Senwes Scenario June / July 2019 - Page 12

Theme: im An al feed AGRICULTURAL ROUGHAGE IN THE FEEDLOT What is the value of roughage and how is it utilized in feedlot diets? Ruminants such as cattle and sheep, evolved as forage consumers. To maintain a healthy digestive system a minimum level of roughage needs to be included in a feedlot feed. Different sources of roughage have various nutritional value and physical characteristics which must be taken into account in formulating feedlot rations.  By Dr Kobus Swart Animal Nutrition Specialist P lant cell walls, which we measure as fiber, cannot be digested by animals, but must be fermented by micro-organisms. Ruminants swallow large particles of forage and selectively retain them in the rumen to allow adequate time for fermentation. They ruminate to enhance digestion and allow passage through the digestive tract. During chewing, they produce salivary buffers that help to maintain the pH in the rumen. In animal production systems there are economic, operational and nutritional reasons for limiting the forage or roughage 10 SENWES SCENARIO | WINTER 2019 content in feedlot rations. Economically, grains and concentrate feeds often are less expensive sources of nutrients than forages. Operationally, roughages create difficulties and additional expense in processing (chopping or grinding), in uniformly mixing and delivering rations, and in obtaining consistent intakes without sorting. Nutritionally, roughages tend to be more variable in nutrient content, which also makes it difficult to use them in rations with consistent results. Thus, there are many practical reasons for minimizing for- age in rations of feedlot cattle. MEASURING FIBER IN FEEDS FOR RUMINANTS In animal feeding, fiber is a term used to define a nutritional, not a chemical or plant anatomical concept. Nutritionally, fiber has both physical and chemical attributes that are related to the mechani­ cal processes of digestion (chewing and passage) and to enzymatic degradation associated with fermentation. The fiber with the smallest magnitude is crude fiber (CF), is not an accurate prediction of fiber and is not used any- more. As acid detergent fiber (ADF) does not contain hemicellulose, it is not an accurate estimate of fiber in feeds. ADF recovers most, if not all, of the polymeric lignin and cellulose in feeds, with some contamination from pectin, hemicellulose, tannin-protein complexes, and ash. Only neutral detergent fiber (NDF) isolates all of the insoluble fiber components in plants (hemicellulose, cellulose, and lignin) with