The data indicated that starch in forage diets has a negative impact on forage digestion, especially when the diet is mostly forage. This would suggest that starch has implication on the microflora in the rumen. Rumen microbes that digest starch, or corn, are different than those that digest cellulose, or forage. So when the diet consists of a combination of starch and cellulose, if rumen microbial protein supplies are inadequate to satisfy the growth needs of all bacteria, the starch-digesting bacteria are favored, creating a depression in forage digestion. Also, the starch will likely cause a reduction in rumen pH, making the rumen more acidic, and this negatively impacts the cellulose digesters.
With small amounts such as three to four pounds of grain in a high forage diet, there will be slight reductions in forage digestion, but likely not enough to see a reduction in performance. In these situations, it is important to meet the degradable intake protein (DIP) needs of the animal as DIP is used by the rumen microbes to make their own protein. In high grain diets there will be some utilization of the forage component, but the microbe population is mostly starch-digesters and the forage is mostly used as a "scratch factor" to keep the rumen healthy.
In the diet that is nine pounds, per head, per day of corn and ten pounds, per head, per day of hay, the hay is not 100 percent utilized. As to how much the digestibility is decreased, my educated guess would be by 20 to 25 percent.
