Soybean production is among the fastest rising of all organic crops grown in the Upper Southeast, but weed control has been an Achilles Heel in sustainable, consistent conservation grain production.

North Carolina State University Organic Crops Specialist Chris Horton says, “Organic animal products comprise the fastest growing sector of the organic food industry. This market has created a large demand for organic soybeans. The selling price for organic soybeans can be more than twice that of conventionally managed crops.”

Controlling weeds is critical to production of soybeans in an organic system and what growers have been doing over the past few years has only worked about half the time, Horton says.

The most often used weed management system among organic growers has been some combination of 3-4 ‘blind’ cultivations in which the soil is lightly disturbed and 2-3 in-row cultivations. At best this has been a 50-50 proposition, with no real answer to controlling perennial weeds in soybeans, he adds.

A more practical and efficacious approach is a multi-faceted one that includes use of cover crops and higher seeding rates to continuously shade out weeds.

A critical component of this approach is fall tillage, which Horton says disrupts development of perennial weeds. By following fall tillage with a cover crop, a grower can make the window of opportunity for perennial weeds very small.

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The North Carolina State researcher says rye has worked well in their tests with organic soybeans. “Keeping the cover crop well fertilized and growing strong throughout the winter and early spring is important. Then, come back, mash the cover crop down good and plant soybeans immediately,” he says.

Roller works at right growth stage

“A big question, Horton says, is can you kill a rye cover crop without herbicides?” As with most things in agriculture, the devil is in the details. If heavy rye cover is in the soft dough stage, and it’s rolled down, it will stay down, he adds.

“If rye is in the flowering stage, whether it stays down or not can go either way. If there is good news to failing, with killing rye cover crops, if you fail, you will know it soon,” Horton says.

If the rye is rolled too early, it can be a season long failure for the following organic crop. It doesn’t have to be that way, because the rye will eventually dye, but it will suck up a lot of moisture before it does. In years like last year, when too much moisture was a problem, missing the optimum rolling date was not a big problem.

Always roll the rye cover crop down in one direction. Growers should expect some lodging problems, but so far in three years of testing, it hasn’t been a big problem with rye cover crops ahead of soybeans,” he says.

With seeding rates on rye up to 90 pounds per acre, getting soybean stands in the heavy mulch can be an issue, but Horton says that even losing some soybeans in the mulch has not significantly reduced yields in his three years of testing.

Getting a good, early stand of soybeans in the rye mulch is critical to good weed control and ultimately in getting a good soybean yield.

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How soybeans are planted into rye mulch can have a big impact on weed control. Weeds find a place to grow like water finds a leak on a roof — you don’t always know why, but it always seems to happen. A common place for weeds to get a start is in the slices made in the soil by row cleaners.

“You don’t ever want to use row cleaners in a heavy mulch cover crop, because that’s the ideal place for the most damaging weeds to get a start,” Horton says. “Regardless of what a machinery dealer tells you, the slicing coulter must have a smooth edge,” he adds.

Fluted coulter can cause problems

Using a fluted coulter, or any kind of rippling can tuck the rye into the seed trench. Then, the seed goes right in on top of the rye, and that’s never good.

An often occurring problem in planting crops under heavy mulches is development of lateral roots. In such an environment, roots tend to spread out rather than go vertical in search of moisture and nutrients.

Anchoring of soybean plants in the soil is an ongoing challenge, even when most other things are right. “Anything the grower can do to promote vertical root development is going to be a plus in the overall weed management program,” he says.

“For example, a bubble coulter, commonly used in conventional production, promotes side wall compaction of soil, which promotes lateral root development. Working against a good anchoring system for no-till, organic soybeans is going to hurt yields,” he adds.

Another part of the North Carolina State organic weed management system is increased soybean seeding rates. If more seed means more plants and more plants mean more shading out of weeds, then it can be an effective weed management tool.

Over several years and multiple locations in North Carolina, Jim Dunphy, a North Carolina State University soybean specialist, has demonstrated that soybean yield does not significantly increase with plant densities exceeding 100,000 plants per acre.

This is the case in conventional production with herbicide use, but for organic soybean producers, increased seeding rates improve early soybean canopy density, which shades out weeds in the early stages of weed competition.

Compared with the cost of genetically modified organism (GMO) seed technology fees, organic soybean producers can increase seeding rates with much less of a negative impact on economic return.

The North Carolina State Organic Cropping Research program investigated the planting rates of 75,000, 125,000, 175,000, and 225,000 seeds per acre for Hutcheson, a Maturity Group V soybean variety at three locations over two years.

At all three locations, organic soybean yields were greatest with the highest planting rate of 225,000 seeds per acre.

At the Goldsboro location, soybean yield with conventional weed management was greater than soybean yields with organic weed management. However, at the Kinston and Plymouth locations, organic and conventional soybean yields were not statistically different.

Horton stresses that these results are likely to be directly opposite of conventionally planted soybeans in most cases.

Conventional growers have the benefit of chemical herbicides, which generally do what they are designed to do — kill weeds. Unfortunately, to be certified and sell organic soybeans for a premium price, synthetic herbicides are not allowed.

Dealing with the ‘Detail Devils’ can be frustrating for organic growers, but reward for their frustrations can be significant in terms of premium prices for their crops.

For those contemplating trying a few acres of organic soybeans, Horton says start small and tweak the weed control system until it works, then begin increasing acreage.

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