LUTEAL FUNCTION AND ESTRUS IN PERIPUBERTAL BEEF HEIFERS TREATED WITH AN INTRAVAGINAL PROGESTERONE RELEASING DEVICE WITH OR WITHOUT A SUBSEQUENT INJECTION OF ESTRADIOL

The objectives of this experiment were to determine if treatment of be ef heifers with progesterone (P-4) using an intravaginal device alone or in combination with estradiol benzoate (EB) would induce estrus and cause development of corpora lutea (CL) with a typical life span. Per ipubertal heifers (n=311) were used when about 40% of the heifers had a functional CL. The heifers were assigned to receive one of the follo wing treatments on Day 0: 1) a sham device for 7 d (C, n=108); 2) an i ntravaginal device containing P-4 for 7 d (P, n=102); or 3) an intrava ginal device containing P-4 for 7 d plus an injection of 1 mg EB 24 to 30 h after device removal (PE, n=101). Serum concentrations of P-4 we re determined on Days -7, 0, 8, 15 and 22. Weight and age of the heife rs at the start of the trial averaged 292 +/- 45 kg and 365 +/- 38 d, respectively. A greater (P < 0.0001) proportion of the heifers from th e PE than P group was in standing estrus (81 vs 37%) and formed normal CL (68 vs 44%) after device removal. Of the heifers exhibiting estrus , a greater (P < 0.05) proportion of PE (94%) than P (80%) heifers was active 1 to 3 d after implant removal. Short-term progesterone treatm ent increased the proportion of heifers in estrus and those forming no rmal CL, and adding EB to the progesterone treatment further enhanced these responses. (C) 1998 by Elsevier Science Inc.