Maximizing Soybean Yields by Improving Agronomic Practices

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Crop Insights by Steve Butzen, Agronomy Information Manager

Summary

Growing soybeans requires more management today than 20 years ago due to earlier planting and increased disease and insect pressure.
Matching soybean varieties to the specific requirements of individual fields is key to maximizing yields. Testing and quickly ramping up acres of top new varieties can also have a significant impact on overall farm yields.
Planting practices, including row width, planting date and seeding rate may have a significant effect on yield. Make decisions based on research results and local factors.
A soybean crop removes 75% as much P and over 50% more K than a corn crop. Soil test and apply fertilizer and lime as required.
Crop rotation is important in soybean production to break disease and insect cycles and increase yields.
Timely weed control can prevent competition with the crop. If weeds compete during critical soybean stages, yield will be lost even though weeds are eventually controlled.
New sterile-carrier inoculants have increased yields in research studies and may merit testing on your farm.
Consider evaluating new production methods in side-by-side plots before using them more broadly.

Introduction

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.

**Figure 1**. U.S. average corn and soybean yields, 1984-2008. Source: USDA.

Variety Selection for Top Yields

Right product on the right acre - Matching soybean varieties to the specific requirements of individual fields is a core practice for maximizing soybean yields. Soil type, tillage system, drainage, geographic location, expected rainfall, potential diseases, and other local factors must all be accounted for in choosing an appropriate variety. Implicit in any variety consideration is the expectation of high yield potential, good standability, and ability to withstand environmental stresses. But in addition, resistance to specific races of SCN, resistance or field tolerance to Phytophthora root rot, other diseases, or iron deficiency chlorosis may be essential to achieving high soybean yields in a particular field. Your local Pioneer sales professional can help you select the appropriate soybean varieties for your individual fields.

Newest varieties - Soybean breeders at Pioneer make yield gains and agronomic improvements every year, using new genetic tools such as Accelerated Yield Technology (AYT) as well as marker-assisted selection. The new Y-series soybeans developed with these technologies represent a step-change in soybean performance. More than 1,800 on-farm comparisons have demonstrated a 5% average yield advantage for these varieties against key competitor products, with some varieties yielding 6 to 10% better than their competitors. Sampling top new varieties each year and ramping these up to significant acreages as soon as possible can have a significant impact on overall farm yields.

Comparing variety performance - When comparing the performance of competing products, it is important to include only products of similar maturity and trait package. Comparing unlike maturities usually results in an advantage for the later variety when both are adapted. Comparing varieties that don't both contain the traits demanded by the field environment can be misleading and futile.

Varieties compared should have the same herbicide resistance, SCN resistance, similarity of other critical traits, and identical maturity. Maturity ratings sometimes vary from company to company, so make sure you have reliable information. After narrowing comparisons to similar products, evaluate performance information from multiple environments to determine the best variety. Ideally, at least two years of data at 10 or more locations should be included. Varieties that win in multiple, diverse environments are more likely to win in next season's unknown environment.

Soybean variety showing SCN resistance and iron deficiency chlorosis tolerance demonstrates the value of variety selection.

Variety at bottom-right with SCN resistance and iron deficiency chlorosis tolerance demonstrates the value of variety selection.

Planting Practices

Planting practices are some of the most important aspects of crop production under the control of the grower. Selection of row width and planting date can affect canopy closure, disease development and yield. Careful monitoring of planting rate, planting depth and seed furrow closure can help ensure a uniform stand. On rough seedbeds, planting speed can also impact uniform stand establishment.

Row Width

Numerous studies over many years have demonstrated an advantage for narrow-row (less than 30-inch) soybeans. Pioneer research over a three-year period from 16 sites showed a 5% advantage for 7-inch rows over 30-inch rows. 15-inch rows yielded almost the same as the 7-inch rows. University studies have generally demonstrated a greater advantage for narrow rows in northern states, and little or no advantage in southern states.

Researchers believe that quicker canopy development is responsible for higher yields in narrow rows. If full canopy develops by flowering, maximum light interception by the crop is possible during the important pod-formation and pod-filling stages. However, this same characteristic has led to higher incidence of white mold in northern areas. Consequently, many growers there have settled on 15-inch rather than 7-inch rows as a practical compromise. The 15-inch rows also allow growers to use a planter instead of a drill, which usually results in more uniform seed placement.

Planting Date

Recent research has shown that soybeans can tolerate a reasonably wide window of planting dates and still produce top yields (Figure 2). However, outside the optimum window, yields are significantly lower and less stable. In Figure 2, note that 6 of the 7 highest yields (those near or above 70 bu/acre) occurred at planting dates from April 21 to May 9 in this study.

Previous Pioneer studies conducted over a 13-year period also showed that highest soybean yields were usually obtained with late-April to early or mid-May plantings. Penalties for planting after June 1 ranged from two to four bushels per week in central states (Nebraska, Iowa and Indiana), and from one to six bushels per week in Minnesota.

Planting very late shortens the soybean's vegetative growth period, resulting in smaller plants and later canopy closure.

