EFFECTS OF MELENGESTROL ACETATE ON ONSET OF PUBERTY, FOLLICULAR-GROWTH, AND PATTERNS OF LUTEINIZING-HORMONE SECRETION IN BEEF HEIFERS

Our objective was to test the hypothesis that short-term (8 days) trea tment of prepubertal heifers with melengestrol acetate (MCA) and subse quent steroid withdrawal would stimulate LH secretion and follicular g rowth. Angus heifers were divided randomly into two groups; MGA-treate d (n = 8) or control (CON; n = 9). Puberty was determined by monitorin g circulating concentrations of progesterone and ovarian morphology du ring a 14-day period following MGA withdrawal. LH secretory patterns w ere assessed upon initiation of MGA (Day 0), during MGA (Day 7), and 1 day after withdrawal of MCA (Day 9). All MGA-treated heifers, versus four CON heifers, exhibited corpora lutea and luteal phase concentrati ons of progesterone within 10 days after treatment (p = 0.01). Mean LH and LH pulse frequency increased (p = 0.005 and 0.0001, respectively) between Days 0 and 9 in MGA-treated heifers. In CON heifers, mean LH concentrations and pulse frequencies did not change. During the same p eriod, diameter of the largest follicle increased in MCA-treated anima ls (p = 0.003) but did not change in the CON heifers. On the basis of these results, we suggest that MGA withdrawal enhances onset of pubert y by stimulating pulsatile LH secretion that accelerates follicle grow th to the preovulatory stage.