Influence of dietary energy intake on prepubertal development of Brahman bulls3

Abstract

Abstract Twelve Brahman bulls (paired by sire, weight and age) were assigned randomly and limit fed to gain either .10 to .25 (moderate gain; MG) or .75 to 1.0 (high gain: HG) kg·hd−1·d−1 to examine the effect of dietary energy on onset of puberty. Hip height (HH), scrotal circumference (SC) and serum samples (20 min for 6 h) were obtained at four times (AGE): 0, 56 and 112 d on feed and after appearance of first motile spermatozoa (FS) in the ejaculate of HG bull of the pair. At FS both bulls of a pair were slaughtered, reproductive tissues were collected and in vitro GnRH release from the median eminence (ME) was measured. Increases in BW, HH and SC were greater (P < .05) in HG bulls. Basal ME GnRH secretion was greater (P < .05) in HG bulls. Serum LH concentrations were unchanged by energy level (P > .10) but increased (P < .01) with increasing AGE. AGE and energy level increased (P < .01) basal, mean and total serum testosterone (T) and these two factors acted synergistically (P < .01). Height and amplitude of T pulses were increased by energy level (P < .003) and AGE (P < .002). Testicular T (P < .08) and development (P < .05) were increased in HG bulls. Growth hormone peak height and amplitude concentrations following feeding increased with AGE (P < .06) but were not altered (P > .10) by energy level. Serum triglycerides (P < .03) and BUN (P < .003) increased with increasing AGE (P > .01). These data indicate that dietary energy level influences onset of puberty most directly at the testicular level.