RELATIONSHIPS OF HORMONAL PATTERNS AND FERTILITY TO OCCURRENCE OF 2 OR 3 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 thre e (3 W) waves of follicular development and associated patterns of con centrations of steroids in jugular serum on pregnancy rate were examin ed. Animals were observed for oestrus (= day 0) twice daily and insemi nated artificially at second oestrus. Follicular development was monit ored by ultrasonography and jugular blood samples were collected simul taneously on alternate days from day 6 after first oestrus until ovula tion after the second oestrus and from day 6 after second oestrus (ins emination) until next ovulation or day 24 of pregnancy. Pregnancy was determined by ultrasonography at 25 days after insemination. From indi vidual 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 2.1 days longer (P < 0.10) and corpora lutea re gressed later (P < 0.01) in animals with 3 W vs. those with 2 W, but m ean 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 f ollicles in animals with 2 W differed from those with 3 W (P < 0.05) i n 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 Ur and 1 00% in cows with 3 W, did not differ (P > 0.05). During the period equ ivalent to an oestrous cycle after breeding, 29 (49%) of the 59 animal s 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 pat terns of concentrations of progesterone on days 6 through 14 or mean o estradiol on day 14 (2.6 +/- 0.2 pg ml(-1)) did not differ. In conclus ion, 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. (C) 1997 Elsevier Science B.V.