Peripheral serum progesterone, 17-hydroxyprogesterone and 20B-hydroxyprogesterone levels in suckled and nonsuckled primiparous Hereford heifers

Abstract

Peripheral serum levels of progesterone (P), 208-hydroxy-progesterone (208-OHP) and 17-hydroxyprogesterone (17-OHP) were determined by radioimmunoassay procedures in suckled (S) and nonsuckled (NS) first-calf heifers from parturition to first estrus postpartum. Blood samples were collected every six hours which accompanied estrous detection. Rectal palpation was conducted every second day and all heifers were weighed every other week. Five of 10 Hereford heifers (NS) had their calves removed immediately after birth while the other five retained their calves (S) and were allowed to suckle their calves at 4-hour intervals. Five heifers (two S and three NS) constituted the final endocrine study. The physiological stress associated with lactation was evidenced by differences in body weight gains following parturition. From four to ten weeks postpartum the S heifers gained 0.4 and the NS heifers gained 0.8 kg/head/day while being fed according to NRC recommendations. Luteinizing hormone (LH) levels were quantitated for these same heifers by a fellow researcher. All five heifers exhibited a sharp LH peak during the period of standing estrus. Seventeen to 20 days prior to standing estrus, elevated LH levels were noted. The significance of elevated LH levels at this time probably results in silent ovulation or follicular luteinization, with or without ovulation, which is evidenced by the presence of subsequent P and 2OB-OHP release..