Starch is the Culprit
It starts with the physiology of the corn kernel and changing rates of starch digestion (often referred to as the mathematical term, Kd). This is why feeding new-crop legume or grass silage soon after harvest is not a concern. The digestion issue revolves around starch rather than fiber, and legume/grass crops are essentially devoid of starch.
The November 2010 Feeding column “Why not all corn feeds the same” on page 765 detailed the anatomy of corn kernel’s endosperm which consists of starch granules encased in zein proteins. Concentrations of these proteins go up with kernel maturation and peak at the time of black-layer formation (about 35 percent kernel moisture). Mother Nature is concerned about the next generation and created these storage proteins with the ability to repel water in order to prevent premature starch hydration that could interfere with germination.
Given that Mother Nature never intended corn to be eaten by a dairy cow, these proteins present a nutritional challenge by interfering with starch digestion. However, over time in fermented storage, silage microbial activity and the chemical action of fermentation acids gradually solubilize the proteins freeing up starch granules for more rapid digestion by rumen microbes. This is why corn in silage or high-moisture grain has higher energy and is more ruminally available than the same hybrid fed as unfermented, dry corn grain.
As the corn kernels in corn silage or high-moisture corn (over 25 percent moisture) begin the fermentation process, they undergo the most rapid changes during the first two to three months in storage. It has now been proven in animal trials in both the U.S. and Europe that the rate of starch digestibility (Kd) is also changing rapidly over this period, providing the scientific basis for our long-held recommendations on waiting to feed new-crop silages.
Perhaps we need to talk more with our beef colleagues. Dairy nutritionists have now learned that for decades, feedlot nutritionists have relied on soluble crude protein (or nitrogen) content of high-moisture corn as a proxy for rate of starch digestion in feedlot cattle. This makes sense because as the zein proteins are gradually solubilized in fermented storage, the level of soluble protein should go up, as well. High-moisture corn with a protein solubility below 40 percent is generally from lower-moisture corn (with less extensive fermentation) exhibiting lower ruminal starch digestibility more similar to that of dry rolled corn. Feeding high-moisture corn with an N-solubility that exceeds 60 percent often results in more acidosis and incidents of steers going off-feed.