Low- and high-quality forage utilization by heifers and mature beef cows

Eight cows (7 to 9 yr old, 522 kg) and six heifers (10 mo old, 169 kg) were fed either alfalfa hay (18.7% CP) or mature brome hay (5.1% CP) to determi ne the effect of cattle age on apparent forage utilization. Cattle were fit ted with ruminal and duodenal cannulas and were individually fed once daily (ad libitum intake, 1000). The split-plot design consisted of age (whole-p lot) and two sampling periods feeding alfalfa or brome hay (subplot). Each period consisted of 28 d: d 1 to 13 for adaptation, d 13 to 20 for feed int ake determination, and d 20 to 28 for sampling. Nylon bags containing NDF s ubstrate from alfalfa or brome hay were incubated ruminally for 0, 3, 6, 12 , 24, 48, 96, and 192 h to determine the rate and extent of fiber degradati on. Ruminal liquid dilution rate and fermentation characteristics were cond ucted on d 27. Ruminal fill was determined by total evacuation at 0800 on d 28. Cows consumed more feed (BW.75; P < .01) and had greater ruminal OM fi ll (P = .04) but had similar fluid fill (P = .88) compared with heifers. Ru minal liquid dilution rate was greater in cows than in heifers (P < .01). T he rate of in situ NDF degradation was 3 and .5% per hour greater in cows t han in heifers when alfalfa and brome hay were fed, respectively (age x hay , P < .01). Ruminal NDF digestibility as a percentage of intake was greater in cows than in heifers (P < .01). Numbers of ruminal cellulolytic bacteri a were not affected by treatment (P > .21). These data indicate that mature cows have a smaller ruminal fluid fill that turns over more rapidly, and t his may be responsible for a faster rate of ruminal fiber degradation in co ws than in young heifers.