Measuring the effectiveness of fiber by animal response trials

Chemical analysis of neutral detergent fiber (NDF) provides a useful description of forages and other feeds. However, use of NDF as the sole measure of the fiber contribution of a feed has proved problematic for two classes of feeds: forages processed into differing physical forms and high fiber by-products. By-products and physically fine forages contribute to the fiber value of dairy rations, but contribute less than long forages do. Therefore, some discount factors must be assigned to these feeds if fiber requirements are to be used in balancing the carbohydrate portion of dairy diets. The effectiveness factors applied to NDF from these feeds provide an improved measure of fiber value. The assumptions and trial designs used to measure the effectiveness of NDF based on any single animal response variable are discussed, and improved approaches are suggested. The use of different response variables to measure physical and overall fiber effectiveness is discussed. Measured effectiveness of a high fiber feed differs when estimated by chewing, by the ratio of acetate to propionate, or by milk fat concentration. In all cases, inclusion of negative control treatments is necessary to measure the effect of removing fiber without introducing a substitute fiber source.