Effect of underfeeding on testosterone-LH feedback in the bull.

This study was conducted to investigate the hypothesis that the low LH pulsatility induced by underfeeding may result from an altered negative-feedback response to testosterone in cattle. Eight 21-month-old Aubrac bulls were divided into two equal groups. The first group (group H) received a high level of nutrition (producing a gain in weight of 570 g/day) and the second group (group L) a low level of nutrition (producing a loss in weight of 330 g/day). After 52 days of underfeeding the bulls were hemicastrated, castrated 7 days later and then injected with 30 mg testosterone daily for 14 days, 10 mg daily for 7 days and 2.5 mg daily for 7 days. After hemicastration, LH pulsatility was higher (P less than 0.001) in group H (9.0 pulses in 24 h) than in group L (5.3 pulses in 24 h). After castration this value was the same in the two groups (24 pulses in 24 h). When the bulls were injected daily with 30 or 10 mg testosterone, the number of LH pulses in 24 h was lower (P less than 0.05) in group L (14.3 and 9.8 pulses in 24 h with 30 mg, and 17.3 pulses in 24 h with 10 mg) than in group H (27.8 and 25.5 pulses in 24 h with 30 mg, and 24.0 pulses in 24 h with 10 mg). When the animals received 2.5 mg testosterone daily, the number of LH pulses was the same in both groups (20.3 pulses in 24 h). These results indicate that the negative-feedback response of the LH-releasing system to testosterone is modified by underfeeding in the bull.