Soybeans are capable of producing yields of 100 bu. an acre or more, insists Richard Cooper, a long-time USDA soybean production researcher at the Ohio Agricultural Research and Development Center in Wooster, Ohio.
“Back in the 1960s, there was a short-lived national soybean yield contest in which the winning entries were around 100 bu./acre,” he says. “As we move into the 70s, rather than seeing yields increase, they began going down.”
The yield contest was discontinued, but the idea that such high yields, while possible, were eluding most producers, pushed Cooper to begin his high yield soybean studies.
“When we couldn’t reproduce the high yields in controlled research plots, we decided there had to be a limiting factor we hadn’t adequately identified,” he says.
What Cooper and his fellow researchers found was not one, but a number of factors that figure into soybean yield, most of which growers can control in one way or another. These factors include:
* Planting Date
* Fertility
* Seeding Rate and Depth
* Variety Selection
* Seed Quality
* Crop Rotation
* Tillage
* Weeds
* Diseases of roots, stems and leaves
* Insects
* Soybean Cyst Nematodes
* Water Availability
* Row Spacing
* Photosynthesis, which is influenced by a number of the above factors.
Early Planting is Critical
“You’re managing a photosynthesis system,” says Jim Beuerlein, an Ohio State University extension agronomist. “For maximum soybean yields, you must take advantage of as much sunlight as possible. If you can’t collect enough sunlight, you’re not going to make 100 bu., so the earlier you can get the crop in the ground and growing, the sooner this process can begin.”
“So the first step in growing high yielding soybeans, contrary to what we may believe, is to put as much emphasis on early planting with soybeans as we do with corn. We’re planting as early as possible after mid April, once soil temperature and moisture conditions allow it.” Beuerlein says. “When planting early, you need to plant shallow. My research planter is set to plant at 3/4 inch.”
Right Variety May Increase Yield 25%
Palle Pedersen, extension soybean agronomist at Iowa State University, says variety selection ranks high among limiting factors. “In 2003, Iowa State University Soybean Variety Trials show that selecting the right variety can boost final yield potential by with more than 25%,” he says.
If you’re not selecting varieties based on both yield and the ability to withstand the disease, insect, and nematode threats on your farm, you’re probably missing out on potential yield. Pedersen says mid-April is usually too early in Iowa, but agrees with the concept of planting as early as possible.
Pedersen says if you don’t know whether you have a problem with Soybean Cyst Nematodes (SCN), don’t just assume one way or the other. “You can have soil samples checked for nematodes by most reputable soil testing labs,” he says.
Where SCN is a problem, not managing properly for it can give you a yield loss of 40-50% in the worst scenarios, Pedersen believes.
“To produce high yields, you must manage diseases, too.” Beuerlein says.
Pedersen figures that diseases are probably the worst yield robbers in soybeans. “If you can see the disease symptoms, you’ve already lost 10% of yield potential. If you’ve got two or three diseases going, you could lose 20 to 30% of your yield without seeing signs of any of them,” Beuerlein believes.
Disease management can be as simple as planting resistant or tolerant varieties. But to push the yields as high as possible, it should also involve monitoring the crop for signs of root, stem, and foliar diseases and implementing control measures when necessary.
Crop rotation is important in managing SCN, most soybean diseases, and other pests as well. In most yield studies, soybeans do best when planted into soil where soybeans haven’t been produced for several years. A two-year corn-soybean rotation is worth about 10% in soybean yields compared to continuous soybean, by Pedersen’s analysis. Beuerlein agrees, adding: “A two year rotation is better than continuous soybeans, but a three-year or longer rotation is better. A couple of farmers I’ve worked with had been in continuous corn for more than 10 years.
The first year they put soybeans back on these fields, they saw yields of around 75 bu. per acre, while their adjacent neighbors, in a corn-bean rotation produced about 55 bu. an acre. The difference was most likely due to the lack of soil borne pathogens and nematode pressure.”
Design Fertility Program For Soybeans Only
Soybeans must be fed properly. That means a fertility program designed for soybeans rather than just allowing the crop to scavenge for remnants left over from corn or wheat production the previous year. Pedersen says if there’s not enough available nutrients to support a high yielding
soybean crop, you won’t have one. “Taking a soil test is the key.” Pedersen says.
Remember, too, that one of the ways soybeans gain access to sufficient nutrients is through symbiosis with rhizobial bacteria in their roots. Research shows little or no improvement in inoculating soybeans in some areas, providing the field has been in soybeans in the past five years. If you want to be sure, though, inoculating soybeans at planting will assure that there’s sufficient rhizobium population to fix sufficient nitrogen for the crop.
Narrow Rows Have A Better Chance
Soybeans do yield better as row spacing decreases, to a point. Variety, seeding rate, and disease pressure can influence this. For example, if conditions are right for sclerotinia (white mold), narrow rows can exacerbate the problem. “University of Wisconsin studies suggest,
however, that in some instances, yields are better with white mold in 7 in. rows than in 30 in. rows where the disease doesn’t occur,” Beuerlein says. “And if you plant in narrow rows and white mold doesn’t develop, yields will be much higher than in 30 in. rows.”
In Cooper’s high yield studies, plots planted in 30-in. rows never approached the 100-bu yield
target. Beuerlein attributes this to the close plant spacing necessary to achieve higher plant populations in wide rows. He says that the more of the field surface used by the crop, the less
competition there is between plants and the higher the yields can go.
Seeding rate must be adequate to produce an early, uniform crop canopy in order to intercept the maximum amount of sunlight for as long as possible during growth and seed
production stages.
“Seeding rate has almost nothing to do with yield, as long as you do this,” Beuerlein says. “If you’re planting in 7-in rows, anything over 80,000 plants is usually adequate, assuming they’re uniformly placed in the rows.”
Pedersen takes issue with this, noting that for the upper Midwest at least, the optimum seeding rate for 7-in. row spacing is close to 200,000 seeds per acre.
“Optimum seeding rate changes with row spacing and we have documented that extensively for many years”, Pedersen says.
Tillage, or a lack thereof, can be a factor in determining yields, although Pedersen says soybeans in general don’t respond to tillage. While tillage can produce a more uniform seedbed and help achieve uniform emergence, particularly in heavy soils, it can also discourage root rots. A fungicide treatment that controls early root and seedling diseases is almost always worthwhile in heavier poorly drained soils. “And while you might get by with lower rates of fungicide in some cases, you’ll need the full rate to help fight phytophthora root rot,” Beuerlein tells.
Some other considerations include soybean sudden death syndrome (SDS) and soybean aphids. SDS is almost always associated with cyst nematode, so if nematodes are a problem; select varieties and manage the crop to anticipate SDS.
Soybean aphids are around in most years, but it takes the right weather and crop conditions for them to develop into the type of problem they were in parts of the soybean production areas in 2003. Since not all areas were equally affected, the general consensus seems to be monitor populations and be prepared to spray if populations hit economic threshold levels.
Cooper has used irrigation to make sure moisture is not a limiting factor in his high yield study plots. Even so, Pedersen figures that moisture stress does not hurt yields on non-irrigated fields in most years in Iowa. However in 2003, he calculated using growth models that 15-18% of the maximum yield potential was lost because of the drought.
Even with all the suggestions made in this article, you can’t control the amount of sunlight reaching the crop early in the season. Cooper says that the more warm, cloudless days you have in April, May and early June, the closer you’ll come to reaching that 100-bu. goal.
And finally, growing a 100-bu. crop doesn’t guarantee you’ll bin a 100-bu. crop, Pedersen reminds. He says the final step is a timely harvest with a combine that’s set and operated properly to harvest as much of the crop as possible.