209 SEASONAL CHANGES IN CHARACTERISTICS OF THE ESTROUS CYCLE IN BOS TAURUS INDICUS (NELORE) HEIFERS

Abstract

To evaluate the possible seasonal variation in estrous cycle characteristics, 8 heifers were synchronized with 2 prostaglandin injections and their estrous cycles were followed during different seasons of the year (autumn, n = 413; winter, n = 8; spring, n = 9; and summer, n = 9) with daily ultrasonographic exams to count and measure follicles ≥3 mm. Blood was collected every 12 h for luteinizing hormone (LH) and progesterone (P4), and every 3 h from estrus until ovulation to characterize the LH peak. Five ovariectomized heifers were injected with 17β-estradiol (2 μg/kg of liveweight) every season and blood samples were collected thereafter at 3 h intervals for LH quantification. The monthly body weight percentile difference (Δ%) did not vary among seasons. Average P4 concentration for the cycle was higher (P = 0.001) and follicle numbers lower during autumn (2.5 ± 0.2 ng mL^-1; 7.8 ± 0.1) and summer (2.9 ± 0.3 ng mL^-1; 6.8 ± 0.2; P = 0.001) compared with winter (1.4 ± 0.2 ng mL^-1; 9.6 ± 0.3) and spring (1.6 ± 0.2 ng mL^-1; 9.7 ± 0.3). During winter, there were more estrous cycles with 3 follicle waves (5/8) and during summer only there were cycles with 2 follicular waves (P = 0.009). Because LH secretion did not vary despite seasonal variation in P4 concentration and because there was a negative correlation between higher P4 values and daily percentile variation of photoperiod (A%; P = 0.0038; r = -0.4533), seasonal variation in the luteal cell sensitivity to LH needs to be evaluated. In the ovariectomized Nelore heifers, the LH circannual concentration without estradiol stimulus was significant (P = 0.02). The LH response to estradiol treatment was lower in summer (0.8 ± 0.2 ng mL^-1 v. 1.3 ± 0.5 ng mL^-1). We hypothesize that seasonal variation exists in hypothalamic sensitivity to estradiol. The present experiment provides evidence that seasonality in reproductive responses remains in Nelore heifers.

FAPESP (07-53539-3), UNESP, and Devani Mariano Pinheiro.