Serum growth hormone release during a 60-hour period in growing pigs

Growth hormone (GH) profiles were measured during a 60-hour period in four castrated male and five female, 20-week-old Yorkshire pigs. During this period, GH release was not affected by feeding when measured at 2, 4 and 8 hr before and after feeding time. A photoperiod of 12 hr light and 12 hr darkness produced a decrease (P less than .05) in baseline mean GH levels from 4.0 to 3.5 ng/ml during periods of darkness. This effect was observed in both genders. Females and castrated males exhibited (P greater than .05) similar baseline GH levels and identical numbers of GH peaks during feeding and photoperiod studies. However, during these periods, the amplitude of the GH peaks and areas under the GH curves were greater (P less than .05) in females. These results indicate that: 1) feeding did not influence GH secretion; 2) darkness produced a decrease in the baseline GH levels in both sexes; and 3) females secreted more GH than castrated males of the same age.