163 Treatment with gonadotrophin-releasing hormone at the time of AI in beef heifers that fail to express oestrus after an estradiol-based synchronisation protocol improves pregnancies per AI

Abstract

Oestrus expression between progesterone (P4) withdrawal and fixed-time AI (FTAI) has been shown to improve pregnancies per AI (P/AI) by 27% in beef cattle. As a result, cattle that do not express oestrus after P4 withdrawal present a challenge to the efficiency of FTAI. The present study was designed to test the hypothesis that administration of gonadotrophin-releasing hormone (GnRH) at the time of AI in heifers that do not express oestrus can improve fertility. Two-year-old Angus heifers (n = 1032) with a condition score of 2.75 to 3.5 at two locations in Argentina were used. On Day 0, heifers received an intravaginal device containing 0.5 g of P4 (Cronipres, Biogenesis) and 2 mg of oestradiol benzoate intramuscularly (Bioestrogen, Biogenesis). On Day 8, devices were removed and heifers received 150 µg of d-cloprostenol intramuscularly (Enzaprost, Biogenesis), 0.5 mg of oestradiol cipionate intramuscularly (Croni-Cip, Biogenesis), and an oestrus detection patch (Estrotect, Rockway Inc.). On Day 10, AI was performed in all heifers 50 to 54 h after device removal. Heifers were categorised based on oestrus expression, and those that did not express oestrus were randomised to receive 100 µg of gonadorelin acetate intramuscularly (Gonasyn, Syntex; n = 158) concurrent with AI or remain as untreated controls (n = 151). Pregnancy was determined using ultrasonography at Days 38 and 111 after AI. Differences in fertility were evaluated using generalised linear mixed models, and the results are shown in Table 1. Oestrus expression between device removal and FTAI was 70.1% (723/1032). Pregnancies per AI at Days 38 and 111 were different between groups (P < 0.01). Heifers that expressed oestrus had greater P/AI than those that did not regardless of treatment (P < 0.01). However, in heifers that did not express oestrus, treatment with GnRH at the time of AI resulted in greater P/AI than in control heifers (P = 0.004). Similarly, P/AI at Day 111 were greater in heifers that expressed oestrus than in those that did not regardless of treatment (P < 0.01). Heifers that did not express oestrus and were treated with GnRH had greater P/AI than those that did not express oestrus and remained as controls (P = 0.02). Pregnancy loss between Days 38 and 111 was not different between groups; however, there was a tendency (P = 0.06) for greater pregnancy loss in heifers that did not express oestrus and were treated with GnRH compared with heifers that expressed oestrus. In summary, treatment with GnRH at the time of AI is a suitable strategy to improve P/AI in heifers that fail to exhibit oestrus after an oestradiol-based synchronisation protocol. However, this strategy appears to increase pregnancy loss, and thus further research is needed to confirm this observation and the mechanisms underlying it.

Table 1.  Pregnancies per AI (P/AI) and pregnancy loss (%; no. in parentheses) in Angus heifers based on oestrus expression and treatment with GnRH