THERIOGENOLOGY SEASONAL VARIATIONS IN CHARACTERISTICS OF ESTROUS CYCLES IN PUBERTAL BRAHMAN HEIFERS R. C. Stahringer,’ D. A. Neuendorff and R. D. Randel Texas Agricultural Experiment Station Texas A&M University Agricultural Research and Extension Center Overton, TX 75684 Received for publication: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO January 31, 1990 Accepted: May 28, 1990 ABSTRACT A group of pubertal Brahman heifers (n = 16) was monitored from October through March to investigate the seasonal changes in estrous cyclicity. The heifers had a mean age of 16.7 + 0.3 mo at the initiation of the experiment. They were kept on pasture with vasectomized marker bulls. Supplemental feed to meet NRC requirements was provided. Estrus occurrence was checked once a day and blood samples were taken weekly by tail venipuncture from heifers that had been in estrus 7 to 14 d earlier. Samples were processed to yield serum and were assayed for serum progesterone by radioimmunoassay. A high proportion of heifers (88%) had abnormalities such as estrus without the formation of a functional corpus luteum (CL) or anestrus, with a distribution of the two abnormalities as follows: October 0 and 0, November 50 and 25, December 0 and 50, January 0 and 50, February 18 and 31 and March 0 and 7%. respectively. Mean serum progesterone concentrations during the luteal phase differed by month: 4.26, 1.50, 3.25, 2.27, 2.65 and 3.70 ng/ml for October, November, December, January, February and March, respectively (P<O.OOl). Heifers that went into anestrus had lower mean serum progesterone concentrations than heifers that had regular estrous cycles throughout the study period (1.35 vs 2.22 ng/ml; P<O.O005). The months with the shortest daylengths (December and January) had the highest incidence of anestrus. Transitional periods (November and February) seemed to occur before and after the months with the highest occurrence of anestrus. During this transitional period a high incidence of estrus without the formation of a functional CL was detected. Serum progesterone concentrations were lower in all heifers during the months with high occurrence of abnormal estrous cycles. Key words: seasonal variation, estrous cycle, Brahman heifers Journal Paper TA 25 18 1, Texas Agricultural Experimental Station. This study was a contribution to the Western Regional Research Project w-112, Reproductive Performance in Domestic Ruminants. 1 Support of the Argentine National Institute of Agricultural Research (INTA, Argentina) is appreciated. AUGUST 1990 VOL. 34 NO. 2 407