Protein. If crude protein content of the consumed forage is less than 7 percent (dormant pasture and crop residue will often be 3 to 6 percent crude protein), the rumen bacteria are being “starved” for nitrogen and will not break down the forage as efficiently as possible. By supplementing with a highly degradable (or DIP) source of protein, such as soybean meal or cottonseed meal, the bacteria will do a more effective job of breaking down the consumed forage, and the animal will get more out of the forage. The secondary benefit of this improvement in digestion is that the animal is able to consume more forage, so the benefit of supplementing protein is that the cow gets more energy from her diet. For example, a 1000-pound cow grazing unsupplemented dormant Flint Hills pasture may consume about 15 pounds of dry matter forage and only digest about 50 percent of it for a net intake of 7.5 pounds of total digestible nutrients (TDN). By supplementing 2 pounds of soybean meal, we can increase consumption to 18 pounds and increase digestion to 55 percent for a net intake of (18 * 0.55 = 9.9 lb TDN) + (2 lb SBM * 0.90 = 1.8 lb TDN) for a total of 11.7 lb of TDN, or a 56 percent increase in energy intake.
Conversely, supplementing low-quality forage diets with grain can have a negative impact on total energy intake because digestion of grain produces acids, which causes a reduction in forage digestion and intake. This could result in less total energy being consumed even though you are supplementing with energy.
