The effect of exogenous acth and pen-confinement on estrous cycle length, plasma steroid levels and ovarian function of beef heifers.

Each of 24 pasture-reared crossbred beef heifers were herded from a large pasture on day 14 of the estrous cycle and assigned randomly to one of four treatment groups as follows: Field Control (FC), Field ACTH (FA), Pen Control (PC) and Pen ACTH (PA). Field groups were maintained in a small field, and pen groups were confined in a pen in a pole barn. ACTH groups received 200 IU of ACTH IM daily for days 17 through 21 of the cycle and control groups received only the gelatin carrier IM on the same cycle days. Average cycle lengths for FA and PA heifers were 25 and 24.3 days with an average period from plasma progesterone decline below 1 ng/ml to estrus of 5.5 days. During the ACTH injection period, follicular growth was suppressed and the proestrus plasma estrogen rise was delayed. Average cycle lengths for FC and PC heifers were 20.8 and 22.8 days respectively. All control group heifers exhibited estrus within 2 days of the plasma progesterone decline below 1 ng/ml. In addition, pen confinement heifers showed a trend for extended luteal function and consequent extended estrous cycle length.