Effect of Monensin on Feed Efficiency for Maintaining Gestating Mature Cows Wintered on Meadow Hay

Abstract

In each of two winters, 48 gravid-mature Hereford cows were stratified by weight and age to two treatments with three replications. Treatments consisted of the control group which received meadow hay free choice plus .45 kg of barley per day and the treated group which received 200 mg of monensin in addition to the control diet. Cows were individually fed the supplemental feed. Hay was weighed in daily with orts weighed back weekly. In trial 1 (1974), monensin fed cows out-gained the controls .43 to .23 kg (P<.01) on less hay intake 11.0 to 11.4 kg (P<.06). In trial 2 (1975), daily gains were .47 and .28 kg (K.05) and hay intake 12.7 and 13.1 kg (P>.05), respectively, for monensin supplemented cows and the controls. Feed efficiency was substantially improved by monensin. Rumen samples were taken from cows on each treatment 4 hr after supplemental feeding for volatile fatty acid (VFA) determination. Total VFA concentration (mM/liter) and relative proportions of acetate, propionate and butyrate (Molar %) for controls and monoensin fed cows were 68.2, 72.2; and 76.2, 72.4; 17.2 22.6; and 6.5, 5.0, respectively. Propionic acid percentage was increased (P<.05) with monensin feeding and acetic and butyric decreased (P<05). Vasectomized bulls with chin ball markers were used to determine frist post-partum estrus. Interval from calving to first estrus was shortened by 12 days (P<.05) with monensin in trial 1, but there were no differences between treatments in trial 2.