EFFECT OF STRUCTURAL AND CHEMICAL FACTORS OF FORAGES ON POTENTIALLY DIGESTIBLE FIBER, INTAKE, AND TRUE DIGESTIBILITY BY RUMINANTS

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Abstract
In view of the large variation found in plant cell wall digestibilities with ruminants, an attempt was made to group 124 feeds into different lignification classes (clusters) on the basis of chemical characteristics. Each feed cluster was described using a structural coefficient (.vphi.) that related the potentially digestible fiber (PDF, %) to the ratio between lignin and cell wall volume. The optimum number of clusters was determined iteratively by performing a regression of the apparent digestiblity of dry matter at maintenance level (DDM1, %) aganist the PDF and cell soluble (SOL, %) contents of feeds. The .vphi. coefficients varied from 0.05 (grains, N=13) to 1.85 (corn silage, N=3) and increased with the maturity of the grasses from 0.88 (legumes, vegetative cool season grasses, N=26) to 1.33 (mature, cool season grasses, N=19). Predicted PDF were closely correlated (r > 0.9, P < 0.01) to in vitro cell wall disappearances (IVCWD). Apparently digestible cell wall in four grasses and four legumes increased linearly with 96-h IVCWD and standrad error (SE) was similar to the SE of predicted apparent digestible SOL from SOL concentrations. Assuming that similarity between SE could be also observed in larger samples. PDF and SOL were used in summative equations to predict apparent dry matter digestibility. DDM1 discounted for intake (DDM1 - 4, %) was regressed against SOL and PDF concentrations of 87 feeds: DDM1 - 4 = ds SOL + df PDF - 14.2 (R2 = 0.99, P < 0.01) with ds and df, the true digestibilities of SOL and PDF. Estimates of ds and df were 0.98 and 0.95 for a zero-production (maintenance) levelof intake, and 0.91 and 0.79 for an intake level four times maintenance. Since the true digestiblity of the PDF component was only 4% - 13% lower than that of the cell soluble component, the concentration of PDF in cell was the major determinant in the variation in apparent digestibility of forages.