VARIABLE PROGESTERONE RESPONSE AND ESTRADIOL SECRETION IN PREPUBERTALBEEF HEIFERS FOLLOWING TREATMENT WITH NORGESTOMET IMPLANTS

Two experiments were conducted to determine the effects of norgestomet ear implants on progesterone response and estradiol secretion in prep ubertal beef heifers. In the first experiment, 47 beef heifers were tr eated with norgestomet. The implants were implanted subcutaneously for 9 d. After implant removal, blood samples were taken from heifers 2 t o 4 d per week for 40 d. Following progesterone determination in jugul ar venous plasma, heifers were classified according to their progester one-response: 1) no response (Group 1); no rise in progesterone above 1 ng/ml throughout the sampling period; 2) one cycle (Group 2); one in crease in progesterone above 1 ng/ml for at least 2 d followed by no f urther increase in progesterone during the sampling period; and 3) two cycles(Group 3); a rise in progesterone above 1 ng/ml for at least 2 d followed by another cycle of normal duration. Heifers treated with n orgestomet were classified as 23 with no response, 9 with 1 cycle and 15 with 2 cycles. Concentrations of estradiol were measured in jugular venous Samples on Day 2 after implant removal. Mean concentrations of estradiol were greater in Group 3 than in Group 1 (P less than or equ al to 0.01). In Experiment 2, 29 prepubertal beef heifers were assigne d randomly to either a 9-d treatment with norgestomet (n=14) or to ser ve as untreated controls (n=15). Blood plasma samples were collected d aily from Days 0 to 44 after implant removal. After progesterone deter mination, heifers were classified as 8 with no response, 4 with 1 cycl e and 3 with 2 cycles in the control group, and 5 with no response, 3 with 1 cycle and 6 with 2 cycles in the norgestomet group (frequencies did not differ; P>0.1). Jugular venous blood plasma was also collecte d at 4-h intervals from 0 h to 96 h after implant removal and concentr ations of estradiol were measured. Patterns of estradiol secretion dif fered (P less than or equal to 0.05) and overall mean concentrations o f estradiol over the first 96 h following implant removal were greater (P<0.01) in norgestomet-treated heifers versus the controls. We concl ude that norgestomet can produce a variable progesterone response with heifers with 2 cycles secreting more estradiol. Implants of norgestom et also causes more acute secretion of estradiol in prepubertal beef h eifers.