PULSATILE INFUSION OF GONADOTROPIN-RELEASING-HORMONE INITIATES LUTEAL ACTIVITY IN NUTRITIONALLY ANESTROUS BEEF-COWS

Thirty-five nonlactating Hereford and Hereford x Angus cows weighing 469 +/- 8 kg, with a body condition score (BCS) of 5.5 +/- .6 and exhibiting normal estrous cycles, were fed to lose 1% of BW per week (n = 2 7) or to maintain BW (n = 8). Blood samples were collected every 7 d to monitor luteal activity and BW and BCS were recorded every 2 wk. When concentrations of progesterone in plasma were < 1 ng/mL for three consecutive weeks, anestrous cows (BCS, 3.1 +/- .7; BW loss, 24 +/- 5%) were assigned to pulses (i.v.) of saline (2 mL every hour), GnRH-4 (2 mug of GnRH every 4 h), or GnRH-1 (2 mug of GnRH every hour). Cows were confined to stalls for 4 d and two jugular cannulas were inserted 2 d before infusion of GnRH. Infusions began on d 0 and continued through d 14. Progesterone was quantified in daily blood samples and LH was quantified in samples collected every 10 min for 4 h on d -1, 0, 2, 4, 6, 8, 10, 12, 14, and 15. On d 16, cows were returned to pasture and blood samples were collected every 2nd d through d 30. Pulsatile infusion of GnRH increased (P < . 0 5) the amplitude of LH pulses but did not affect pulse frequency. During infusion of GnRH every hour or every 4th h, concentrations of LH in serum were increased compared with those in saline-treated cows (P < .001), and cows receiving GnRH hourly had greater (P < .005) LH than cows given GnRH every 4 h. Among cows infused.with saline, fewer had luteal activity by d 10 (P < .05) and d 30 (P < .005) compared with those given GnRH-4 and GnRH-1. More GnRH-1 cows (P < . 1) than GnRH-4 cows initiated luteal activity by d 25. After it was initiated, more GnRH-1 cows maintained luteal activity during (P < . 1) and after (P < .005) infusion compared with GnRH-4 cows. Pulsatile infusion of GnRH every hour induced luteal activity in nutritionally anestrous beef cows.