Remember what’s important when it comes to harvesting Corn Silage

Remember What’s Important When it Comes to Harvesting Corn Silage
Robb D. Wock, DPC – Purina Animal Nutrition LLC (507) 696-6351

With Corn Silage harvest only a month to a month-and-a-half away it’s important to re-focus and concentrate on the REAL reasons why we feed Corn Silage and how this relates to the ideal time to harvest.  “Whole-Plant-Moisture” is one of the easiest and fastest ways to evaluate the general area of maturity of the Corn plant.  However the REAL basis behind understanding “Whole-Plant-Moisture” and its relationship to the timing of harvest is in our understanding of “Ruminal-Starch-Digestibility”.  I will attempt to explain this in better detail within the remainder of this article.


During a normal growing season your typical variety of Corn grown for silage purposes contains somewhere between 55 and 65% of its energy within the corn kernel itself.  This energy is primarily in the form of “starch” which is a key component of cow performance.  Dairy Cows require a precise amount of starch & energy in order to maximize milk production and maintain good health.  Therefore many ration decisions are at least in part, based not only on the amount of total starch that is fed but particularly on the amount of “Ruminal-Degradable-Starch” – as this quantity reflects how much starch the cow can actually utilize.


One of the methods Purina Animal Nutrition has developed to help evaluate “Ruminal-Starch-Digestibility” is called a GPN test using proprietary technology to measure how digestible & available the starch will be to rumen microbes.  Ingredients sampled receive a score based on an index of 1 to 11, with 1 being very slow digesting and 11 being very fast.  It is important to realize that Ruminal-Starch-Digestibility varies by year & location, and is often influenced by many factors including stage of maturity, kernel processing, storage time, hybrid selection and growing conditions.


At the time of harvest much of the data we have collected on Corn Silage using the GPN technology has shown us that the range of starch content often varies significantly, from as low as 18% to as high as 48%, with averages often from 30-34%.  However, again as mentioned above, the actual maturity of the plant, plant varieties, and many growing conditions including soil-type all play a role in the starch content at the time it is sampled.  Starch digestibility ratings using GPN often range from 5.5 to 7.5, with averages somewhere around 6 to 6.5 or so. 


The bottom line is that there is a clear linear decrease in rumen-starch-degradability as crude starch content increases.   In other words starch content & digestibility are inversely related, which means waiting to let the ear fill completely to maximize TOTAL starch content is not likely to provide optimum starch digestibility for your dairy cows.  Some “high-starch” varieties (40%+) measuring low in GPN are hard & dense kernels that are almost impossible to process and pass right through the rumen.


GPN scores increase as moisture content increases, supporting the theory that harvesting more immature silage to capture more digestibility is likely the best strategy.  As the plant matures & kernel starch-fill continues, the digestibility of the starch declines.  Although less mature corn silage may contain lower quantities of starch, the starch is more digestible in the rumen, the kernels process better, and the additional moisture results in more favorable packing & fermentation. 


So this is the reason why we utilize “Whole-Plant-Moisture” to help determine plant maturity & harvest timing – there is a direct relationship between plant moisture & ruminal-starch-digestibility.  Dr. Randy Shaver, professor of Dairy Science & Extension dairy nutritionist at the University of Wisconsin-Madison, says a good rule of thumb for harvesting Corn Silage is when the kernel is at half milk line (half milk & half dough).  “Once the kernel is dented, it’s time to start measuring kernel movement & dry matter” he explained.


Keep in mind that the most desirable corn hybrid is one offering both high starch and high digestibility.  From a performance standpoint, personally I have seen anywhere from a 2 to 5 pound daily milk response per cow when starch is efficiently utilized.  This is huge – particularly now in a depressed milk economy. 

In order to help you understand the stage of maturity of your Corn Silage I would encourage you to join us at All-American Coop during our Corn Silage Burn-Down events later this month and in September.  We can help you decide when you should plan your harvest so that you can provide the most digestible nutrients to your Dairy enterprise.

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