Dale Fink has often wished he could plant soybeans with his silage corn to spike silage protein content. But, until recently, no herbicide was available to control weeds in both crops.

Roundup Ready technology changed that. Last year, Fink planted 56 acres of Roundup Ready soybeans between just-planted rows of Roundup Ready corn. The resulting silage is higher in protein than corn silage, but not nearly as high as he had hoped. The corn shaded the soybeans, hindering their growth.

"I was disappointed, but it was not a complete failure," says Fink, of Trempealeau, WI. "And I think it can be improved."

This year he'll reduce the corn population and perhaps leave every fourth row unplanted, opening the canopy. That should increase the soybean portion of the silage mix, he says.

It'll help soybean growth and bump up silage protein content, but will cut the total yield, says Dan Undersander, extension forage agronomist at the University of Wisconsin. A better option, he adds, is to plant corn and soybeans separately in the same or adjacent fields, then mix them when they're chopped.

"When the two are planted together, one or the other usually dominates, depending on early spring conditions. A high percentage of the time, they don't grow well as a mixture."

The basic problem, according to Undersander, is that corn does better than beans under cool conditions. Corn germinates when the soil temperature rises above 50 degrees, while soybeans start growing at about 60 degrees. So the corn will get a head start most years, and interplanted soybeans won't do well.

Sorghum is a better match for soybeans because it also starts growing at about 60 degrees. But sorghum-soybean plantings usually aren't very successful, either, he says.

Fink planted corn in 30" rows at 30,000 seeds/acre, then planted soybeans halfway between the corn rows. He used 102- and 107-day hybrids in different parts of the field, and a Group III soybean variety. He figured the late-maturing variety would grow tall for a high silage yield, and would mature at about the same time as the corn.

He planted the Roundup Ready mixture in a field where he had a shattercane problem. One Roundup application did the job on that and other weeds, at less cost than the postemergent herbicide (Accent) he normally uses for shattercane control in conventional corn.

"That's one reason we went this route; it lowered the cost," says Fink.

When the corn was ready for chopping, the soybean pods were filled but still green. There were no harvesting problems, and the mixture ensiled well.

Yield was unaffected, says Fink. It was impressive - 28-29 tons/acre - but he has brought in corn silage-only yields as high as 34 tons/acre from the same field.

The silage is only about 1.5 percentage points higher in protein than corn silage. But Fink says his Holstein steers seem to like it better, and are gaining well this winter with corn-bean silage and haylage as their only feeds.

Corn-soybean silage is "a very good feed," Undersander agrees. At the time they're chopped for silage, soybean plants are 20-25% protein, and corn plants are around 8%, he says. So a 50-50 silage mixture of the two crops usually tests 16-18% protein, plus it's high in energy - almost a complete feed.

However, the silage won't be half soybeans if you grow the two crops together, says Undersander. While good corn silage hybrids can produce 8-10 tons of dry matter per acre (25-30 tons of 65%-moisture silage), the best soybeans only yield 2-3 tons of dry matter per acre. And they're not likely to do that well when interplanted with corn.

If you do plant corn and soybeans together, he recommends pairing a late-maturing vine-type soybean variety with a good silage hybrid, but not one that grows exceptionally tall.