After your silage corn has been chopped, processed and packed into a bunker, bag or tower silo, you're done until you load it into a TMR mixer, right?

Wrong, says Mike Hutjens, University of Illinois extension dairy nutritionist. You need to evaluate the silage before feeding it to make sure it'll function as expected in the ration. Improperly processed kernels and cobs, or stalk and leaf pieces that are too long or short, can have a big impact on ration formulation, says Hutjens.

The analysis should take place once a month or whenever there's a major change, such as a switch to silage from a new field or bag, he says. The main tool needed for the five-minute job: a Penn State forage particle separator.

Hutjens is high on crop processing as a way to increase the energy value of corn silage. But the ensiled crop should be evaluated to determine the degree of processing that actually took place. Most of the kernels should be broken, but not into fine pieces.

“I like to see in the range of 95% of the kernels broken,” says Hutjens.

Unbroken kernels likely will pass through your cows undigested.

“What that simply means to me is those kernels have no feed value,” says Hutjens. “You're going to have to replace the lost nutrients. It'll probably be corn or some other starch or grain equivalent to keep the ration balanced.”

Knowing how much to add is a challenge. Be cautious to avoid rumen acidosis: Add a pound or two at a time and watch the response.

An easy way to separate the kernels from the rest of the silage: Put a couple of handfuls of silage into a water-filled bucket. The kernels will sink while everything else floats. Dump out the water and floating plant parts, and you can quickly determine the percent broken kernels.

In the particle separator, processed kernels should drop through the top screen to the middle box. If many end up in the bottom box, the crop was processed too fine (rollers too tight), and the starch will ferment rapidly in the rumen. To maintain an optimal rumen pH, you may need to reduce the total amount of starch in the ration.

“If I'm looking at 27-28% starch in the total ration dry matter, and the grain in my corn silage has been massacred, I might back down my grain a pound or two to avoid a rumen acidosis, sore feet or off-feed problem,” says Hutjens.

Cob pieces should also end up in the middle box. They should be ¼-½" in size — small enough so cows can't separate them and leave them in the bunk, but big enough to function as fiber in the rumen.

“The key is that they're not sortable,” he says. “The cows will consume them and they've got some functionality as well.”

For grain-type hybrids chopped at the optimal moisture level, the recommended theoretical length of chop is ¼". In most cases, that'll put 10-20% of the silage in the top separator box, 50-60% in the middle box, and less than 30% in the bottom one.

However, particle length may be reduced going into storage or coming out. That can happen when frozen silage is removed from a top-unloading tower silo, for example, or when chopped corn is bagged.

“We have strong evidence that when corn silage goes through a bagger, we probably take a quarter of an inch off the theoretical length of chop,” says Hutjens.

Some Illinois dairy producers using baggers chop a bit coarser to compensate. But if corn silage or haylage comes out of storage too short, you may need to add a fiber source to the ration. Baled hay and fuzzy cottonseed are logical choices, says Hutjens.