THE USE OF FIBER CONCENTRATIONS FOR RATION FORMULATION

Dairy cattle require fibrous feedstuffs in the diet. However, defining the fiber requirements requires knowledge of many interacting compone nts within the diet as well as how the feed is allocated and processed , To meet the demands of high producing dairy cows for energy, diets c onsist of large quantities of concentrates and high quality forages co ntaining relatively low amounts of fiber. However, to maintain normal rumen function and milk fat percentage, a large portion of the fiber n eeds to come from forage. Current National Research Council recommenda tions are a minimum of 25 to 28% dietary neutral detergent fiber, 75% of which is supplied from forage. Chemical measures of fiber alone are not adequate for ration balancing; fiber varies in its effectiveness at stimulating chewing. This variation is greater when feeds contain h igh amounts of by-products in place of forages. The effectiveness in s timulation of chewing of fiber is variable as the particle size and re tention times of indigestible and digestible fiber vary. Currently, li ttle information exists about the fiber requirements of the cow from i mmediately postcalving until peak lactation. Early lactation cows are clearly more limited by physical fill than are cows in mid and late la ctation. Rations based on nonforage fiber sources (>45% neutral deterg ent fiber on a dry matter basis) are being evaluated for early lactati on cows. Information is needed to determine the variation in the amoun t of fermentation acids produced, chewing activity, and amount of sali va secreted as these all contribute to the physical effectiveness of f iber.