Although dramatic new yield records for soybeans have excited growers in recent years, yield increases for soybeans have not kept pace with those of corn (Figure 1). Over the last 25 years, US average corn yields have increased by 1.6% per year while soybeans have only achieved a 1.27% per year gain. Some experts suggest that this discrepancy exists because many growers do not apply the same level of management to their soybeans as to their corn.
Soybeans were once considered much easier to manage than corn. Because they were planted later, they didn't need fungicide seed treatments. In the Midwest states, post-emergence disease and insect control were seldom needed. Soil fertility, harvest and storage were easier than for corn. Although some of these advantages still apply, others have changed. For example, soybeans are now planted earlier, so seed treatments may provide a significant return. Increasing soybean aphid and bean leaf beetle problems mean that scouting and spraying may now be necessary. Spread of soybean cyst nematode, sudden death syndrome and other diseases have increased management requirements for soybeans. Growers still taking a "minimal management" approach to soybean production are likely missing a significant opportunity to increase their bottom line.
This Crop Insights will examine agronomic practices that may help increase soybean yields and profits. These include variety selection, planting practices (row width, planting date and seeding rate), soil fertility, crop rotation, weed control, use of inoculants and other practices. A second article in this series will address overcoming disease and insect obstacles to maximizing soybean yields.
Crop rotation is important in all crops to break disease and insect cycles and increase yield, and soybean is no exception. Diseases such as soybean cyst nematode, white mold, brown stem rot and sudden death syndrome survive in the soil or in crop residue, and readily attack a successive soybean crop. Most soybean diseases survive more than one or two years in the soil, so rotation does not eliminate the problem. But time away from soybeans diminishes the amount of disease inoculum available to infect the next crop, and thereby lessens its severity. For this reason, two or more years away from soybeans is preferable to just one, in terms of disease impact on the crop.
Rotation studies in Minnesota and Wisconsin showed that soybeans in a corn/soybean rotation yielded 8% more than continuous soybeans. These studies were conducted in good growing environments where moisture was not severely limiting. Soybeans following five years of continuous corn yielded 15 to 17% more than continuous soybeans.
Tillage has long been used to bury crop residue, prepare a seedbed and control weeds. Current planting equipment and herbicides now allow growers to achieve excellent soybean stand establishment and weed control with little or no tillage. No-till or reduced till practices can help minimize soil loss and increase organic matter levels that contribute to long-term productivity. Research studies have demonstrated that soybeans yields are similar across conventional till, minimum till and no-till. For this reason, growers can choose a tillage system that makes sense economically, environmentally and logistically, and focus on optimizing other management practices within that tillage system.
In this age of soybean varieties with the Roundup Ready® trait, the issue of timely weed control and its effect on soybean yield is often ignored. But if weeds compete with soybeans for moisture, light and nutrients during the critical development period from the second trifoliate stage to beginning flowering, yield may be reduced, even if weeds are ultimately controlled.
To prevent weed-induced yield losses, growers should control weeds in a timely manner. In some instances where well-timed applications of Roundup® branded herbicides may be jeopardized by workload demands or weather and field conditions, use of a pre-emergence herbicide may be justified. In other cases, two applications of Roundup may be required for timely weed control.
In some soybean fields, rank growth of winter or spring annual weeds can pose challenges at planting time and beyond. This dense weed growth often slows soil drying and warming in the spring and affects seedbed quality and crop establishment, including timely planting. Competition may also be a problem, as some of these weeds persist well into summer. A fall application program which includes a residual herbicide such as Canopy® XL is an excellent way to control emerged winter annuals and provide residual control of later-germinating winter and spring annuals.
Growers using "traditional" (non-sterile, peat-based) rhizobia inoculants have rarely seen an advantage in fields with a recent history of soybean production. Newer soybean inoculant products now offer several advantages over traditional products. The new formulations deliver high populations of bacteria, on the order of 10 to 100 times more than traditional products. Use of sterile carriers prevents competition from other bacteria, and the ability to adhere to the seed has been improved. Also, newly available rhizobia strains have demonstrated improved nitrogen-fixing ability in some studies. Finally, the addition of "extenders" prolongs the life of bacterial cells when inoculants are applied to seed long before planting or when seed is treated with a fungicide.
Pioneer and university¹ researchers have tested many of the "new" inoculant products over the last decade:
These positive research results should encourage growers to at least test these new inoculant products on their farms to determine if they merit more widespread usage. This applies even in fields with a recent history of soybean production, such as corn-soybean rotations.
No-till soybeans planted in high crop residue with cooler, wetter soils are expected to benefit the most from new inoculant products. Inoculant providers can advise growers on how to prevent reduced performance when using inoculants along with fungicide seed treatments. If a yield monitor will be used to harvest the proposed test plot, consider using the Pioneer split-planter method to conduct the comparison.
Good harvest practices, including timely harvest with a well-adjusted combine, prevent yield from being left in the field. Soybeans dry down very rapidly after physiological maturity, and pods readily lose and re-absorb moisture. After several cycles of wetting and drying, pods are predisposed to shatter. To prevent shattering losses, harvest soybeans the first time they reach 13 to 14% moisture. Close monitoring of seed moisture as soybeans dry down is important to accomplish timely harvest.
Correct concave clearance and cylinder (or rotor) speed are essential to proper threshing and prevention of machine losses. Closely follow your operator's manual for initial machine settings, then check losses and re-adjust if necessary. Changes in weather and/or crop conditions usually require combine adjustments.
ApronMaxx® and CruiserMaxx® are registered trademarks of a Syngenta Group Company.
R - Contains the Glyphosate Tolerant trait. Always follow grain marketing, stewardship practices and pesticide label directions. Varieties with the Glyphosate Tolerant trait (including those designated by the letter “R” in the product number) contain genes that confer tolerance to glyphosate herbicides. Glyphosate herbicides will kill crops that are not tolerant to glyphosate.