Effect of nutrient composition of feeds on digestibility of organic matter by cattle: a review

Estimates of nutrient availability, calculated as TDN for 106 different feedstuffs generated from various published equations, were compared with TDN for similar feeds listed in the 1961 text by F. B. Morrison titled Feeds and Feeding. Incomplete analysis of carbohydrate fractions limited accuracy of evaluations. Although published equations may satisfactorily rank feeds in energy value, the absolute values, correlations, and SE of the estimates revealed that most equations were inaccurate. Across all feeds and forages, TDN was related most closely to crude fiber (R(2) = 0.68) within data sets from Morrison's text and from the NRC publications concerning Nutrient Requirements for Dairy and for Beef Cattle in 1989 and 2000, respectively. Within the latter data set, of the total variation, ADF accounted for 65% of the variation in TDN across all feeds, 62% of the variation in TDN for concentrate feeds, but only 41% of the variation in TDN of forages. Within the 2001 publication for dairy cattle from the NRC, ADF content was related most closely to TDN for all feeds, but nonfiber carbohydrate was most closely related to TDN of forages (R(2) = 0.81 and 0.69, respectively). To separate true from apparent digestibility of nutrients, fecal excretions of components (i.e., CP, fat, crude fiber, nitrogen free extract) were regressed against concentrations of these nutrients in feedstuffs and summed to estimate fecal loss. Metabolic fecal loss of OM (MFOM), the difference between true and apparent OM digestibility, was correlated closely with crude fiber content of feedstuffs (R(2) = 0.86) and increased from 7 to 25 g/100 g of diet as dietary crude fiber concentration increased. This may explain why most TDN equations are based on crude fiber or ADF. Whether dietary NDF similarly increases metabolic OM excretion is not certain, but when humans were fed NDF-enriched diets, fecal excretion of nonfiber carbohydrate increased markedly. The impact of crude fiber on nutrient availability of feeds appears to be related more closely to its adverse effect on apparent digestibility of other nutrients than to the amount of energy that fiber itself contributes. Refinements to laboratory methods for measuring fiber digestibility that match apparent in vivo digestibility coefficients for fiber by ruminants is needed, and the origin, composition, and cost of replacing MFOM need additional research.