2754 1 Contribution no. 97-218-J, Kansas Agric. Exp. Sta., Manhattan. Appreciation is expressed to Betty Hensley for her assistance with data collection and laboratory analyses. 2 To whom correspondence should be addressed (phone: 913-532-1243; fax: 913-532-7059; E-mail: jstevens@oz.oz- net.ksu.edu). 3 Present address: Eastern Oregon Agric. Res. Center, Union Station, Union, OR 97883. Received January 5, 1997. Accepted June 2, 1997. Luteinizing Hormone Release and Reproductive Traits in Anestrous, Estrus-Cycling, and Ovariectomized Cattle After Tyrosine Supplementation 1 J. S. Stevenson* ,2 , J. R. Jaeger †,3 , I. Rettmer ‡ , M. W. Smith*, and L. R. Corah* *Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201; † KSU Agricultural Research Center-Hays 67601; and ‡ Institut fuer Anatomie, 18055 Rostock, Germany ABSTRACT: The role of rumen-protected L-tyro- sine 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 mg 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 sup- plementation. 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-17b. 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 PGF 2a -synchronized estrus). Tyrosine increased ( P = .05) the percentage of cows in estrus after PGF 2a 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 PGF 2a and tended to reduce intervals to first postpartum ovulation. Key Words: Tyrosine, LH, Naloxone, GnRH, Progesterone, Cattle J. Anim. Sci. 1997. 75:2754–2761 Introduction Pulsatile LH secretion is a key regulator of ovarian follicular function in cattle (Lucy et al., 1992) and is governed by pulsatile hypothalamic GnRH secretion (Levine et al., 1991). Studies indicated that pulsatile LH release is linked to nutritional status of animals (Steiner et al., 1983; Schillo, 1992). However, no single metabolite has been identified to influence LH secretion. The amino acid tyrosine, a blood-borne metabolite, may be involved in stimulating GnRH release because availability of tyrosine influences synthesis of norepinephrine (Wurtman et al., 1981; Gibson and Wurtman, 1986; Acworth et al., 1988), a neurotransmitter that stimulates GnRH release (Ramirez et al., 1984; Terasawa et al., 1988). Exogenous tyrosine decreased age at puberty in rats (Hammerl and Ru ¨ sse 1987a); increased litter size in rats (Hammerl and Mu ¨ller, 1988) and swine (Ham- merl and Ru ¨ sse, 1987b); increased LH pulse fre- quency in growth-restricted lambs (Hall et al., 1992); induced follicular growth, estrus, and ovulation in anovulatory dairy cows (Wetzel, 1985; Hammerl, 1986; Mu ¨ nsterer, 1987); and improved expression of estrus in sows (Hammerl and Ru ¨ sse, 1987b). The amino acid tyrosine may act as a nutritional signal influencing hypothalamic GnRH secretion. In the present study involving four experiments, we tested the hypothesis that tyrosine alters secretion of LH and(or) other reproductive traits in cows.