Effects of monensin and forage: concentrate ratio on feed intake, endocrine, and ovarian function in beef heifers

The objective of this study was to determine the effects of monensin (M) an d altered forage:concentrate ratio (F:C) on feed intake, hormonal profiles, and ovarian function in beef heifers. Four ruminally cannulated heifers we re maintained in metabolism crates and fed twice daily. Diets contained 0 o r 200 mg per day M in diets of 70:30 (high forage, HF) or 30:70 (high conce ntrate, HC) F:C. Diets were limit fed to achieve isocaloric intake of calcu lated ME. Monensin supplementation had no effect on feed intake, but heifer s fed HF had greater refusals compared with heifers fed HC (P < 0.005). Ser um insulin, progesterone. and estradiol concentrations did not differ betwe en diets or treatments (P > 0.10). Diet and/or treatment tended to alter th e size or number of follicles on either or both ovaries. Monensin resulted in more waves of follicular development per estrous cycle with HF diets, bu t fewer with HC diets (P < 0.07). The interovulatory interval, lifespan of the corpus luteum, and the size of the dominant follicle in the first two f ollicular waves were not different due to diet or treatment. However. the H C diet tended to produce a larger ovulatory follicle compared with the HF d iet (HF = 12.25, HC = 13.5, P < 0.08). We conclude that feeding monensin af fects estradiol secretion and a higher proportion of concentrates alters ov arian dynamics, which could affect fertility in beef heifers. (C) 2001 Else vier Science B.V. All rights reserved.