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Published online by Cambridge University Press: 24 November 2017
A rising plane of nutrition prior to mating increases ovulation rate in pigs (Anderson and Melampy, 1972), but this potential for increased Utter size is negated by the subsequent detrimental effect of high plane feeding on embryo survival. The mechanism underlying this effect may involve an increased rate of steroid clearance from the liver in animals on a high plane of nutrition (Symonds and Prime, 1989). The resultant lower circulating steroid levels would be expected to elevate pituitary gonadotrophin levels and consequently increase ovulation rate. However, after mating, such low steroid (particularly progesterone) concentrations may compromise embryo survival.
A further factor which may affect the ability of a gilt to carry a large number of fetuses to term is the spacing of fetuses within the uterus. Dzuik (1985) proposed that a uniform distribution of fetuses would optimise litter size, fetal growth and survival.