| Summary: | An experiment was conducted with Brangus cows to evaluate the effect of loss of body weight and condition on pituitary responsiveness to gonadotropin releasing hormone (GnRH) stimulation during late lactation. The treatment groups were lactating intact (LI), lactating ovariectomized (LO), nonlactating intact (NLI), and nonlactating ovariectomized (NLO). The study was carried out in two separate blocks, each one consisting of 3 periods. During period 1 the cows were fed a ration that supplied 90% and 88% of the NRC recommendations for TDN in lactating and nonlactating cows, respectively. This period lasted 170 in block 1 and 130 days in block 2. During period 2 the TDN was reduced to 55% or 52% for lactating and nonlactating cows, respectively. Period 2 lasted 100 days for cows in block 1 and 63 days for cows in block 2. At the beginning of period 3 TDN was further reduced to 25% or 27% for the lactating and nonlactating cows, respectively. Cows in block 1 were challenged with GnRH 40 days after the beginning of the 1st energy reduction, 30 days later and 7 days after the 2nd energy reduction. The cows in block 2 were challenged with GnRH 30 days after the 1st energy reduction, 30 days later and 25 days after the 2nd energy reduction. At the end of the study body composition parameters and organ gland weights were determined. No significant differences in the weights of the cows among the treatment groups were found. All cows were, however, losing weight through the course of this study. The nonlactating cows maintained higher body condition (P < .05) than lactating cows from 31 days after ovariectomies were performed until the end of the study. The pituitary glands were significantly heavier in the lactating ovariectomized (2.3 g vs. 1.7 g, P < .05) than the nonlactating intact cows. The weight of the adrenals per unit of body weight of LO cows was significantly higher (.057 g/kg vs. .040 g, P < .05) than among NLO cows. The percent of carcass lipid was significantly higher (P < .05) in nonlactating as compared to lactating cows. Percent moisture and protein were higher (P < .05) in lactating cows. Amount of LH released after GnRH stimulation tended to be higher in lactating than nonlactating cows. The magnitude of the LH peak did not differ significantly among the treatment groups at each of the dates GnRN was injected. Ovariectomized cows (LO and NLO) responded more rapidly (P < .05) to GnRH stimulation than intact cows (LI and NLI). Time on reduced TDN did not affect cow's response pattern after GnRH injection.
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