Dairy producers feed dairy cows a variety of grains, including dry corn, high moisture corn (HMC) and snaplage. These grains are stored in bins, bags, silos and bunkers for variable lengths of time and are processed by a multitude of systems.
Traditional forage testing procedures have not worked very well on feed grains because many nutrients such as ADF, NDF or lignin are extremely low in feed grains and not well related to the digestibility of grain in lactating dairy cows. The digestibility of feed grain is influenced by other factors, such as grinding, steam-flaking or – in the case of HMC – fermentation itself. In the past five years, we’ve increased our knowledge of factors that influence feed grain utilization by dairy cows.
As a general rule, dry corn is one of the slower-digested grains fed to lactating dairy cows. The principle components which alter dry corn digestibility in lactating dairy cows are particle size and hardness of the endosperm. Typically, 50 percent of the starch in dry corns is digested in the rumen and 40 to 45 percent of the starch is digested in the small intestine. As a result, dry corn will have less influence on rumen pH as compared to feeding well fermented high moisture grains.
The key to feeding dry corn is to quantify and understand processing. Typically, roller mills can process dry corn between 600 and 800 micros and hammer mills can process dry corns between 500 and 600 microns. At these levels of processing, starch digestibility will be 92 to 94 percent, but some starch will go undigested. Procedures to evaluate particle size and adequacy of processing of dry corns are readily available in feed and forage testing labs.
The hardness or vitreousness of dry corn is harder to quantify, but can be crudely evaluated by conducting a prolamin test on the dry corn. Prolamins are proteins solely associated with starch in all feed grains. Prolamin proteins are very resistant to digestion and cross-link encapsulate starch into a water-tight matrix.
Feed grain evaluation systems, such as the UW Feed Grain v2.0, integrate particle size and the influence of these proteins into an estimate of digestion potential. In addition, seven-hour in vitro starch digestibility measurements are available and can serve as an index of starch digestibility potential, but they do not integrate the effects of processing on dry corn digestibility.
High moisture corn
When it comes to evaluating and feeding HMC, the focus shifts from particle size and hardness to the degree and extent of fermentation, which is more of a driving force in the digestibility of high moisture corn.
The hard-to-digest prolamin proteins which are associated with starch in HMC can be significantly degraded in the fermentation process. Degradation of these proteins is dependent on the moisture content at ensiling, the temperature at ensiling, and the duration of fermentation.
For example, a dry HMC ensiled at 24 percent moisture will undergo a very slow, poor fermentation. It is going to take a long time for the prolamin proteins to degrade. If it is also coarsely processed (greater than 2,000 microns), starch digestion will be very slow and the potential for excessive fecal starch will be high.
In contrast, a wet snaplage ensiled at 40 percent moisture may ferment very rapidly and, if finely processed (less than1,000 microns), may result in excessively fast ruminal starch digestion. This may lead to rumen acidosis or decreased milk component production.
The most important thing to remember is there is no such thing as a universal HMC. The digestion of HMC or snaplage is dynamic and dependent on processing and especially dependent on the intensity and length of fermentation. HMC may feed poorly in the fall, well in the spring, and become excessively digestible in the rumen the following summer. The digestibility of high moisture corn or snaplage is always a moving target.
Keeping an eye on HMC
Fortunately, these events now can be monitored. The key is to watch particle size (because it will get finer with advancing ensiling time) and the soluble protein or ammonia content of high moisture corns throughout the ensiling period.
As the tougher prolamin proteins are degraded, they first become soluble proteins followed by total degradation (called deamination) to ammonia-nitrogen. HMC with ammonia-nitrogen (NH3-N) content below 1.0 percent is not fermented and will feed like dry corn. When ammonia-nitrogen contents of HMC are above 5.0 percent of protein, they are well fermented, indicating extensive protein breakdown and increased starch digestion potential.
University of Wisconsin and University of Delaware researchers have similarly defined these relationships and an example of the relationship between NH3-N concentration and seven-hour in vitro starch digestibility of high moisture corns is presented in Figure 1.
Finally, feed and forage testing laboratories now have tools to integrate these effects into high moisture corn evaluation programs.
Figure 1. Relationship between ammonia-N and 7-h ruminal in vitro starch digestibility in high moisture corn. Courtesy of L. Ferraretto and R.D. Shaver, University of Wisconsin-Madison.