Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/1999
Publication Date: N/A
Interpretive Summary: Mature beef cows, greater than 4 years old, have a lower requirement for energy than young growing heifers. Older cows also maintain body weight and condition better than weaned calves when grazing low quality forage. The objective of this study was to determine if mature cows differ from young heifers in their ruminal metabolism of low and high quality forage. Mature cows consumed 27% and 50% more alfalfa and brome hay respectively, per unit metabolic weight, than 10 month old heifers. The ruminal liquid dilution rate was faster in cows than heifers. This explains the faster rate of fiber degradation in cows than heifers, 3% and .5% per hour respectively, when alfalfa and brome hay were fed. This information provides an explanation why mature beef cows are able to use low quality forages better than young heifers and why young heifers will require diet supplementations before mature cows.
Technical Abstract: 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 determine the effect of cattle age on apparent forage utilization. Cattle were fitted with ruminal and duodenal cannulae and were individually fed once daily (ad lib, 1000 h). The split plot design consisted of age (whole plot) and two sampling periods feeding alfalfa or brome hay (sub-plot). Each period consisted of 28 d; 1 to 13-d adaptation, 13 to 20 d feed intake, and 20 to 28 d sampling. Nylon bags containing NDF substrate from alfalfa or brome hay were incubated ruminally for 0, 3, 6, 12, 24, 48, 96, and 192 h to determine rate and extent of fiber degradation. Ruminal liquid dilution rate and fermentation profile were conducted on d 27. Ruminal fill was determined by total evacuation at 0800 on d 28. Cows consumed more feed (BW .75; P < .01) had greater ruminal OM fill (P = .04), but had similar fluid fill (P = .88) compared to heifers. Ruminal liquid dilution rate was greater in cows than heifers (P < .01). The rate of ruminal NDF degradation was 3 and .5% per h greater in cows than heifers when alfalfa and brome hay were fed, respectively (age x hay P < .01). Ruminal NDF digestibility as a percent of intake was greater in cows than heifers (P < .01). Number of ruminal cellulolytic bacteria were not affected by treatment (P > .21). These data indicate that mature cows have a smaller ruminal fluid fill which turns over more rapidly and results in a faster rate of ruminal fiber degradation than young heifers.