113 Different doses of equine chorionic gonadotrophin on pregnancy and twin rates of Bos taurus beef heifers subjected to J-Synch protocol for fixed-time artificial insemination

Abstract

The objective was to evaluate the effect of different doses of equine chorionic gonadotrophin (eCG) on pregnancy (P/AI) and twin rates of beef heifers in a progesterone and oestradiol-based treatment (J-Synch protocol) for fixed-time AI (FTAI). A total of 1,404 Bos taurus beef heifers with a body condition score 4.8±0.5 (mean±s.e.m., scale 1–8) were used in three different locations. The presence of a corpus luteum (CL) was determined by ovarian ultrasonography (5.0MHz) at the beginning of the J-Synch protocol. On Day 0, the heifers received 2mg of oestradiol benzoate (intramuscular, IM) associated with an intravaginal device containing 0.5g of progesterone. At device removal, on Day 6, 500µg of cloprostenol sodium was administered IM and the females were randomly assigned to one of two experimental groups to receive 200 (n=702) or 300 (n=702) IU of eCG (Zoetis) IM. Heifers were also tail-painted. On Day 0 females with paint rubbed-off were FTAI 64 h after device removal, and those that remained with the paint received 8 μg of buserelin acetate 64 h after device removal to be FTAI 8 h later. FTAI was performed at 64 to 72h after device removal, and pregnancy diagnosis was performed by ultrasonography 30 to 35 days after insemination. Data were analysed by logistic regression using GLMM. The results are shown in Table 1. Pregnancy rate was not different between heifers treated with 200 or 300IU of eCG (P=0.513) and was not affected by the presence of CL at the beginning of the protocol (P=0.513). Twin rate was greater in heifers treated with 300IU than with 200IU of eCG (P=0.005), and it was not affected by the presence of CL (P=0.362). Interaction between eCG treatment and presence of CL was not significant for pregnancy or twin rates (P=0.532). In conclusion, although 300IU of eCG increased the twin rate compared with 200IU, no differences were found in P/AI. We suggest that the dose of eCG in the J-Synch protocol could be adjusted to 200IU in Bos taurus beef heifers. Table 1. Pregnancy per AI (P/AI) and twin rates obtained with different doses of eCG (200 or 300IU) in a J-Synch protocol for fixed-time AI in cycling (CL) and anestrous (no CL) Bos taurus beef heifers Variable 200 IU 300 IU P-value CL no CL Total CL No CL Total eCG CL eCG×CL P/AI (%) 61.6% (135/219) 59.8% (289/483) 60.4% (424/702) 60.2% (133/221) 59.5% (286/481) 59.7% (419/702) 0.513 0.513 0.513 Twin rate (%) 3.0% (4/135) 3.5% (10/289) 3.3% (14/424) 6.8% (9/133) 9.4% (27/286) 8.6% (36/419) 0.005 0.362 0.532