Sex differences in plasma cortisol and growth in the bovine

Tame, pubescent beef calves (six bulls; six heifers) were used to examine the relationship between sex, increase in body weight (BW) and plasma cortisol (C). In bulls (B), plasma testosterone (T) was also examined. The two groups were fed a concentrate to 1.5% BW and Coastal bermudagrass hay ad libitum. From 7 to 12 mo of age, at 28-d intervals, the animals were weighed and jugular vein blood was collected every 15 min for 6 h. Onset of puberty was determined weekly by measuring scrotal circumference in B and assaying for plasma progesterone in the heifers (H). Acceptable growth occurred in both B (.83 kg/d) and H (.67 kg/d). These growth rates were different (P less than .02). Plasma C concentrations were significantly different between the sexes over the time span examined. Whereas C remained between 1.6 and 1.8 ng/ml between 7 and 12 mo of age in B, C rose from 2.9 to 6.0 ng/ml in H. The experiment means for B and H were 2.8 +/- .3 (mean +/- SE) and 5.7 +/- .4 (P less than .01). There were significant correlations between plasma C and age or weight in H, but not in B. Mean plasma T in B was greatest at 9 and 10 mo of age (1.4 and 1.3 ng/ml) and lowest at 7 and 12 mo of age (.76 and .74 ng/ml). Plasma T was not correlated with plasma C or age. The lower plasma C concentrations in B compared with H is commensurate with the greater growth that occurred in B.(ABSTRACT TRUNCATED AT 250 WORDS)