Senwes Scenario June / July 2019 - Page 13

AGRICULTURAL | LANDBOUKUNDIG AGRICULTURAL There are many practical reasons for minimizing forage in rations of feedlot cattle. some protein contamination. NDF is not an ideal nutritive entity because its digestibility varies with lignin concentration and other factors in the feed. Variation in dry matter digestibility (DMD) is related primarily to the concentration and digestibility of NDF in feeds. It is only NDF that measures the differences within and among feed types and has the potential for developing a system of general feeding recommendations across all feeds. EFFECTIVENESS OF FIBER The concept of effective fiber (EF) was developed by nutritionists to formulate rations that would maintain milk fat per- centage in dairy cows and is not appro- priate over the full range of feeds fed to ruminants. A roughage value unit (RVU) system was proposed for measuring the effectiveness of feeds in stimulating chew- ing activity that was based on a chemical measure of fiber (NDF) and a physical measure of particle size. A standard or reference value with an RVU of 100 was defined as a hypothetical feed containing 100% NDF with all particles large enough to stimulate chewing (a long grass hay containing 100% NDF). A direct rela- tionship between the NDF content plus physical length of fiber and stimulated chewing activity were found. To clarify these concepts definitions was proposed for both effective NDF (eNDF) to main- tain milk fat and physically effective NDF (peNDF). The peNDF of a feed is related to the physical properties of its fiber (pri- marily particle size) that stimulates chew- ing activity and establishes the biphasic stratification of ruminal contents (floating mat of large particles on a pool of liquid and small particles). However, peNDF is a feed attribute that is based on a fixed scale (0 to 1) and reference value (long grass hay). The peNDF of every feed can be calculated taking the NDF value and physical fiber length into account. DETERMINING MINIMUM peNDF REQUIREMENTS FOR RUMINANTS Feedlot production is a terminal process that rarely lasts beyond 180 days. Given the short-term nature of feedlot production and the need to maximize animal perfor- mance, the minimum fiber requirements for feedlot cattle may be substantially lower than that for dairy cows. Because fiber has lower productive energy density than concentrates and is poorly digested in high concentrate diets, lower fiber con- centration in feedlot rations may improve animal performance and reduce manure excretion. It appears that there are positive relationships between ADG and peNDF within trials when peNDF is less than 10% of ration DM and negative relationships within trials when peNDF is greater than 18%. The peNDF that maximizes ADG was determined as 15.3%. However, there is little difference in ADG when peNDF in the ration is between 12 and 18%. The optimum peNDF in the ration to minimize liver abscesses was about 22%, and the peNDF that maximized intake was about 25%. The relatively broad range in acceptable peNDF (12 to 18% of ration DM) suggests that recommendations can be modified to match multiple objectives and account for other factors that may influence minimum peNDF requirements for feedlot cattle. Another problem in ration consistency is related to particle size, mixing effec- tiveness and selection by animals. It is difficult, if not impossible, to uniformly mix and deliver rations containing concen- trates and coarsely chopped roughage, especially if the ingredients are dry. In addition, animals will selectively eat roughage and grain when the forage is chopped coarsely even if a uniform ration is delivered. Thus, there is a tendency to reduce the particle size of the roughage to improve handling, mixing and delivery. When this occurs it is important that the peNDF value of the roughage be adjusted to reflect its effectiveness. CONCLUSIONS AND RECOMMENDATIONS Both the chemical and physical charac- teristics of rations are important in deter- mining animal performance. Physically effective NDF attempts to take into account both the chemical and physical nature of fiber that influences the chewing activity and ruminal function of ruminants. Although chewing activity is important in providing salivary buffers for controlling ruminal pH, it is also an indicator of the physical environment of the rumen that helps to establish optimal ruminal fermen- tation and production. Om die produktiwiteit van produksiestelsels vir beeste te verbeter. Adviesdiens vir herkouervoeding Voerprosessering, voedingsbestuur en formulering van voere vir volhoubare produksieprestasie Formuleer en verskaf konsentrate om formulasies te komplimenteer Dr. Kobus Swart 083 262 0946 • SENWES SCENARIO | WINTER 2019 11