EVIDENCE FOR STRESS-MEDIATED HIGH PROGESTERONE RELEASE IN PREPUBERTALCALVES

Plasma progesterone (P4) and cortisol (CS) concentrations were determi ned in venous blood of female Holstein calves taken by jugular punctur e at the ages of 6 and 26 weeks, after an overnight fast. Plasma conce ntrations of P4 and CS were also determined in blood collected by jugu lar catheterization during metabolic challenge tests (with GRF, adrena line, and glucose) at the age of 17 and 38 weeks. Calves at those ages were brought into a negative energy balance over the 4 days preceding and during the 1-day challenge tests. P4 and CS concentrations were d etermined for all animals in the first (prechallenge) sample of the te st-day. At 6 and 26 weeks of age, all animals had P4 concentrations wi thin the normal prepubertal range (< 0.5 ng/ml). However, at 17 weeks of age, 37 out of 152 calves showed P4 concentrations > 2 ng/ml, and a t the age of 38 weeks 14 out of 154 calves had > 2 ng/ml. At both 17 a nd 38 weeks, eight animals were found with concentrations > 8 ng/ml. S ix to seven animals in both age groups of 17 and 38 weeks with high (> 0.5 ng/ml) and low (<0.2 ng/ml) P4 concentrations in the first sample were selected for further establishing P4 and CS profiles over the wh ole test-day. Five out of 7 calves with high P4 concentrations in the first sample at 17 weeks showed a decrease to prepubertal levels in la ter samples on the same test-day, but elevations were found around mom ents of handling. In four out of seven calves with high P4 concentrati ons at 38 weeks of age, P4 concentrations were high in all samples tak en on the test-day (< 5 ng/ml). Calves with low P4 prechallenge concen trations at 17 and 38 weeks showed low values over the whole test-day, although small increases were found around moments of handling. Value s for CS concentration had a low correlation with P4 in spot samples a t 6 weeks (-0.05) and a moderate one in first samples of 38 weeks (0.2 9). However, this correlation was high in spot samples at 26 weeks and for test-day samples at 17 weeks, over animals as well as over test-d ay samples within animals (0.63-0.73). It is concluded that stress-med iated high P4 concentrations may be found in calves long before onset of puberty.