Last winter, when Dave Hemann read about 6'-tall, USDA-developed soybean varieties in Hay & Forage Grower, he called us to find out when they'll be available.

He figures tall soybean plants - to be introduced in a year or two - will increase both the yield and protein content of sorghum-soybean mixtures. He's been growing the two crops together for his 100-cow dairy herd for 15 years.

Hemann, who farms with his son, Allen, near Greenwood, WI, says the ensiled mixture already is higher in quality than corn silage. It normally tests 15-18% crude protein and around 30% ADF and 40% NDF. Yield usually runs a respectable 15-18 tons per acre.

They plant the crop around June 1, mixing the two types of seed together in their grain drill seed box. They plant 60-72 lbs per acre of Corsoy 79, a tall soybean that matures late in their area, and about 15 lbs per acre of forage sorghum.

The Hemanns cut the combo when the soybeans start forming pods and before the sorghum heads - usually in mid-August. They field-dry it for a day or two, chop it and put it in a silo with their other silage - a clover and grass mix.

"The biggest drawback is if you mow it and get so much rain that you can't get it off," says Hemann. "Once it starts going out of condition, you don't have anything but manure."

The Hemanns don't grow much alfalfa because it doesn't do well on their farm. So the sorghum-soybean mix is their best forage.

"When we hit the sorghum and soybeans, we usually go up a pound or two (per cow) on milk and a point on fat and protein," Hemann reports.

Sorghum and soybeans can make a high-quality forage for dairy cows, agrees Dan Undersander, University of Wisconsin extension forage agronomist.

"It's not as good as alfalfa - a little lower in protein and a little higher in fiber - but it's probably betterthan corn silage," says Undersander.

However, he says the two species should be grown separately and harvested together.

"The data we've collected suggest that, over the long haul, you can run into several problems if you try to mix the two in the same field at the same time," says Undersander.

In warm, dry growing seasons, sorghum tends to outcompete soybeans, lowering the protein content of the silage, he says. Conversely, soybeans might be dominant in years with cool springs, reducing energy content and yield.

"In most years, it will work pretty good to mix them," he says. "But you can get extreme years where one grows madly and outcompetes the other. If you plant them separately, you don't have that problem."

Since sorghum needs fairly high temperatures, the risk of poor performance is greatest in the extreme North, such as in northern Wisconsin and Minnesota, he adds.

Another concern: If weeds invade a soybean-sorghum crop, no herbicides are available to control them.

For growers who plant the two crops together, Undersander recommends grain-type sorghum varieties, which don't grow as tall as the forage type.

"If you use tall sorghum, you'll end up with mostly sorghum and not much soybeans."