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The Influence of Dietary Energy Intake on Progesterone Concentrations in Beef Heifers¹

University of Nebraska, Lincoln 68583

Abstract

An experiment was conducted with 50 cyclic Angus X Hereford heifers that were 16 mo of age at the time the experiment was initiated. The heifers were randomly assigned by weight stratification to one of three treatment groups. Heifers were fed low (N=17), maintenance (N=17) or high (N=16) energy diets for 150 d. All heifers were synchronized using the progestogen implant, Norgestomet, before initiation of dietary treatments. The feeding of dietary treatments was started at the time of progestogen implant removal. Blood samples were collected every other day for progesterone determination throughout the experimental period. Serum progesterone concentrations, when analyzed as area under progesterone curves or as peak progesterone concentration, increased linearly (P<.01) as energy level increased when analyzed for the six estrous cycles that occurred during the treatment period. Corpora lutea were collected through a high lumbar incision during the fourth and seventh estrous cycles following initiation of the experiment to determine progesterone production in vitro. The experiment was designed to collect corpora lutea from six heifers in each treatment group on the 10th d of the fourth estrous cycle and in addition from six different heifers in each treatment group on the 10th d of the seventh estrous cycle. Due to cessation of estrous cyclic activity the number of corpora lutea obtained from heifers fed low and maintenance diets during the seventh estrous cycle was limited. Progesterone production by corpora lutea in vitro when the cells were incubated with human chorionic gonadotropin was less in heifers fed low than the maintenance diet at the fourth estrous cycle. The limited number of corpora lutea available during the seventh estrous cycle did not allow for a similar comparison of progesterone production. In addition, comparisons of in vitro progesterone production during the fourth and seventh estrous cycle could not be made due to the limited number of observations from the seventh cycle. The length of estrous cycles was consistent up until the time of cessation of cyclic activity. The average length of the estrous cycle before cessation of cyclic activity was only 1 d shorter when compared with previous cycles. Progesterone concentrations were also similar during the cycle immediately preceding cessation of cyclic activity when compared with previous cycles in heifers.

¹ Published as Paper No. 6740, Journal Ser. Nebraska Agr. Exp. Sta.

² Anim. Sci. Dept. Acknowledgement is made to Jane A. Ossenkop and Diana J. Smith for assistance in preparation of the manuscript, to John C. Fudge for assistance in data collection, to Dennis R. Brink for assistance in preparation of diets, to Kenneth A. Pearson for technical assistance in laboratory procedures and to Stephen R. Lowry and Merlyn K. Nielsen for assistance in the statistical analysis of data.