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Effect of Melatonin on Induction of Estrous Cycles in Anestrous Ewes^1,2,

Purdue University^,2 West Lafayette, IN 47907

Abstract

To examine the influence of melatonin on seasonality of reproduction, 97 multiparous Suffolk and Suffolk-cross ewes were randomly assigned to one of four treatment (TRT) groups in a 2 x 2 factorial arrangement. Treatments were as follows: 1) control (C); 2) melatonin (M); 3) progestogen (P) implant of norgestomet plus pregnant mare serum gonadotropin (PMSG) injection (P+PMSG) and 4) M plus TRT 3 (M+P+PMSG). From April 3 to June 24 an oral dose of 2 mg M was administered once daily at 1600 to each ewe in TRT groups M and M+P+PMSG. On April 30, ewes in groups P+PMSG and M+P+PMSG were implanted in the ear with 2 mg norgestomet for 13 d. Immediately following implant removal, each ewe was injected with 500 IU PMSG. Blood samples were collected from all ewes twice weekly from March 22 through June 24. Number of estrous cycles per ewe during the TRT period of 82 d (April 3 to June 24) was higher (P<.05) for M+P+PMSG (2.1 ± .2) than for C (.3 ± .2), M (1.5 ± .2) and P+PMSG (1.1 ± .2). Control ewes had fewer (P<.05) estrous cycles per ewe than either M or P+PMSG. Following the induced estrus, 40% of ewes in the M TRT had one estrous cycle; 32% had two or more cycles. For ewes treated with M+P+PMSG, 24% had one cycle, and 32% had two estrous cycles. In the C group, one (4%) ewe had an estrous cycle and none of the ewes treated with P+PMSG had an estrous cycle after the induced estrus. In summary, a daily oral dose of M at 1600 induced estrous cycles and maintained cyclicity in ewes treated with M and M+P+PMSG.

¹ Journal paper no. 11159, Purdue Univ. Agric. Exp. Sta.

² Dept. of Anim. Sci. The authors express appreciation of CEVA Laboratories Inc., Overland Park, KS for SYNCRO-MATE-B, Dr. G. D. Niswender, Colorado State Univ., Ft. Collins, for supplying antisera to progesterone and melatonin. The authors also wish to thank Doris Blair, Henry Kohl and Gerald Kelly for their technical assistance.

³ Present address: Dept. of Anim. Sci., Univ. of Idaho, Moscow, 83843.

⁴ Address reprint requests to this author.

⁵ Present address: Dept. of Anim. Sci., Univ. of Wyoming, Laramie, 82071.