Does fermentation change corn silage processing?

By Dr. Luiz Ferraretto, University of Wisconsin-Madison assistant professor and extension specialist

As soon as the silo is sealed and oxygen is used up, anaerobic bacterial fermentation begins, acids accumulate (primarily lactate and acetate), and the pH drops. Although these are the most important events for forage conservation, much more is happening within the silo. As you may know, starch digestibility spikes during initial fermentation and continues to increase gradually with prolonged storage (not as pronounced as the initial spike). This phenomenon is driven by enzymes that breakdown zein proteins, which surround starch granules and inhibit its digestion. About 60% of these enzymes come from bacteria, whereas 30% are enzymes found inside corn kernels and are activated by a low pH. Breakdown of zein proteins by these enzymes during fermentation could separate starch granules, reducing the particle size of corn kernels.

During the 2014 corn silage season, we conducted two studies to answer if fermentation can change corn silage processing score (CSPS). CSPS, or the percent of starch passing through the 4.75 mm sieve, is the only assay commercially available to determine kernel breakage (and to some extent particle size) and was used across our studies. Fermentation increased CSPS from 50% at harvest to 60% after 30 days of storage, on average, in our first study. Our second study evaluated corn silage fermented after 240 days, and CSPS increased gradually from 60% at harvest to 68% after 240 days of storage.

More recently, we conducted another study to test if the initial CSPS drove this response. This study evaluated corn silage harvested at three maturities with two roll gap settings and was fermented for 240 days. Prolonged storage improved CSPS across all treatments, but the magnitude of the response was not related to the initial CSPS. In contrast, a recent study from Cornell University with four hybrids grown at two locations for two consecutive years did not present a consistent effect on CSPS, suggesting this effect may not always occur.

Another unknown fact was the fate of intact or slightly damaged kernels during silage fermentation. One study compared very poorly processed silage (less than 30% of starch passing through a 4.75 mm sieve) at harvest or ensiled for 120 days. This poor processing reflected a silage containing intact or slightly damaged kernels only as intended. CSPS values were identical for fresh and fermented samples. These two studies highlighted that prolonged storage length will not replace adequate processing at harvest.

Going back to our question, does fermentation change CSPS? The answer is yes and no. Yes, most trials reported benefits of fermentation and prolonged storage on CSPS and an improvement throughout fermentation is expected. But no, fermentation and prolonged storage will not compensate for inadequate processing at harvest. Therefore, targeting a 70% to 75% CSPS at harvest is suggested.