Influence of monensin fed with a starch-based energy supplement on forage digestibility and intake by range cows during drought

Management strategies that maintain livestock performance while reducing intake in times of low forage availability, such as drought, are valuable. The hypotheses of this experiment were that a grain-based supplement would decrease forage intake and digestibility, whereas a grain-based supplement plus monensin would decrease forage intake and increase forage digestibility. Crossbred cows (n = 36) grazing native range pasture during the late summer were used in a randomized complete block design for 2 yr (average forage quality for yr 1 = 8.7% CP, 77% NDF, 47% ADF; for yr 2 = 8.0% CP, 76% NDF, 45% ADF). Treatments were control (no supplement; NS), cracked corn (S; 0.91 kg/cow per day, as-fed basis), and S with monensin added at 385 mg/kg of corn (SM). Diet samples were collected from 6 ruminally cannulated cows to calculate digestibility. Cows were dosed with Cr2O3 boluses to determine fecal output, OM intake, and digestible OM intake during 2 sampling periods each year. Forage intake and digestible OM intake as percentages of BW were greater (P < 0.01) for SM than other treatments in yr 1 (1.61, 1.37, and 1.24% for forage intake as a percent of BW and 0.93, 0.69, and 0.59% for digestible OM intake as a percent of BW for the SM, NS, and S treatments, respectively), with no differences (P > 0.05) in yr 2. Starch-based supplement alone reduced OM and NDF digestibility in period 1 of both years compared with NS or SM treatments (P < 0.001). In period 2 of both years, OM digestibility was greatest for the SM treatment (P < 0.001). In period 2 of yr 1, NDF digestibility was greatest in SM (P < 0.001), but similar between SM and NS in yr 2. Under our experimental conditions, monensin fed with a starch-based energy supplement did not decrease forage intake during drought; however, moderate forage quality and type and amount of supplement provided may have affected responses.