LUTEINIZING-HORMONE RELEASE AND REPRODUCTIVE TRAITS IN ANESTROUS, ESTRUS-CYCLING AND OVARIECTOMIZED CATTLE AFTER TYROSINE SUPPLEMENTATION

The role of rumen-proteeted L-tyrosine as a nutritional signal alterin g EH release and other reproductive traits in cattle was studied. In E xp. 1, 28 suckled crossbred cows were assigned randomly to five treatm ents (0 or 40 g of tyrosine daily in feed for 3 d followed by a single i.v. injection of 200 mu g of GnRH or 1 mg/kg of naloxone on d 26 +/- 1 postpartum; no tyrosine plus an injection of saline was the control ). Peak LH after GnRH was greater(P < .001) in GnRH-treated cows regar dless of tyrosine supplementation. Compared to COWS receiving saline, days to first postpartum ovulation were reduced (P < .05) by naloxone, tyrosine, and GnRH. In Exp. 2, 47 suckled crossbred cows were assigne d randomly to six treatments (0, 20, or 40 g of tyrosine daily far 3 d before GnRH or saline was injected i.m, on d 23 +/- 1 postpartum). In jection of GnRH increased (P +/- .001) LH. An interaction (P = .08) of tyrosine and GnRH tended to reduce days to first Postpartum ovulation . In Exp. 3, tyrosine (40 g) administered once daily for 3 d to ovarie ctomized cows (six cows per treatment) had no effect on any characteri stic of LH before or after estradiol-17 beta. In Exp. 4, suckled cows (n = 136) were allotted randomly to two treatments (0 or 30 g of tyros ine daily for 3 d before a PGF(2 alpha)-synchronized estrus). Tyrosine increased (P = .05) the percentage of cows in estrus after PGF(2 alph a), but reduced ( P = .05) AI conception rate, These results fail to s upport the thesis that tyrosine alters LH release in cattle. Supplemen tal tyrosine increased expression of estrus in suckled cows after PGF( 2 alpha) and tended to reduce intervals to first postpartum ovulation.