Effects of Monensin on Growth and Reproductive Performance of Beef Heifers2

Abstract

Thirty-nine Hereford (H), 32 Brahman (B) and 113 BxH (F-l) heifers were randomly allotted to one of 3 Experiments (E): I. Continuous grazing (wheat-oats-ryegrass); II. Part-time grazing (wheat-oats-ryegrass 4 hr daily, 3 days a week for 59 days followed by part-time grazing 4 hr daily); III. Drylot confinement. E groups were divided into subgroups which respectively received 200 mg or 0 mg monensin in .91 kg of range cubes per head daily. At scheduled times the heifers were measured for weight (W), height (Ht), length (L), heart girth (C), examined for ovarian activity by rectal palpation and rumen samples were collected. The average on test W for F-l, H and B were 227, 180 and 200 kg, respectively. E I control heifers readily consumed the range cubes but the monensin heifers did not. After 59 days, E I heifers were taken off test. There was a 30.9 and 25.1% increase in ruminal propionic acid (C₃) of E II monensin over control (P2) and butyric acid (C₄) were decreased by 11.2 and 23.5%, respectively, in E II monensin compared to control (P<.05) and 6.8 and 22.3%, respectively, in E III monensin compared to control (P<.005). Monensin did not affect total volatile fatty acid (VFA) production. Period differences were observed for VFA (P<.05), C₂ (P4 (Pvs 2.26 kg/head; Pvs 14.49 kg feed/kg gain). No heifer in E II reached puberty during the 158 day test. More E III monensin heifers reached puberty than controls (P<.02) during the 171-day test; 92% (22/24) and 58% (15/26), respectively. Conception rates for E III monensin and controls were 55% (12/22) and 47% (7/15), respectively (NS). All pregnant heifers produced healthy, normal calves with no incidence of dystocia. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.