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* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409 and
and
New Mexico State University, Clayton Livestock Research Center, Clayton 88415
2 Correspondence: Box 42141 (phone: 806-742-2453; fax: 806-742-2427; E-mail: mgalyean{at}ttacs.ttu.edu).
Intake by beef cattle fed high-concentrate, grain-based diets is likely controlled primarily by metabolic factors and not limited by bulk fill. Nonetheless, small increases (e.g., 5% of dry matter or less) in the concentration of bulky roughage and changing from less fibrous to more fibrous sources of roughage typically increase dry matter intake (DMI) by feedlot cattle. Reasons for increased DMI with changes in roughage level and source are not understood fully. Energy dilution effects caused by added dietary fiber might be responsible for altered DMI, but the quantity of dietary net energy for gain provided by roughage shows little relationship to changes in DMI with roughage source and level. Altered rate of ruminal acid production as a result of roughage source and level might affect DMI through various mechanisms, including increased chewing and/or rumination with increased saliva flow; altered ruminal and/or intestinal digesta kinetics; and altered site and extent of digestion. We hypothesized that much of the effect of roughage source and level on DMI by feedlot cattle could be accounted for by changes in dietary neutral detergent fiber (NDF). Data from 11 published trials involving roughage source and level effects on intake by feedlot cattle were compiled. The dataset included 48 treatment means, with roughage sources such as hays, straws, byproducts, and silages, and with roughage levels ranging from 0 to 30% of dry matter. Effects of dietary roughage level (percentage of dry matter), NDF (percentage of dietary NDF from roughage), or effective NDF (eNDF, percentage of dietary eNDF from roughage) and the random effects of trial on DMI (percentage of body weight) were evaluated using mixed-model regression procedures. Tabular values were used to obtain estimates of NDF and eNDF. Using trial-adjusted means, dietary roughage level accounted for 69.9% of the variation in DMI, whereas the percentage of dietary NDF and eNDF supplied by roughage accounted for 92.0 and 93.1%, respectively, of the variation in DMI. The relationship between dietary NDF (percentage supplied by roughage) and DMI (percentage of body weight) for trial-adjusted data was given by: DMI = 1.8562 – (0.02751 x NDF) (P < 0.01; root mean square error = 0.0447); intercepts differed (P < 0.02) among trials, but slopes did not (P > 0.18). Based on these results, the percentage of NDF supplied by roughage in diets can be used to predict effects of roughage source and level on DMI by feedlot cattle, and NDF supplied by roughage might be a useful method for exchanging roughage sources in finishing diets.
Key Words: Beef Cattle • Fiber • Intake • Roughage
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