It may not appear to be exotic or at the cutting edge of research, but the amount of ammonia in corn silage is a key nutritional driver in some ration programs and nutritional models such as the Cornell (CNCPS) Model.
Recently, nutrition modelers at Cornell University determined that it was more advantageous to use ammonia as the definition of non-protein nitrogen (NPN) in feed and forage materials. Lab procedures to correctly determine NPN in forages are difficult to execute and are not routinely run by forage labs.
On the other hand, many labs do determine ammonia by assays that are repeatable and inexpensive.
Cumberland Valley Analytical Services (CVAS) reviewed chemistry data on more than 4,000 corn silage samples across four years to determine expected levels of ammonia in corn silage and to see if there were particular characteristics of silage that produced higher levels of ammonia. Knowing expected levels will allow the users of this information to evaluate data from the lab for correctness. Where information on ammonia is not available, judgments can be made on how best to adjust library values.
Ammonia is generally reported on a "crude protein equivalent" basis. Ammonia is not protein, but the nitrogen in ammonia can be used by rumen bacteria to generate protein that is used by the cow. For purposes of evaluation and ration balancing, ammonia is generally considered as a percent of crude protein or as a percent of the soluble protein.
For the crop years of 2007 through 2010, analysis at CVAS determined the average level of ammonia in corn silage as a percentage of crude protein was 11.2 percent, with a standard deviation of 3.8 percent. On a soluble protein basis, the average was 20.3 percent with a standard deviation of 5.4 percent. Ammonia levels in fresh or unfermented corn silage are typically less than 1.5 percent as a percent of crude protein.
Silage characteristics having the most significant relationship to the level of ammonia in corn silage were the total level of VFA, acetic acid, and number of months in storage. For every unit increase in the acetic acid level in corn silage, ammonia as a percent of crude protein increased by 0.47 percent. Fermentation in corn silage that is less efficient and more prolonged in initial pH drop tends to have a higher level of acetic acid. Management practices that produce lower packing densities would also be expected to have potentially higher levels of acetic acid and ammonia as well. It was interesting to note that there was no statistical relationship between dry matter and ammonia in corn silage.
The number of months that corn silage was in storage significantly impacted the level of ammonia. Even after the fermentation stabilizes (three to four months), there are continued changes in the silage including proteolysis (protein breakdown) which creates ammonia. During the first six months of fermentation, each month of storage was associated with an increase of 0.85 percent ammonia %CP which declined to 0.49 percent during subsequent months.
While we do not want higher levels of ammonia, some researchers believe that the increase in ammonia may be related in part to breakdown in the zein protein surrounding corn silage starch. This may be the mechanism by which corn silage starch becomes more rapidly degradable in the cow with increased storage times.