RESUMO
The objectives of this study were to determine the effect of supplemental progesterone on fertility in lactating dairy cows lacking a corpus luteum (CL) at the initiation of the timed artificial insemination (AI) program. Holstein cows were subjected to the 5-d timed AI program (d -8 GnRH, d -3 and -2 PGF2α, d 0 GnRH and AI). Cows had their ovaries scanned by ultrasonography on d -8 and those bearing a CL were considered to be in diestrus (DI; n=946). Cows that lacked a CL on d -8 were assigned to remain as untreated control (CON; n=234) or receive 2 controlled internal drug release (CIDR) inserts containing progesterone (2CIDR; n=218) from d -8 to -3, as a single insert has been proven insufficient to modulate fertility in cows without CL. Blood was analyzed for progesterone and estradiol concentrations. Pregnancy was diagnosed on d 34 and 62 after AI. Progesterone concentrations during the timed AI program were lowest for CON, intermediate for 2CIDR, and highest for DI. Supplementation increased progesterone concentrations between d -7 and -3 compared with CON (2.65 vs. 0.51 ng/mL). Ovulation to the first GnRH was not affected by treatment. However, a greater proportion of CON and 2CIDR cows had a new CL on d -3 compared with DI cows (66.7 vs. 61.9 vs. 52.0%). In cows with a new CL, the diameter of the ovulatory follicle was larger for CON than 2CIDR, and intermediate for DI (18.7 vs. 16.5 vs. 17.7 mm). Concentrations of estradiol on d -3 did not differ among treatments; however, DI cows had greater estradiol concentrations at AI compared with CON or 2CIDR cows. Pregnancy per AI was less for CON compared with 2CIDR or DI on d 32 (30.8 vs. 46.8 vs. 49.9%) and 64 (28.6 vs. 43.7 vs. 47.3%), indicating that supplementation with progesterone reestablished fertility in cows lacking a CL on d -8. A greater proportion of nonpregnant CON cows had a short reinsemination interval compared with 2CIDR or DI (11.1 vs. 3.5 vs. 5.7%). Treatment did not affect pregnancy loss between d 34 and 62 of gestation. A single ultrasound exam was effective in identifying a low-fertility cohort of cows based on the absence of CL at the first GnRH injection of the timed AI protocol. Progesterone supplementation with 2 CIDR inserts increased progesterone in plasma to 2.65 ng/mL and restored fertility in lactating dairy cows lacking a CL at the initiation of the timed AI program similar to that of cows in diestrus.
