11 FERTILITY RESPONSE IN SUCKLED BEEF COWS SUPPLEMENTED WITH LONG-ACTING PROGESTERONE AFTER TIMED ARTIFICIAL INSEMINATION

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Corpus luteum (CL) and progesterone (P4) secretion are affected by preovulatory follicle (POF) size. Increased circulating P4 during early diestrus has a positive effect on embryo development in beef cattle. However, the combined effects of the POF size and P4 supplementation during early diestrus on fertility of beef cows are not known. The objective was to evaluate the effects of POF size and supplementation of long-acting P4 after timed-AI on pregnancy rates (P/AI). Suckled Nelore cows (n = 596) were evaluated twice by transrectal Doppler ultrasonography (US) 10 days apart to detect the cyclic status. In Study 1, anestrous cows (absence of CL on both exams) received an intravaginal P4 device and an oestradiol benzoate (EB) injection on Day –10 (day of the second US). Devices were removed and sodium cloprostenol [prostaglandin F2α (PGF2α)], oestradiol cypionate, and eCG were given on Day –2. Cows were timed-AI on Day 0 and assigned to receive placebo (control group, n = 187) or 150 mg of long-acting P4 on Day 4 (P4 group, n = 189). In Study 2, cyclic cows (presence of CL) received a PGF2α injection on Day –20 (first US). Cows with a new CL on Day –10 received an intravaginal P4 device and an injection of EB and were split to receive an injection of PGF2α [large follicle (LF); n = 109] or not [small follicle (SF); n = 111]. Devices were removed and PGF2α was injected on Day –2. Ovulation was induced with buserelin acetate, and cows were timed-AI on Day 0 and split to receive placebo (LF/control group, n = 55, and SF/control group, n = 55) or 150 mg of long-acting P4 on Day 4 (LF/P4 group, n = 56, and SF/P4 group, n = 54). Ultrasonographic scanning was done on Days 0, 4, and between 35 and 40 to detect the POF and CL sizes and P/AI, respectively. Data were analysed using PROC GLIMMIX (SAS Institute Inc., Cary, NC). In anestrous cows, P/AI was reduced in POF with <11 mm. The P/AI was greater in the P4-treated group than in the control group for all cows (55.6% v. 46.0%; P = 0.05) and for ovulated cows (59%, 105/178 v. 49%, 86/173; P = 0.08). For cyclic cows, POF size (mm) on Day 0 (13.5 ± 0.3 v. 11.2 ± 0.2), ovulation rate (90% v. 77%), and CL area (cm2) on Day 4 (1.46 ± 0.05 v. 1.25 ± 0.05) were greater (P < 0.007) in the LF group than in the SF group. There was a main effect of follicle group on P/AI (54%, LF group v. 38%, SF group; P < 0.01). Moreover, P/AI were greater (P < 0.05) in the LF/control (56%) and LF/P4 (52%) groups than in the SF/control group (31%), whereas no difference was detected between the SF/P4 group (45%) and the other groups. Among cows that ovulated, P/AI was lower (P = 0.05) in the SF/control group (41%, 17/41) compared with the LF/control group (62%, 31/50) and were similar for the SF/P4 group (56%, 25/45) and LF/P4 group (57%, 28/49) compared with others. We suggest that P4-stimulated embryotrophic effects improved fertility in anestrous beef cows supplemented with long-acting P4 on Day 4 after timed-AI. Also, the presence of a functional CL during follicle growth results in smaller POF and CL and reduces the ovulatory and P/AI rates in cyclic cows. Post-AI P4 supplementation may attenuate the negative effects of small POF/CL. Research was supported by CNPq, FAPESP, Ouro Fino Agronegócio, and Innovare.