A new test can help dairy producers determine how well their corn silage was processed.

Processing rolls break or nick many kernels, making them more digestible. But some remain undamaged and may pass through cows undigested.

“Kernels that pass out of the cow can represent a significant loss of digestible starch and energy,” says Dave Mertens, a USDA-ARS dairy scientist with the U.S. Dairy Forage Research Center in Madison, WI.

Mertens developed the test in cooperation with Dairyland Laboratories, Arcadia, WI, and Pioneer Hi-Bred International. The result, called the Corn Silage Processing Score (CSPS), is the percentage of a sample's starch that has been processed for adequate digestion.

To get such a score, send a quart-sized silage sample to Dairyland Labs. Each sample is dried overnight to 7-8% moisture. Then a vertical shaking method is used to quantify its physical structure. The method involves vigorous shaking in a pan with nine sieves, each with different-sized openings.

After shaking, the proportion of particles collected on each sieve is measured. Particles greater than 4.75 mm (about ¼") are considered coarse, 1.18-4.75 mm particles are medium and all others are fine.

Next, starch analyses using wet chemistry or NIR are done on the whole sample and its coarse component. “We need those analyses to compare the starch of the whole sample to the amount collected on the coarse sieves,” says Jerry Dekan, Dairyland Labs' technical director.

The results are used to determine the CSPS, which is the percentage of starch passing through the coarse screens. If greater than 70% pass through, the processing is optimal. Scores between 50% and 70% are considered average; less than 50% is inadequate.

“We typically assume that the larger the CSPS, the better, because it suggests that starch digestion will be improved,” notes Mertens. “However, farmers may need to adjust rations when the CSPS is greater than 70% because the starch in the silage will be rapidly fermented. That's especially true if the ration is already low in fiber and high in fermentable carbohydrates.”

The score is helpful in two ways, says Mertens.

“First, you have a measurement of how well the crop processor on your chopper is functioning. Or, if you hire a custom harvester, it helps you know if the level of processing was adequate.

“Second, it provides an indication of starch and energy utilization in your silage. If your silage wasn't processed very well, you may need to increase the energy value of your ration — perhaps by adding some ground corn.”

Ideally, the test should be done at harvest so that the processing rolls can be adjusted, if necessary. Or it can be done occasionally during the year, whenever there's a major change, such as a switch to silage from a new field or bag.

“If a farmer uses an upright silo, he can actually damage kernels while unloading, depending on what type of system he has. That could change the CSPS,” says Mertens.

The CSPS lab report also lists the sample percentages that are coarse, medium and fine. Those figures can be used to evaluate the silage's fiber contribution to the ration. Processing crushes cobs, preventing cows from sorting them out and resulting in higher fiber intakes. But fine pieces may not function as fiber in the rumen.

“Fiber in the small fraction (less than 1.18 mm) probably passes through cows very quickly and contributes little to the effectiveness of fiber in stimulating chewing activity,” says Mertens. “The proportion of particles on the large and medium sieves may be a good indicator of the physical effectiveness factor for NDF.”

The test costs $16/sample plus the cost of a starch analysis, and requires at least two days for completion.

For more information, contact Dairyland Laboratories, Inc., 217 E. Main, Arcadia, WI 54612. Phone: 608-323-2123.