The effect of hormonal stimulation scheme and season on the efficiency of estrus synchronization in Romanov ewes (Ovis aries L.)

Estrus synchronization in sheep is an important element in the organization of reproductive programs, including those based on assisted reproductive technologies. The diversity of sheep breeds, the differences in environmental conditions of breeding zones and the specificities in hormonal status of animals not allow the finding a universal protocol for the management of reproductive cycles. The aim of our work was to identify the conditions of estrus synchronization in Romanov ewes in different seasons of the year. For the first time, a comparative analysis of two schemes of hormonal stimulation of estrus was carried out, involving either two consecutive injections of prostaglandin F2a (Scheme 1), or initially injection of gonadotropin releasing hormone followed by prostaglandin F2a treatment (Scheme 2). The study was carried out on mature Romanov ewes ( n = 160). The first group ( n = 121) underwent hormonal treatment using the Scheme 1, with two injections of cloprostenol (125 μg per injection) 13 and 2 days before the expected day of estrus (day 0). In the second group of animals ( n = 39), the Scheme 2 was used, which included injections of 15 μg of lyuliberin acetate and 125 μg of cloprostenol at days 9 and 2 before expected day of estrus, respectively. In the first group, the results of estrus synchronization were analyzed by seasons, the autumn-winter period ( n = 73) and spring-summer period ( n = 48). For detail analysis, the data were studied by two-month periods: Sept-Oct ( n = 24), Nov-Dec ( n = 26), Jan-Feb ( n = 23), Mar-Apr ( n = 32), and May-June ( n = 16). The efficiency of hormonal treatment was evaluated in all experimental animals based on appearance of estrus in 24 and 48 hours after the last injection. In some of the animals that showed estrus response ( n = 80), visual assessment of the ovaries for the presence of corpora luteum (CL) was performed using laparotomy or laparoscopy 96 hours after estrus detection. The variance analysis showed a reliable effect of the hormonal treatment scheme ( F = 5.21, p = 0.024) as well as the season ( F = 13.82, p = 0.0003) on the estrus response. The average number of CL was subjected to greater variability by the year of experimental studies (p ≤ 0.05) and the season (p ≤ 0.01), without significant effect of the treatment scheme. Using the Scheme 1 revealed the trend of higher estrus response comparing to Scheme 2, 80.17 % vs 66.67 %. Along with that, in the second group the CL were found in all ewes with the estrus signs, while in the first group the CL were found in 90.77 % of animals (p £ 0.05). Synchronization results for ewes in the autumn-winter period were better than in the spring-summer period: the estrus response was 92.50 % compared to 64.58 % (p £ 0.01), the average number of CL for ewes in the estrus stage was 2.02 vs 1.28 (p £ 0.01), and the average number of CL in ewes with identified CL was 2.18 vs 1.62 (p £ 0.05). A more detailed analysis by the two-month periods showed that the values of the above-mentioned indices were relatively stable during the autumn-winter period, after which they decreased in Mar-Apr and reached minimum values in May-June. Thus, for stimulating the estrus in Romanov ewes we recommend two consecutive injections of cloprostenol (13 and 2 days before the expected estrus). The efficiency of the method is higher in autumn-winter season, after which a decrease occurs in the response of animals to hormonal treatment.