Several models and markers are available for digesta kinetic studies. In this study, kinetic estimates derived from chromium-mordanted hay or pellets were compared to estimates derived from rare earth markers (Yb, Dy, or Er) applied individually to samples. Twelve yearling rams (52 kg) were given ad libitum access to either hay or a commercial pelleted diet in a crossover experiment. Digesta kinetic estimates were obtained both by nonlinear analysis with two age-independent rates (G1G1) or with gamma time dependency in the fast compartment (G2G1 to G4G1) and by linear regression of natural log transformed fecal marker concentrations (LN method of estimate). Model did not influence total tract (P> .21) or nominal mean retention times (P> .87). Partitioning of total retention time was similar (P> .21) for the LN, G3G1, and G4G1 models, but the GlGl and G2G1 models did not adequately fit these data. Nonlinear models overestimated, and LN underestimated, fecal DM output by 9% (SEM = 4.7) for the hay diet. All the nonlinear models provided fecal DM output estimates that were within 5% of actual fecal DM output, but the LN model underestimated it by 18% (SEM = 3.3) for the pelleted diet. Ruminal outflow rate was slower (P < .01) and both ruminal and total mean retention time estimates were longer (P < .01) for Cr than for the rare earths. Despite the marker chosen, relative diet effects were similar. The three rare earth markers gave identical results for digesta kinetic estimates, indicating that they are useful for simultaneous study of more than one ingredient or particle, but direct comparison of rare earth and Cr-mordant passage rates is not advisable.
Two completely random digestion trials were conducted, each with 12 beef steers (325 kg initial weight), to measure changes in digestibilities of fat and of forage components when fat was added to diets containing 62 to 76% wheat straw. Trial 1 diets contained either no added fat or 6.3% added fat from whole cottonseed (30% of the diet), cottonseed oil or animal fat; diets were formulated to contain equal levels of cottonseed hulls and cottonseed meal. Trial 2 diets contained 0, 2, 4 or 8% added animal fat. In all forms and at all levels, added fat increased apparent digestibility of dietary lipid (P less than .05). However, estimated true digestibility of lipid decreased (from 94 to 71%) as added fat was increased from 0 to 8% (P less than .05). Up to 6.3% added fat increased digestible energy (DE) content of the diet. Fat additions of 2 and 4% increased daily DE intake (P less than .05) and did not depress digestibility of diet components (P greater than .05). Fat additions of 6.3% or greater, either as free fats or as whole cottonseed, reduced (P less than .05) mean acid detergent fiber digestibility from 40 to 28%. In addition to depressing fiber digestibility, 8% added fat reduced (P less than .05) digestibilities of dry matter (from 54 to 47%), organic matter (60 to 52%) and gross energy (60 to 51%). Oil fed as whole cottonseed caused digestibility depressions similar to free fat addition at the same level.(ABSTRACT TRUNCATED AT 250 WORDS)
Soybean hulls have been successfully fed to ruminant animals as an economical substitute for hay. This feedstuff is a source of highly digestible fiber that does not contain starch. The purpose of this trial was to evaluate soybean hulls as a replacement fiber in horse diets. Four cecally cannulated Quarter Horse geldings, aged 6 to 10 yr and averaging 502 kg, were used in a 4 x 4 Latin square design with 21-d periods. Diets consisted of alfalfa/bromegrass hay (14.4% CP, 58.1% NDF, 39.1% ADF; DM basis) with the replacement of either 0, 25, 50, or 75% (as-fed basis) unpelleted soybean hulls (13.1% CP, 60.6% NDF, 43.7% ADF; DM basis). Diets were offered at 1.8% of BW (DM) daily and body weights were measured weekly. Cecal samples (90 min after feeding) and total fecal collections (3 d) were taken at the end of each treatment period. Fecal collection bags were emptied every 6 h and 10% of the total amount was frozen for later analysis. Total cecal VFA production increased linearly (P = 0.02) from 70 mM to 109 mM as proportions of soybean hulls in diets increased. Proportions of propionate increased linearly (P < 0.01) with means of 15.7, 18.0, 16.6, and 21.9 mol/100 mol total VFA for the 0, 25, 50, and 75% soybean hulls diets respectively. Proportions of butyrate decreased linearly (P < 0.01) from 5.3 to 3.9 mol/100 mol total VFA. The acetate:propionate ratio decreased linearly (P = 0.02) and cubically (P = 0.03) with means of 4.9, 4.2, 4.9, and 3.3. Apparent digestibility of DM (P = 0.95), OM (P = 0.70), NDF (P = 0.34), ADF (P = 0.31), cellulose (P = 0.93), and hemicellulose (P = 0.25) did not differ among treatments. Apparent digestibility of N decreased linearly (P < 0.01) as concentrations of soybean hulls increased in the diet, and this response was associated with increased cecal fermentation and microbial biomass production. Cecal pH decreased linearly (P = 0.01) from 7.00 to 6.45 as the level of soybean hulls increased, but there was no change (P = 0.68 for the linear effect) for cecal ammonia (mean concentration of 3.85 mM). Soybean hulls stimulate cecal fermentation and are a suitable replacement for hay in equine diets.
