Dr. Bill Weiss, Dairy Nutrition Extension Specialist, The Ohio State University (top of page) pdf file
The high moisture (HM) corn and corn silage fed today was made 9 or 10 months ago, and they are not the same feedstuffs as they were last winter. If HM corn and corn silage are properly made and well-preserved, the concentrations of the major chemically-defined nutrients such as crude protein (CP), neutral detergent fiber (NDF), and starch probably changed very little during the last 9 months of storage. However nutrient fractions measured by biological methods probably changed substantially. A study from Europe reported that in vitro starch digestibility (an index of starch digestion in the rumen) in corn silage increased 30% (not percentage units) and in vitro protein degradability increased 20% over 10 months of storage (Newbold et al., 2006, J. Dairy Sci. 89:190). Starch degradability in wetter silages (<30% dry matter) did not change greatly over storage, probably because it was very high initially, but starch degradability in drier corn silage (>35% DM) changed greatly. A study from Nebraska (Benton et al., Nebraska Beef Cattle Report, 2006) revealed that in situ DM disappearance in HM corn after 10 months of storage was 25 (corn with 24% moisture) to 33% (corn with 30% moisture) greater than it was after 1 month of storage. Because starch is the major component of HM corn DM, the change in DM disappearance likely reflects a change in starch disappearance. In situ protein degradation followed a similar pattern of change. Researchers at the University of Wisconsin (Hoffman et al., 2011, J. Dairy Science 94:2465 ) studied changes in specific proteins in HM corn (approximately 30% moisture) over 8 months of storage and found that some specific proteins decreased by 50% over that time. Those proteins are hydrophobic (very low solubility in water) and are part of the starch-protein matrix that make up corn starch granules. The concentrations of these proteins probably have a negative correlation with starch degradability. The results of these 3 studies indicate that a much larger proportion of starch in HM corn and corn silages that have been stored for many months will be degraded in the rumen compared with newer HM corn and corn silage. Cows fed aged HM corn or corn silage could be at increased risk for acidosis, and diet modifications may be needed to reduce this risk. The concentration of total starch in a diet may have to be reduced as HM corn and corn silage ages or some dry ground corn will need to replace HM corn to prevent excessive amounts of starch from being fermented in the rumen. The increase in protein degradability may mean that diets with older corn silage and HM corn may need additional rumen undegradable protein. Sources of rumen degradable protein (e.g., urea) may not be needed when feeding older corn silage and HM corn. Just because the concentration of starch or protein in a feed does not change, this does not mean its nutritional value remains constant.