Though planting dates for soybeans have been trending earlier, there is little or no research that shows an advantage for planting soybeans prior to April 20 in the Midwest US. Farther north, (e.g., Minnesota, Wisconsin and northern-most Iowa), planting soybeans prior to May 1 is usually not recommended. Planting earlier exposes the crop to a number of risks, including frost, damping off, crusting, delayed emergence, and higher disease incidence. Diseases that increase with early planting include Phytophthora, sudden death syndrome, and a virus vectored by bean leaf beetle - the bean pod mottle virus.

Seeding Rate

Seeding rate decisions should be based on soybean seed cost, grain market price and individual production practices and seedbed conditions. To help determine optimum seeding rate recommendations, Pioneer Agronomy Sciences researchers conducted studies from 2006 to 2008. Seeding rate effects were evaluated with and without fungicide and insecticide seed treatments. Based on these studies, optimum economic seeding rates for 30-inch rows are shown in Table 1 (higher rates would be needed for 15-inch or drilled soybeans). Consider these values only as a starting point for seeding rate decisions, and adjust based on local soil type, residue levels, planting date, etc.

Table 1 . Approximate economic optimum seeding rates/acre for 30-inch rows, using various seed costs and market prices*.

Approximate economic optimum soybean seeding rates/acre for 30-inch rows. — * Seed treatment cost is included in seed cost/unit.

It is commonly understood that soybeans have a greater ability than corn to compensate for reduced stands. However, too much reliance on this ability can lead to poor stands and the need to replant in some situations. Higher seeding rates may need to be maintained to help prevent potential yield reductions or replanting when seedbed conditions, weather or pests are likely to reduce soybean stands.

Soil Fertility

Phosphorus and Potassium. Many soybean producers depend on residual corn fertility to supply nutrients to their soybean crop. When soils are routinely maintained at high or very high levels of P and K, this may be a safe strategy, but often it is not. Some extension soil fertility specialists have indicated that soil test levels of P and K in their states are often not adequate for optimum soybean yields.

A 50 bu/acre soybean crop would remove about 42 lbs P₂O₅ and 65 lbs K₂O from the soil in the grain². This is only 25% less phosphorus than a 150 bu/acre corn crop removes. But even more surprising to some growers, it is 60% more potassium than a 150 bu/acre corn crop removes in the grain. Soil testing can determine if field levels are adequate to supply these or other required amounts.

Soil pH. Soybean growers should also be aware of their soil pH. A soil pH of 6.0 to 7.0 is considered optimum for soybean production. Liming acid soils or utilizing varieties with good iron deficiency chlorosis scores on high pH soils will help prevent yield reduction.

Nitrogen. Soybeans are high in protein and therefore in nitrogen. In fact, soybeans remove about 4 lbs of N from the soil for each bushel of grain produced. This compares to less than a lb of N removed per bushel of corn grain produced. The soybean plant's N needs are usually supplied by nitrogen-fixing rhizobia bacteria associated with its roots, so adding nitrogen fertilizer is usually unnecessary under normal production practices.

However, research in some irrigated, high-yield environments has demonstrated that applications of N during the pod or seed stages of soybean development can increase yield. Lower yielding, non-irrigated sites have typically not shown an economic benefit from this practice. N applications during vegetative soybean growth stages have been shown to reduce early nodulation, and were not beneficial to yield.

Foliar Fertilizer, Banding. In studies conducted in Iowa, foliar feeding increased yields only 15 to 20% of the time. However, it may be useful when soil nutrients are inadequately supplied, such as production on sandy soils or high-yielding irrigated fields.

Studies in Iowa and Minnesota with banding fertilizer close to the row have not proven beneficial. On the contrary, stands were reduced and yields were not improved by this practice.

Crop Rotation

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.

Other Practices for Increasing Soybean Yields

Tillage

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.

Weed Control

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.

Soybean Inoculants

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:

In 2008, Pioneer studies in 7 locations showed a 1.0 to 1.9 bu/acre advantage for inoculant products. In 41 Pioneer comparisons in the early 2000's, the average yield advantage for inoculants vs. un-inoculated seed was 2 bu/acre.
Ohio State University also reported a 2 bu/acre average advantage in 64 trials.
In 155 product evaluations over 7 years, North Dakota State University found a 2.7 bu/acre advantage.
Purdue University reported an average advantage of 1.0 bu/acre for inoculants in 11 years of testing.
Other universities have also shown positive responses for inoculants.

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.

Harvest Practices

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.

Testing New Practices on Your Farm

An assortment of management options have been presented here for soybean producers striving for maximum yields. Many growers will want to test various planting practices, seed treatments or inoculants, fertility options or other factors prior to using them in full-scale production. Below are some tips for conducting your own treatment comparisons:

Identify objectives: what do you want to measure and why?
Design treatments to represent a single specific practice, and be careful to control other variables.
Pick a uniform site to conduct the comparison, but do not always choose the most productive soil by default.
Be sure the plot is large enough to identify small differences, but not so large as to make weighing it difficult.
Position comparisons in the field so all treatments have an equal chance. Replicate if possible.
Measure yield, but also record other supporting observations.
Do not depend solely on results from one location and one year. When possible, consider results over several years and locations.
To compare two treatments across an entire field or field area, use Pioneer's split-planter comparison tool (uses a yield monitor at harvest.)

¹Beuerlein, J. 2006. Soybean Inoculation; its Science, Use and Performance

²Int'l Plant Nutrition Institute 2005.