Nutrition-endocrine interrelationships in the lactating and weaned sow

Previous studies had identified the effects of nutrient restriction on delayed occurrence of estrus when body condition is low at farrowing or levels of intake fall below 2.5 kg (8.35 Mcal ME/h/d) during lactation. It appears that the failure via the GnRH pulse generator to stimulate sufficient LH pulsatility to trigger steroidogenesis may play a major role in the lack of reinitiation of cyclicity. Base-line concentrations, because of their variability between individual animals, may not be a major factor in predicting the time to return to estrus. Prolactin appears to be definitely antigonadotropic during lactation but may be involved in folliculogenesis effects during the postweaning period. Previous results would indicate that metabolic factors in concert with the major reproductive hormones play a key role in reestablishing the cycle postweaning. To achieve a greater understanding of the complexities of these two systems--nutrition and reproduction-greater emphasis must be placed on more precisely regulating the integral factors modulating the return to cyclic behavior.