The high moisture corn and corn silage fed today was made nine or 10 months ago, and they are not the same feedstuffs as they were last winter, says Bill Weiss, Ohio State University dairy nutrition extension specialist. "If high moisture 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 nine months of storage," he says.
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 percent (not percentage units) and in vitro protein degradability increased 20 percent over 10 months of storage (Newbold et al., 2006, J. Dairy Sci. 89:190).
Starch degradability in wetter silages (less than 30 percent dry matter) did not change greatly over storage, probably because it was very high initially, but starch degradability in drier corn silage (greater than 35 percent DM) changed greatly.
A study from Nebraska (Benton et al., Nebraska Beef Cattle Report, 2006) revealed that in situ DM disappearance in high moisture corn after 10 months of storage was 25 (corn with 24 percent moisture) to 33 percent (corn with 30 percent moisture) greater than it was after one month of storage.
Because starch is the major component of high moisture 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 high moisture corn (approximately 30 percent moisture) over eight months of storage and found that some specific proteins decreased by 50 percent 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 three studies indicate that a much larger proportion of starch in high moisture corn and corn silages that have been stored for many months will be degraded in the rumen compared with newer high moisture corn and corn silage, says Weiss.
Cows fed aged high moisture corn or corn silage could be at increased risk for acidosis, and diet modifications may be needed to reduce this risk, he warns. "The concentration of total starch in a diet may have to be reduced high moisture corn and corn silage ages or some dry ground corn will need to replace high moisture 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 high moisture corn may need additional rumen undegradable protein. Sources of rumen degradable protein (like urea) may not be needed when feeding older corn silage and high moisture corn. Just because the concentration of starch or protein in a feed does not change, this does not mean its nutritional value remains constant, he concludes.
Source: Buckeye Dairy News, July issue