Luteinizing hormone release and reproductive traits in anestrous, estrus-cycling, and ovariectomized cattle after tyrosine supplementation.

The role of rumen-protected L-tyrosine as a nutritional signal altering LH release and other reproductive traits in cattle was studied. In Exp. 1, 28 suckled crossbred cows were assigned randomly to five treatments (0 or 40 g of tyrosine daily in feed for 3 d followed by a single i.v. injection of 200 micrograms 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 regardless 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 assigned randomly to six treatments (0, 20, or 40 g of tyrosine daily for 3 d before GnRH or saline was injected i.m. on d 23 +/- 1 postpartum). Injection 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 ovariectomized cows (six cows per treatment) had no effect on any characteristic 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 tyrosine daily for 3 d before a PGF2 alpha-synchronized estrus). Tyrosine increased (P = .05) the percentage of cows in estrus after PGF2 alpha but reduced (P = .05) AI conception rate. These results fail to support the thesis that tyrosine alters LH release in cattle. Supplemental tyrosine increased expression of estrus in suckled cows after PGF2 alpha and tended to reduce intervals to first postpartum ovulation.