Monensin Effects on Beef Heifers Grazing Bahiagrass Pastures and Receiving Molasses Supplementation

The objective of this study was to evaluate the effects of monensin on forage characteristic and performance of beef heifers. The experiment was conducted in Ona, FL from August to November 2016. Treatments were the addition of monensin (200 mg/animal/d) in the supplement or control (no monensin) distributed in a complete randomized design with five replicates. Heifers received 2.3 kg/heifer/d of liquid sugarcane (Saccharum officinarum) molasses (11% CP, 76% TDN) supplement. The daily supplement level was multiplied by 7 d and divided by 3 feeding events/wk (Mondays, Wednesdays, and Fridays). Ten pastures (experimental units; 1.2 ha) of bahiagrass (Paspalum notatum) were used with a fixed and continuous stocking rate. Three Brangus heifers with initial BW of 389±36 kg were allocated per experimental unit. Herbage mass (HM) and nutritive value were estimated every 14 d and the heifers were weighed every 28 d. There was no effect of the treatments on HM (P = 0.11, SE = 200, mean = 3100 kg/ha), CP (P = 0.37, SE = 0.6, mean = 8.1%) and in vitro digestible organic matter (P = 0.72, SE = 1.0, mean = 50.9%). Molasses disappearance 34 h after feeding was greater for control than monensin (98 vs. 93%, P = 0.01, SE = 0.2); however, there was no difference in heifers ADG (P = 0.58, SE = 0.07, mean = 0 kg/d), final BW (P = 0.87, SE = 18, mean = 385 kg), and BCS (P = 0.51, SE = 0.27, mean = 6.2). Heifers receiving monensin tended to have less blood glucose (86 vs. 94 mg/dL, P = 0.07, SE = 2.8) and greater BUN concentrations (12.0 vs. 9.4 mg/dL, P = 0.06, SE = 1.3), but there was no difference in blood IGF – 1 (P = 0.76, SE = 4.2, mean = 72 ng/mL). The addition of monensin in molasses supplement may decrease the rate of consumption of the supplement but the magnitude of the decrease may not be sufficient to affect ADG of heifers grazing warm-season grasses with limited nutritive value.