Journal
JOURNAL OF ANIMAL SCIENCE
Volume 95, Issue 6, Pages 2736-2746Publisher
OXFORD UNIV PRESS INC
DOI: 10.2527/jas.2016.1011
Keywords
cattle; digestion; dried distillers' grains; forage; ionophore; methane
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Effects of monensin inclusion and cattle subspecies on utilization of bermudagrass hay (13.7% CP, 77.3% NDF, and 38.8% ADF) were evaluated using ruminally cannulated steers (5 Bos taurus indicus [BI] and 5 Bos taurus taurus [BT]; 398 kg BW). Subspecies were concurrently subjected to a 2-period, 2-treatment crossover design. Treatments were 0 (CON) or 200 mg.steer(-1).d(-1) monensin (MON) in 0.91 kg dried distillers' grains with solubles. Periods were 70 d in length: 20 d of adaptation, 22 d of sample collection, and 28 d for withdrawal of treatment. Steers were group housed during adaptation and moved to individual covered pens for sampling. Hay, ort, and fecal grab samples were collected d 21 through 25 for determination of intake and digestion. Ruminal fluid was collected with a suction strainer 0, 2, 4, 8, and 12 h after feeding on d 42 for pH, VFA, and ruminal NH3-N (RAN) analysis. Additionally, at h 2, ruminal fluid and contents were collected for determination of rate of NH3 production and CH4 production rate. No subspecies x monensin interactions were observed (P = 0.12). Monensin had no effect (P = 0.16) on intake or digestibility parameters. No subspecies effect (P = 0.11) was observed for forage OM intake, total OM intake, or OM digestion. Total digestible OM intake tended to be greater (P = 0.06) for BT steers than for BI steers (14.0 vs. 12.2 g/kg BW). There was an effect of hour after feeding (P = 0.01) on pH, total VFA, acetate: propionate ratio, and molar percent acetate and propionate. Total VFA concentration was greater (P = 0.01) in CON steers than in MON steers (66.5 vs. 62.0 mM). Monensin decreased molar percent acetate (P = 0.02) from 72.5 to 71.2% and increased molar percent propionate (P < 0.01) from 16.9 to 18.7%, resulting in a reduced (P < 0.01) acetate: propionate ratio (from 4.34 to 3.85). Although not significantly (P = 0.19), monensin numerically reduced the CH4 production rate by 15.8%. Greater (P = 0.07) CH4 production rate tended to be observed in BI steers than in BT steers (21.4 vs. 16.6 mu mol CH4.mL(-1).h-1, respectively). Monensin had no effect (P = 0.32) on pH, RAN, or rate of NH3 production. A subspecies x hour after feeding interaction was observed for RAN, with BT having greater RAN at h 0 and 4, whereas BI had greater RAN at h 2, 8, and 12. Overall, monensin decreased the acetate: propionate ratio and total VFA concentration but had no effect on forage utilization. Bos taurus indicus steers consumed less digestible OM and had a greater CH4 production rate compared with BT steers, suggesting BT were better able to utilize the available forage resource than BI.
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