PHYSIOLOGICAL-EFFECTS OF UNDERNUTRITION ON POSTPARTUM ANESTRUS IN COWS

The focus of this review is the physiological effects of undernutritio n on ovarian follicle growth and the occurrence of ovulation in postpa rtum cows. Evidence suggests that moderate levels of underfeeding, bef ore or after calving, may interfere with the mechanism(s) of final fol licle maturation and ovulation, whereas more pronounced nutritional de ficiencies may affect the mechanism(s) regulating dominant follicle si ze and the dynamics of dominant follicle growth and regression. Severe undernutrition may result in the absence of follicles > 5 mm in diame ter. These changes are consistent with likely effects of reduced LH or FSH secretion that have been associated with inhibition of both tonic and surge release of GnRH from the hypothalamus. As yet, there is no direct evidence for local effects of undernutrition on ovarian functio n in cows, and studies examining this issue often ignore dynamic inter dependencies between the hypothalamus, anterior pituitary gland, and t he ovaries, and possible nutritional modulation of ovarian feedback me chanisms that affect gonadotrophin secretion. Further research is requ ired to elucidate nutritional effects on the positive feedback regulat ion of gonadotrophin secretion and on the expression of behavioural oe strus. Further studies are also needed to determine whether nutritiona l or metabolic state may influence ovarian function directly in a mann er that affects the duration of postpartum anoestrus and fertility in postpartum cows.