Butyric acid in grass or alfalfa silage can predispose cows to ketosis.

"Feeding butyric acid to cows may cause ketosis, or let's say it's a risk factor for ketosis," says Gary Oetzel, veterinary researcher at the University of Wisconsin.

Butyric acid can occur when the silage has been put up too wet. It is a byproduct of clostridial bacteria fermentation, which can give the silage a rancid odor.

Grass or alfalfa silage is more susceptible to this problem than corn silage.

If producers have silage that smells bad, they should have it checked by a lab. As a result, rations may have to be re-formulated, so that no cow eats more than 50 grams of butyric acid per day; otherwise, the risk for subclinical or clinical ketosis can increase.

Daily intakes of 50 to 100 grams of butyric acid can cause subclinical ketosis, and doses exceeding 200 grams may cause clinical ketosis, Oetzel says.

It's best if there is absolutely no butyric acid in the feed, he adds.