– Francis L. Fluharty, Ph.D., Department of Animal Sciences, The Ohio State University (excerpted from Optimizing Performance of Cattle by Increasing the Digestion of Forages with Protein Supplementation)
Ruminant animals in grazing situations need to maximize forage digestion in order to increase performance parameters such as average daily gain or milk production. Factors that limit the animal’s ability to reach production goals may include the forage’s energy and protein content, or availability. These factors are impacted by the forage species, maturity, lignin concentration, and ruminal ammonia requirements of cellulose digesting bacterial species.
However, unlike grain-based diets, there is a time period, referred to as the lag phase, required for cellulose digesting bacteria to attach to forage particles. This creates a situation where protein availability in the rumen must match the timing of energy availability in order to achieve optimum microbial digestion.
Several factors have been shown to alter bacterial degradation of protein, and, in turn, the amount of microbial protein reaching the ruminant small intestine. In production situations where energy is limiting, either because of relatively low-quality forage such as native tall grass prairie, mature fescue, or corn stover, etc., or in production situations where there is reduced dry matter intake, microbial protein reaching the small intestine may be insufficient to maximize animal growth, and ruminally undegradable intake proteins (UIP, or bypass protein) may be warranted.
In diets based on crop residues, and low-quality or mature forages, sufficient evidence is available to justify feeding combinations of ruminally available (DIP) protein sources such as urea or soybean meal (SBM) in combination with UIP sources that mostly bypass rumen degradation but are available for enzymatic degradation in the small intestine if not over-heated during drying. Common sources of UIP include corn gluten meal, distillers grains, feather meal, fish meal, or blood meal.
One way to make more of the cellulose and hemicellulose, the primary carbohydrates in forage, available would be to grind the forage and thereby increase the amount of carbohydrates available for immediate attachment by bacteria. However, in many production situations, it is not possible or feasible to grind forage. In these situations, it is simply more economical and easier to provide N for the ruminal bacteria and use bypass protein sources, in combination, to maximize performance.
Protein supplementation costs can be reduced if a portion of the DIP comes from non-protein nitrogen (NPN) sources such as urea or biuret. In fact, cellulolytic bacteria prefer ammonia (NH3) as their N source, so substituting NPN for a portion of the degradable true protein in supplements for range cows should be a viable option.
In summary, supplying combinations of DIP and UIP could best meet the animal’s amino acid requirement through maximizing microbial growth and cellulose digestion, as well as providing amino acids from both microbial and feed origin to the small intestine. This, in turn, can maximize forage digestion in order to increase performance.