Reuse of norgestomet implants in an eCG-based superovulation protocol administered to Nelore (Bos taurus indicus) cows

Abstract

This study assessed the reuse of norgestomet implants in Nelore cows that were superstimulated with eCG. In a crossover design trial, eight cows were randomly divided into two experimental groups and twice superstimulated: Group 1 — half of the cows received a new norgestomet implant and 2mg estradiol benzoate (EB) on Day 0; Group 2 — remaining cows received two once-used norgestomet implants and 2mg EB also on Day 0. On Day 4 all cows received a single dose of 2000IU eCG, and on Day 6 cows were treated with two doses of PGF2α 12h apart. Ovulation was induced with 12.5mg pLH 12h after implant withdrawal (on Day 7), and fixed-time artificial inseminations were carried out 12 and 24h later. Seven days after pLH injection embryos were recovered and blood samples were taken to determine circulating progesterone. Ultrasound examinations were performed at pLH administration and at embryo recovery. The number of large follicles (≥8mm) was greater (P<0.05) in Group 1 (17.1±1.8) than in Group 2 cows (9.7±1.6). The mean number of corpora lutea was greater (P<0.05) for Group 1 (13.8±1.8) as compared to Group 2 cows (5.4±1.0) and the percentage of large follicles that ovulated following pLH administration also differed (P<0.05) between treatments (80.3% and 53.8% for cows in Groups 1 and 2, respectively). However, no differences (P>0.05) were found between treatments (respectively, for Group 1 and Group 2 cows) in the follicle diameter (10.5±1.6 and 10.2±1.7mm), corpus luteum (CL) diameter (15.7±1.8 and 14.4±1.7mm), CL volume (2232±1356 and 1893±1828mm3), ova/embryos recovered (6.3±1.1 and 4.0±1.9), transferable embryos (4.0±1.8 and 2.5±1.0) and plasma progesterone concentration (36.1±8.3 and 35.1±6.9ng/mL). In conclusion, the stimulatory effects on the ovaries provided by the use of two once-used implants were less intense than the ones verified after using a new implant.