The move to big rectangular bales in the humid Midwest is likely causing some hefty hay losses - even when a preservative is used.

In two years of University of Wisconsin research, dry matter and quality losses in propionic acid-treated big bales were almost the same as when no preservative was used. And when the hay was baled at 19% moisture or higher, those losses were significant.

However, ag engineer Kevin Shinners, who headed the study, doesn't discourage the use of propionic acid in big bales. Hay often can't be field-dried far enough to store safely in big bales without a preservative, he says. And propionic acid does reduce heating, which can harm protein availability.

"Right now we have almost nothing to offer producers except propionic acid," says Shinners. "Most of them are using it and it's a wise move. It's not as good as baling the hay dry, but since it's difficult to get it dry, we'd better consider propionic acid as our ally."

He doesn't necessarily suggest that growers go back to small bales, either.

"You would get a better-quality product if you made small square bales. But you have to balance that against the productivity of the big baler and the labor situation."

Instead, he suggests that growers "work hard" to field-dry the hay to 16% moisture or lower, so it can be stored in big bales without serious losses.

"That means setting the conditioner up correctly, spreading the crop out as wide as possible, raking at the right moisture content and using the right type of rake to get a nice, fluffy windrow," says Shinners. "And even if we do all those things, it's still really tough to get it down to 16% moisture."

In his study, Shinners used 3 x 3' bales. He baled alfalfa at moisture levels ranging from 14% to 28%, applying a preservative when bale moisture exceeded 16%. The preservative was a commercial product containing 75% propionic acid. The application rate was 0.8%, (16 lbs/ton), which cost about $15/ton.

He surmises that if he had applied the preservative at 11/2 or 2% of the hay's weight, it would have done a better job. But that would have bumped the cost up to $30/ton or more.

"I don't think you can afford to put an extra $30/ton into your hay here in the Midwest."

After four months in storage, treated and untreated bales averaged dry matter losses of 13% when hay was baled at 27% moisture. Losses were 6% in 19%-moisture hay and 3.5% in hay field-dried to 16% moisture or lower.

"You're always going to have losses," he says. "If you get in that 3-4% range, you're doing pretty good. But if your losses are in the 9-12% range, that's too high."

At the highest moisture levels, treated and untreated bales also were higher in fiber than untreated low-moisture hay. Propionic acid treatment had improved protein availability, although not to the level of low-moisture bales.

Shinners points out that the preservative reduced the amount of heating, which removes moisture. So treated bales retained more moisture than untreated bales, perhaps lengthening the period of microorganism activity. That may explain why propionic acid didn't perform better, he says.

As part of the same study, he evaluated the benefits of ventilating each bale with either a 3"- or 41/2"-diameter hole through the center. Those bales were stacked with spaces between tiers to permit air movement.

Bale ventilation didn't consistently reduce losses, although it did show some promise. Shinners thinks a bigger hole, or four holes through each bale, might be more effective.

"I'm intrigued by that, and I'm intrigued by the concept of propionic acid plus ventilation."

He adds: "There's more work to be done on preserving big square bales because our losses in storage are still too high."

Check Preservative Label Before Buying

When shopping for a preservative for big bales, look for a product with a high percentage of buffered propionic acid. Then use the recommended application rate to figure out how much acid is actually in it.

That advice is from Jeff Roberts of Harvest Tec, Hudson, WI. His company sells hay preservatives and drying agents, and is the leading supplier of application equipment for both types of products.

Roberts says commercial propionic acid preservatives are 40-75% acid. But the amount of acid in a product can be quite different from the stated percentage. That's because some companies include the buffering agent and other inactive ingredients in the acid percentage.

"We tell farmers to look at the label and see how much it tells you to use," says Roberts. "Concentrate on the application rate because the company has to stand behind that."

Most of today's propionic acid preservatives are buffered, but the acid still causes oxidation, he points out.

"It will oxidize metal somewhat. But the big concern is that it discolors the hay."

If hay color is important, spend a little more for a preservative with an antioxidant, Roberts suggests.