Abstract

Increasing incidence of prolonged oestrus has been reported among dairy cattle especially after the popularization of artificial insemination (AI). The condition is often considered as a simple deviation of the physiological manifestation and the underlying causes and mechanisms have not been adequately explored. However, considerable impairment of fertility has been reported and various interventions are being carried out to restore normal fertility. Hence objective of the present communication is to describe the causative factors for prolonged oestrus with emphasis on human interventions in animal breeding such as oestrus detection and AI management. Lack of males in the herd, intensive system of management, minimal opportunity for social interactions and stress of high milk production have made the oestrus manifestations unusual and misleading. Hence, expulsion of mucus from the reproductive tract is highly relied upon for oestrus detection, which makes the distinction of oestrus from pro- oestrus very difficult and thus increasing the proportion of prolonged oestrus cases. Owing to the shorter fertilizable life of cryo-preserved spermatozoa, better synchrony of insemination with ovulation results from later AI adhering to AM:PM rule. However, the chance of infection increases considerably with late insemination contributed by inadequacy of tubular defense mechanisms and harmful effects of more anterior deposition. Thus, it appears that the major underlying reason for the increasing occurrence of prolonged oestrus is the use of cryo- preserved semen together with adherence to AM:PM rule for AI and the resultant indirect selection of animals having longer oestrus period as parents of the future generations.

Keywords : Cattle, Prolonged oestrus, Artificial insemination, AM:PM rule, Ovulation