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

Angus (n = 6), Brangus (5/8 Angus × 3/8 Brahman, n = 6), and Brahman × Angus (3/8 Angus × 5/8 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 PGF2α protocol (25 mg i.m. Day −14 and 12.5 mg i.m. Day −3 and −2 of experiment). Heifers received 100 μg GnRH i.m. on Day 6 of the subsequent estrous cycle. Blood samples were collected at −60, −30, and −1 min before GnRH and 15, 30, 60, 90, 120, 150, 180, 240, 300, 360, 420, and 480 min after GnRH to determine concentrations of serum LH. Estradiol concentrations were determined at −60, −30, and −1 min before GnRH. On Day 6 and 8, ovaries were examined by ultrasonography to determine if ovulation occurred. On Day 13, heifers received 25 mg PGF2α 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 ± 0.8 ng/mL) compared to Brangus (4.6 ± 0.8 ng/mL) and Brahman × Angus (2.9 ± 0.8 ng/mL), which were similar (P > 0.10). Mean LH peak-height was similar (P > 0.10) for Brangus (13.9 ± 3.4 ng/mL) compared to Angus (21.9 ± 3.4 ng/mL) and Brahman × Angus (8.0 ± 3.4 ng/mL), but was greater (P < 0.05) for Angus compared to Brahman × Angus. 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.