A recent field study of fresh chop corn silage shows that waiting for that last 3 to 5 percent of starch fill may not be worth it – especially if you intend to feed that silage to lactating dairy cows.

As the plant matures and kernel starch fill continues, the rumen digestibility of the starch decreases, explains Dr. David Weakley, director of forage research for Calibrate® Technologies. In dairy cows, when the amount of rumen degradable starch is less than expected, milk production can suffer.

Weakley and his team tested 300 fresh cut corn plant samples using Calibrate’s proprietary GPN technology. (GPN is a measure of rumen degradable starch obtained from an analytical rapid NIR test.) All of the samples were obtained from a number of Texas growers who had planted approximately 30 corn silage hybrids.

The starch content of all 300 samples averaged 34 percent, but individual samples ranged from 16 percent to 47 percent. The GPN score averaged 6.8 but individual samples ranged from 5.8 to 7.8.

“We saw a clear linear decrease in rumen starch degradability as crude starch content increased,” says Dr. Weakley. These results showed that starch content and GPN are inversely related. That means waiting to let the ear fill completely to maximize crude starch content may not provide optimum starch digestibility for the cows.

In addition to lower rumen digestibility, more mature starch also means that kernel processing may not be as effective. That, in turn, could further decrease the amount of starch that is available to the cow.

The researchers also tested moisture content. When Dr. Weakley’s team analyzed the data and compared moisture content with GPN scores they found that the two were positively correlated. GPN score increased as moisture content increased, thus lending support to the conclusion that harvesting more mature corn silage to capture more starch and total yield may not be the best strategy for dairy producers.

Making good corn silage is a tradeoff, says Dr. Weakley. When corn silage is less mature, it will contain less starch, but the starch will be more digestible in the rumen, the kernels will process better, the higher moisture results in better packing and you will have better aerobic stability after opening the bunker. However, if corn silage is harvested too wet you can get abnormal fermentation with high butyric acid.

If you waited for greater starch fill in the ears before harvesting your corn silage this year, you may want to let it ensile a little longer for starch availability to increase before you feed it. The good news is that corn grain is relatively cheap this year and other sources of highly-digestible starch, such as dried or flaked corn, are also cost-effective at this time. That gives you more options to make up for lower, but more digestible, starch corn silage, if more rumen digestible starch is needed.

Test your corn silage before you start feeding it and again every two weeks. Then use that information to provide a ration with optimal amounts of rumen degraded starch to maximize your cows’ performance.