When is ‘adequate’ kernel processing not adequate? – Jon Urness and Pat Hoffman, Vita Plus
A kernel processing score between 60 and 70 has been widely considered as “ideal.” However, that might not necessarily be true depending on harvest moisture. Wetter corn silage has the potential for greater starch digestibility while drier silage may have far less starch digestion potential given the same processing score.
Real word experience
This point was driven home to me this past fall when I received a call from a producer in southern Wisconsin. Milk had dropped 8 to 10 pounds when the farm got into new-crop corn silage. Granted, the change in crops would normally have some effect on milk production, but an 8- to 10-pound loss seemed excessive.
My first questions centered on harvest moisture content and kernel processing. The producer admitted the corn silage was drier than ideal at 60-percent moisture, but insisted that processing was good because his custom harvester had just invested in a new chopper equipped with the latest kernel processing technology.
The next day, I visited the farm and concentrated on three things: corn silage moisture, kernel processing and fecal starch. The silage didn’t appear to be well processed. The silage coming out of the bunker was on the dry side and the manure showed excess corn coming through. Based on analysis, moisture was confirmed at 60 percent. The kernel processing score was 61 and fecal starch came back at 8.0 percent.
The key here is the fecal starch result. With 8.0 percent starch in the manure, we can conservatively predict a milk loss of 3 to 4 pounds based on research by Dr. Jim Ferguson at the University of Pennsylvania. But why are we getting elevated fecal starch with what is considered an “adequate” processing score of 61? Pat Hoffman tackles that question…..read on.
What’s actually going on?
By Pat Hoffman
It is easy to lose sight that kernel processing is only one step in a dynamic process that increases corn silage starch digestibility. For example, let’s assume that whole unprocessed corn silage kernels have an average total tract starch digestibility (TTSD) of 80 percent in a lactating dairy cow. If corn silage kernels are wet, TTSD could be 84 percent and, if kernels are dry, TTSD could be 76 percent.
The first process in increasing TTSD is accomplished by the chopper knives, which typically improve TTSD by five to six percentage units. In our example, that would increase TTSD to 81 to 89 percent. But, as we know, lots of whole kernels can pass through the knives of a chopper.
The second process is the kernel processor, which can increase the TTSD approximately four more percentage units by mitigating remaining whole kernels in pre-ensiled corn silage. As such, the knives plus processing yield TTSD values of 85 to 93 percent. Again, the variance is due to moisture, which is a benchmark of starch chemical maturity.
It is at this point that starch digestibility in corn silage is a bit odd to understand because TTSD values of 85 to 93 percent seem high, but they are actually low because starch digestibility in a lactating dairy cow can be nearly complete at 98 percent.
New research information has shown us that, as fermentation takes over, proteins in the kernel that hold starch together are degraded, which actually yields the largest increase in TTSD. Fermentation can increase TTSD anywhere from 6 to 12 percentage units with increases occurring over a long fermentation time. Thus, intensity and duration of fermentation of corn silage has a great effect on corn silage TTSD. But fermentation effects on TTSD are aided by kernel processing because kernel processing provides fermentation bacteria access to the proteins that surround starch.
A rule of thumb
So what are the take-home messages from all these complex dynamics? They are actually pretty simple.
First, the drier corn silage is, the greater processing and longer fermentation times are required to fully improve TTSD. When corn silages are harvested wetter, less processing and shorter fermentation times are required to reach optimum TTSD.
These dynamics can be kept straight by using a little thumb rule. Kernel processing scores should be 2 times the dry matter minus 5. So, for our producer’s corn silage (40-percent DM), a kernel processing score of 75 percent (2 x 40 – 5) would have been a goal because the corn silage was so dry at harvest.
But, in the end, only a long fermentation time will fully improve starch digestibility in our producer’s corn silage. The milk yield loss and higher-than-normal fecal starch make sense when the full dynamics of corn silage starch digestion are kept in mind.
Feed quality and nutrition