Response to GnRH on day 6 of the estrous cycle is diminished as the percentage of Bos indicus breeding increases in Angus, Brangus, and BrahmanxAngus heifers

摘要

Angus (n=6), Brangus (5/8 Angusx3/8 Brahman, n=6), and BrahmanxAngus (3/8 Angusx5/8 Brahman, n=6) heifers exhibiting estrous cycles at regular intervals were used to determine if the percentage of Bos indicus breeding influenced the secretory patterns of LH in response to a GnRH treatment on Day 6 of the estrous cycle. Heifers were pre-synchronized with a two-injection PGF(2alpha) protocol (25mg i.m. Day -14 and 12.5mg i.m. Day -3 and -2 of experiment). Heifers received 100mug GnRH i.m. on Day 6 of the subsequent estrous cycle. Blood samples were collected at -60, -30, and -1min before GnRH and 15, 30, 60, 90, 120, 150, 180, 240, 300, 360, 420, and 480min after GnRH to determine concentrations of serum LH. Estradiol concentrations were determined at -60, -30, and -1min before GnRH. On Day 6 and 8, ovaries were examined by ultrasonography to determine if ovulation occurred. On Day 13, heifers received 25mg PGF(2alpha) i.m. and blood samples were collected daily until either the expression of estrus or Day 20 for heifers not exhibiting estrus to determine progesterone concentrations. There was no effect (P>0.10) of breed on ovulation rate to GnRH as well as size of the largest follicle, mean estradiol, and mean corpus luteum volume at GnRH. Mean LH was greater (P<0.05) for Angus (7.0+/-0.8ng/mL) compared to Brangus (4.6+/-0.8ng/mL) and BrahmanxAngus (2.9+/-0.8ng/mL), which were similar (P>0.10). Mean LH peak-height was similar (P>0.10) for Brangus (13.9+/-3.4ng/mL) compared to Angus (21.9+/-3.4ng/mL) and BrahmanxAngus (8.0+/-3.4ng/mL), but was greater (P<0.05) for Angus compared to BrahmanxAngus. Interval from GnRH to LH peak was similar (P>0.10) between breeds. As the percentage of Bos indicus breeding increased the amount of LH released in response to GnRH on Day 6 of the estrous cycle decreased.