11 STRATEGIES TO MODULATE THE PERI-OVULATORY ENDOCRINE MILIEU BY CONTROLLING PROGESTERONE CONCENTRATIONS PRE- AND POST-INSEMINATION IN BEEF COWS

Abstract

Recent evidence indicates that progesterone (P4) secretion from corpus luteum (CL) is affected by preovulatory follicle (POF) size and that increased circulating P4 during early diestrus has a positive effect on embryo development and pregnancy rates in cattle. Despite the embryotrophic effects, early administration of P4 may impair CL growth and anticipates luteolysis. Owing to develop favourable peri-ovulatory endocrine milieus to pregnancy success, we evaluated in beef cows (1) the effect of CL presence during dominant follicle growth on follicular vascularization, size and ovulation rate and (2) the effects of POF size and supplementation of long-acting P4 post-AI on CL development and regression. Sixty-six Nelore cows received an intravaginal P4 device and an injection of oestradiol benzoate on Day 10 (5–10 days post-oestrus), and were split to receive sodium cloprostenol [prostaglandin F2α (PGF2); large follicle (LF); n = 31) or not [small follicle (SF); n = 35]. Devices were removed and PGF2 was injected on Day 1.75. Ovulation was induced with a gonadotropin-releasing hormone (GnRH; buserelin acetate) on Day 0. Cows were inseminated twice on Days 0.5 and 1. Cows that ovulated were assigned to receive placebo (LF/control group, n = 14; and SF/control group, n = 9) or 150 mg of long-acting P4 on D4.5 (LF/P4 group, n = 13; and SF/P4 group, n = 12). Colour-Doppler ultrasound scanning was done on Days 10, 6, 2, 0.5, 1, 1.5, 4.5, 7.5, and 15.5. Structural luteolysis was detected by 25% decrease in CL area and 50% in luteal blood flow of subsequent scanning. Data were analysed for the main effects of POF size and supplementation and their interaction using the PROC MIXED procedure of SAS (SAS Institue Inc., Cary, NC, USA). Follicle diameter (mm) on Day 2 (11.4 ± 0.4 v. 8.4 ± 0.3) and POF (14.1 ± 0.3 v. 12.5 ± 0.3) were greater (P < 0.05) in LF group. Percentage of luteal blood flow in follicle wall was greater (P < 0.05) on Day 0.5 (33.9 ± 3.4 v. 22.8 ± 3.6) and 1 (48.9 ± 4.8 v. 28.3 ± 4.3) in the LF group. Ovulation rate was lower (P < 0.05) in the SF group (60%, 21/35) than in the LF group (90%, 28/31), suggesting that a functional CL during dominant follicle growth is less favourable to pregnancy success due to 30% reduction in fertilizing potential of non-ovulated follicles. For luteal area (cm2), an effect (P < 0.05) of POF size was detected on Day 4.5 (1.7 ± 0.1 v. 1.3 ± 0.1), 7.5 (2.8 ± 0.2 v. 2.2 ± 0.1), and 15.5 (2.5 ± 0.1 v. 2.2 ± 0.2) in the LF and SF groups, respectively. Luteal blood flow was greater (P < 0.05) in the LF groups (40.4 ± 1.7 and 47.6 ± 3.4) than in the SF groups (35.7 ± 1.5 and 37.9 ± 4.1) on Day 4.5 and 15.5, respectively. This indicates that larger and more vascularized POF may be associated with greater CL vascularization in addition to greater CL size. The frequency of cows with early luteolysis did not differ (P > 0.1) between P4-treated (5/25) and placebo-treated (1/23) cows. We suggest that the P4 supplementation did not severely induce early luteolysis in inseminated cows and that the P4-embryotrophic effects may down-regulate the mechanisms involved in the earlier luteolytic process caused by greater exposure of uterus to P4. Support for this study was provided by the São Paulo Research Foundation (FAPESP, Sao Paulo, Brazil) and Ourofino (Cravinhos, SP, Brazil).