Put Soybeans Back Into Your Crop Rotation

Put Soybeans Back Into Your Crop Rotation

Is it time to increase the ratio of soybean acres to corn acres in your rotation?

By Mark Johnson

Editor's Note: Mark Johnson is the Iowa State University Extension field agronomist for north central Iowa. He can be contacted by email at [email protected]; his phone number is 515-979-9578.

In recent years the profitability of corn has been lucrative and has led many Iowa farmers to choose to plant more acres to corn and less to soybeans. In fact some farmers in many parts of the state have gone to planting all their crop acres to corn the last few years. A contributing factor has been that many producers felt their corn yields were continuing to increase while their soybean yields had leveled off.

BENEFITS OF BEANS ADD UP: It may be time to look at adding soybeans back into your crop rotation. You can expect disease pressure to be less in fields following continuous corn. Shifting to soybeans also helps eliminate problems such as increased corn rootworm pressure and resistant rootworm populations. Corn following soybeans yields roughly 10% more than continuous corn; and soybeans following continuous corn typically yield more than rotated beans—so it's a "win-win" in terms of yields for both corn and beans.

As with any monoculture, this has brought on some challenges. We now have more corn disease, more insect pressure and more weed pressure on the continuous corn acres. Herbicide rotation is more difficult with continuous corn. And continuous corn provides a great environment for many diseases.

Some of these diseases show up each spring in the corn seedlings. Some are foliar diseases that are more problematic in corn-following-corn because the pathogen survives in the corn residue. And some cause various stalk or ear problems in the fall at crop maturity. Many of these diseases may require the application of foliar fungicides to minimize damage. There are similar problems with regard to more continuous corn and an increase in insect pests.

Corn has seen dramatic increases in corn rootworm pressure the past several years

BEAN YIELD BOOST: Iowa State University studies have shown that putting beans in rows narrower than 30 inches can improve yield potential. This data was the result of studies conducted by Pella Pedersen, former Iowa State University Extension soybean agronomist. Pedersen and his graduate students conducted these studies in fields in eastern, central and western Iowa, in cooperation with the Iowa Soybean Association.

During the last few years many producers are seeing resistance to corn rootworm (CRW) Bt traits in corn hybrids. This has become a large problem and each year in any given problem field, it becomes more economically damaging. This is costing producers not only yield, but also wear and time on the combine.~~~PAGE_BREAK_HERE~~~

Some farmers have responded to this situation by planting corn that has the Bt CRW resistance traits and also applying a soil insecticide with that corn—a two-pronged approach to rootworm control. And now, researchers have documented cross resistance; that is, resistance to more than one Bt event.

All three pest problems (diseases, insects, weeds) affect both the cost of production and yields negatively. In turn, they affect profitability negatively.

This is a good time to take a hard look at adding soybean acres back into your crop rotation

If a field has been out of soybean production for several years, and especially if it has been 5 or more years since soybeans were grown, the farmer would likely be pleasantly surprised by the soybean yield. Routinely, farmers find those fields that have been in continuous corn for several years or more to be their highest yielding soybean fields when they rotate them back to beans. Soybean cyst nematode numbers would have decreased dramatically (but not totally disappeared) if numerous years of corn had been grown consecutively.

Whether a field has been out of soybean production for several years or not, the best way to improve soybean yields in the upper Midwest is to narrow the row width to 20 inches or less, preferably to 15-inch rows. Iowa State University studies show an average 4.5 bushels-per-acre yield advantage for 15-inch rows over 30-inch rows. See Figure 1.

Studies in Minnesota, Wisconsin, Michigan, Indiana, Illinois, Kentucky and Kansas have also all shown similar results to narrow rows, which is defined as 20-inches or less when compared to 30-inch row spacing. The goal is to have 90% to 95% light interception at the R1 growth stage of the soybean plant. You want to have row closure by the time the soybean plant flowers or shortly after flowering.

A new multi-state state, multi-year study has evaluated ways to increase soybean yield; it shows that narrow rows add bushels

Chad Lee, professor and Extension agronomist at the University of Kentucky, gave a presentation on "How To Increase Soybean Yields" when he spoke at the recent annual Integrated Crop Management Conference at Iowa State University Dec. 4 and 5. He collaborated with colleagues at 9 other universities in a multi-state, multi-year study to find the top ways to increase soybean yield.~~~PAGE_BREAK_HERE~~~

At the recent ICM conference at ISU in Ames, Lee showed results where he separated out the northern states and in each case, narrowing of the rows contributed more to yield increase than any other practice. That includes but is not limited to, the use of fungicides, insecticides, foliar feeding, use of inoculant and additional soil fertilization above and beyond normal recommendations.

Those studies also found no advantage to higher stand populations than 100,000 plants per acre actual stand, nor any advantage to adding nitrogen fertilization.

One of the best ways to control corn rootworm is to grow soybeans; rootworms must feed on corn roots to survive

If a farmer has had severe corn rootworm injury during the last year or two in his continuous corn fields, taking those problem fields out of corn for a single year will allow a "reset." That is, the corn rootworm larvae will not survive without corn roots so if you plant beans in that field in 2014 there will not be a population of rootworm larvae in that field in 2015. The field in 2015 would not need any CRW Bt trait corn planted and would not need a soil insecticide applied.

Planting the field to beans in 2014 would also allow that farmer's cost of production in 2015 to be lower than he has experienced the last couple of years. In addition to lowering his costs, his yields would go up and his harvest would go more smoothly.

Crop budgets for 2014 are showing Iowa soybean production to be as profitable as corn production

The past few months saw the price of corn drop more than the price of soybeans and the futures market prices reflect the same thing. So as we look to 2014, we have a new scenario to consider.

Using projected grain prices for 2014, even if relative yield levels remain constant, crop budgets are showing Iowa soybean production to be as profitable as corn production, in many cases for 2014. When you add in the crop rotation effect discussed above, and also the opportunity to rotate herbicides—between corn and beans—the balance tips even more in favor of soybean acres in 2014.

Farmers who have moved away from a corn-soybean or corn-corn-soybean rotation will want to take a close look at crop budgets this winter and make a sound business decision as to whether it is time to plant more acres to soybeans in 2014.

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