EFFECT OF SEASON ON FOLLICULAR DYNAMICS AND PLASMA-CONCENTRATIONS OF ESTRADIOL-17-BETA, PROGESTERONE AND LUTEINIZING-HORMONE IN LACTATING HOLSTEIN COWS

Lactating Holstein cows (n=16), averaging 64.1 d in milk, were utilized over 4 replicate months (April, June, August and November) in a shade management system to examine the effects of season on follicular dynamics and plasma concentrations of estradiol (E(2)), progesterone (P-4) and luteinizing hormone (LH). Cows were synchronized to estrus using a combination of Buserelin (GnRH, 8 ug) and prostaglandin F-2 alpha (PGF(2 alpha) 25 mg) given 7 d apart. Follicular development was monitored daily by ultrasonography, and plasma concentrations of E(2), P-4 and LH measured by radioimmunoassay. The replicate month had no detectable effects on estrus interval (3.1 +/- 0.3 d) or percentage of cows (78.1 +/- 9.4%) that expressed estrus following GnRH and PGF(2 alpha) treatment. Preovulatory follicles grew at faster rates (P<0.01) in June (2.0 +/- 0.6 mm/d), than in April (1.1 +/- 0.6 mm/d), August (1.0 +/- 0.6 mm/d) or November (1.2 +/- 0.6 mm/d). First wave dominant follicles were consistently larger in April than in June, August and November. The larger and more persistent size of the first wave dominant follicle in April was associated with an earlier regression of the largest subordinate follicle and a sharper decrease in the number of medium size follicles (6 to 9 mm) by Day 9 of the estrous cycle. Conversely, growth of the first wave dominant follicle was slower and the largest subordinate follicle was more persistent in August than in April, June or November. The proestrous rise in plasma E(2) occurred faster (P<0.01) in August (10.1 +/- 2.1 pg/d) than in April (4.6 +/- 2.1 pg/d), June (5.3 +/- 2.1 pg/d) or November (5.9 +/- 2.1 pg/d). Concentrations of P-4 in plasma increased and reached higher (P<0.01) luteal values in August (15.1 +/- 0.6 ng/ml) and November (16.0 +/- 0.6 ng/ml) than in April (6.1 +/- 0.6 ng/ml) and June (10.6 +/- 0.6 ng/ml). There was no detectable effect of month on LH pulse characteristics 48 h post-PGF(2 alpha). The maximum size of the corpus luteum (CL) was greatest in November and was related positively to diameter of the ovulatory follicle of the preceding cycle. Results indicated that ovarian follicular development and dominance may be altered during summer months. However, it is uncertain whether these changes can be related to the well-documented low breeding efficiency during warmer months of the year in subtropical environments.