fertility thus, animals may suffer blindness before their reproductive organs are affected (by keratinisation of the vagina or degeneration of the testes). An extreme and long-lasting deficiency of vitamin A may prejudice embryonic development and the ovarian cycle of mares, although this rarely occurs. Recently 3-carotene has been claimed to have a specific effect on fertility (i.e. independent of its role as a precursor of vitamin A), but the claim has yet to be adequately confirmed (see Chapter 6). 

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