Relationships of hormonal patterns and fertility to occurrence of two or three waves of ovarian follicles, before and after breeding, in beef cows and heifers.

Ovarian follicular waves were characterized before and after breeding in 52 lactating beef cows and 16 heifers. Effects of two (2 W) or three (3 W) waves of follicular development and associated patterns of concentrations of steroids in jugular serum on pregnancy rate were examined. Animals were observed for oestrus (= day 0) twice daily and inseminated artificially at second oestrus. Follicular development was monitored by ultrasonography and jugular blood samples were collected simultaneously on alternate days from day 6 after first oestrus until ovulation after the second oestrus and from day 6 after second oestrus (insemination) until next ovulation or day 24 of pregnancy. Pregnancy was determined by ultrasonography at 25 days after insemination. From individual patterns of growth or regression of the largest follicles, more of the 59 animals with oestrous cycles of 17 to 25 days had 2 W (51, 86%) than 3 W (8, 14%) during the oestrous cycle before breeding (P < 0.01). Cycles averaged 1.1 days longer (P < 0.10) and corpora lutea regressed later (P < 0.01) in animals with 3 W vs. those with 2 W, but mean oestradiol between 7 and 2 days before second oestrus (3.7 +/- 0.3 pg ml-1) did not differ between cycles with 2 W or 3 W. Ovulatory follicles in animals with 2 W differed from those with 3 W (P < 0.05) in day of detection (12.3 +/- 0.3 vs 16.5 +/- 0.5), growth rate (1.0 +/- 0.1 vs 1.5 +/- 0.1 mm day-1), interval from detection to ovulation (9.3 +/- 0.3 vs 6.3 +/- 0.7 days) and duration of dominance (4.0 +/- 0.2 vs 2.1 +/- 0.6 days). Pregnancy rates, 82% in cows with 2 W and 100% in cows with 3 W, did not differ (P > 0.05). During the period equivalent to an oestrous cycle after breeding, 29 (49%) of the 59 animals had 2 W and 30 (51%) had 3 W. Fewer animals with 2 W than 3 W after breeding became pregnant (16/23, 70% vs 26/27, 96%; P < 0.05), but patterns of concentrations of progesterone on days 6 through 14 or mean oestradiol on day 14 (2.6 +/- 0.2 pg ml-1) did not differ. In conclusion, fewer animals had 3 W than 2 W before breeding and fertility did not differ. During the equivalent of one oestrous cycle after breeding, approximately equal numbers of animals had 2 W or 3 W and fertility was greater for animals with 3 W.