Indian J Dairy Sci 74(5): 379-386 379 INVITED REVIEW Prolonged oestrus as a cause of infertility in dairy cattle – A review Arsha Shaji, Kamaraj Elango and Arumugam Kumaresan Theriogenology Laboratory, Veterinary Gynaecology and Obstetrics, Southern Regional Station of ICAR- National Dairy Research Institute, Bengaluru, Karnataka 560030, India Arumugam Kumaresan() Theriogenology Laboratory, Veterinary Gynaecology and Obstetrics, Southern Regional Station of ICAR- National Dairy Research Institute, Bengaluru, Karnataka 560030, India. Mobile: +91 - 9671673108; Email: ogkumaresan@gmail.com Received: 13 August 2021 / Accepted: 18 August 2021 / Published online: 31 October 2021 Indian Dairy Association (India) 2021 © Abstract: Prolonged oestrus is one of the conditions associated with perturbed follicular dynamics that culminates into substantial economic losses to dairy industries through low success rate of artificial insemination, increased inter-calving period, reducing total milk yield, lowering calf production, shortening reproductive life of the animals, lowering fertilisation rate and abnormal embryonic development. Prolonged oestrus is characterized by the exhibition of extended duration of oestrus in various breeds of cattle. Cows having prolonged oestrus need to be examined at frequent intervals for determining the correct time of artificial insemination. This makes it challenging to inseminate the animals at the correct time, which is a major inconvenience for the field veterinarians and farmers. Since the follicular dynamics in cows is intricate to comprehend, the disorders in which also often go unnoticed and not treated/managed wisely in the field conditions. Therefore, it is of paramount importance to understand the aetiology of prolonged oestrus so that strategies to improve fertility could be designed. Alterations in hypothalamo- hypophyseal-gonadal axis, which is also influenced by some other extrinsic factors like stress, nutrition, age, parity, breed etc., apart from the endocrine alterations could cause the prolonged dominance of follicle. The presence of suprabasal progesterone and luteal insufficiency are considered as the main cause of this condition among cattle. In this review, we attempted to analyse the prior art on the topic and to delineate the possible causes, and to suggest managemental strategies to mitigate the prolonged oestrus condition in dairy cows Keywords: Cattle, Follicular dynamics, Fertility, Suprabasal progesterone, Prolonged oestrus, Repeat breeding Introduction The profitability of dairying depends to a large extent on the efficiency of reproduction. Maximizing reproductive efficiency requires the matching of genotypes to the production environment in order to ensure that the calving intervals are short and the rates of conception to artificial breeding are high. However, when the reproductive efficiency of the dairy cows are analysed, based on the records, it was observed that the conception rates are low (country’s average is 35%) and the calving intervals are quite high. One of the important reasons for extended calving interval is repeat breeding. It has been reported that the incidence of repeat breeding in cattle ranged from 5.5 to 33%. It is also reported that one of the most significant reason for repeat breeding in high producing crossbred cows is prolonged oestrus and associated alterations in the endocrine profile. Prolonged oestrus is a condition in which the duration of oestrus and the interval between the onset of oestrus and ovulation interval both are exceeding the normal range. In some cases, the duration of oestrus has been reported to be greater than 36 hrs, which can vary from 2-7 days (Bage et al. 2002; Bloch et al. 2006). At field conditions, the inseminators/veterinarians are baffled to precisely time the insemination. Prolonged oestrus condition can increase the number of services per conception to more than 4 against the optimum of <2. In addition, it also increases the inter- calving period and decreases the total milk yield, fertilisation rate, embryo development and calf production (Government of Kerala-Economic Review, 2015). Prolonged oestrus has been reported to be observed in 50% of repeat breeder cows (Cummins et al. 2012; Ghuman et al. 2014; Arun et al. 2020). The conception rate in cows showing prolonged oestrus decreased drastically as the duration of oestrus increased (Nebel et al. 2000). The https://doi.org/10.33785/IJDS.2021.v74i05.001