Forages comprise 35% to 70% of the dry matter (DM) in diets for lactating dairy cows. Upsetting a cow’s diet consistency, variability in forage quality can impact DM intake, diet energy density, dietary grain and protein supplementation amounts, feed costs, lactation performance and cow health. It’s critical for dairy producers to continually test their forage supply, so they can adjust for variability and maintain a consistent ration for peak production.

With alfalfa and grass silage, producers should expect to see high variability due to multiple cuttings being stored at different times during the growing season, not to mention that individual fields will vary greatly in quality. That means frequent sampling and testing are necessary.

“Most producers will test for DM content at least weekly as well as any time they sense moisture or DM content is changing,” said Randy Shaver, professor and dairy nutritionist at the University of Wisconsin-Madison Extension. “Testing allows producers to adjust the feeding array of wet forages to keep a consistent amount of DM in the ration. Sending alfalfa and grass silage samples in for nutrient analysis every other week is a good practice, along with using a rolling average when reformulating rations.”

Corn silage tends to be more consistent although there’s more variation in DM (moisture) than neutral fiber detergent (NDF) and starch. Corn silage DM testing is usually needed every week, but NDF starch and crude protein may only need to be tested monthly.

Recent research at The Ohio State University indicates there’s a lot of sampling error that can occur.

“To come onto a farm and only take one sample on one day can be somewhat inaccurate, particularly for NDF,” Shaver said. “Ohio State’s research suggests taking two samples on the same day to account for some of the sampling error. There is always day-to-day variation and analytical variation. However, when you take two separate samples, it takes out some of the inaccuracies.”

When taking a sample from silage bags, 6" to 12" depth samples should be taken from around the 10' to 12' diameter opening.

“If producers are peeling it down with a skid steer, you can usually take several different grab samples, mix it together well into one sample and send that sample in to the lab,” Shaver said.

For a silage bunker or pile, it’s a little more difficult because there’s variation across the face from the top, middle and bottom. It’s also very unsafe to walk up to the face of a silo and take a sample. The current recommendation is to skim down the face with a face peeler or an end loader. Then from the pile that’s been skimmed down, staying back from the face for safety, take several different grab samples and combine into a single, homogenized sample and send to the laboratory.

“The goal is to try to mimic what’s actually being fed to the cows or what’s going into the total mixed ration mixture, and it should be a blend across the full face. Only taking a grab sample can be very inaccurate and also very dangerous,” he said.