Breaking the bean yield barrier
Each winter I get the question from farmers, “How do I break the yield barrier and increase my soybean yields?” And each time I start formulating my answer by trying to figure out what practices are being used and reinforcing best management practices that are known to provide yield responses. For this column I decided to scour the Web to see if somewhere, someone has a different answer or different idea than what is currently being used to try to boost bean yields.
Across nearly a dozen reputable sources, there is agreement that soybean management should focus on variety selection, fertility, planting date, row spacing, seeding rate and pest management. Some sources include other things such as no-tillage, irrigation, seed treatments, inoculants, soil compaction and location. Again, I say, a “silver bullet” to higher soybean yields has not been found. Having more products, available at great cost, hasn’t provided a clear answer to greatly improving soybean yields. I believe it’s just as important now as always to make sure we’ve grabbed the low-hanging yields while we stretch for the higher-yield levels.
• Variety selection. First and foremost, I can’t stress enough the importance of variety selection. Yield performance should be the No. 1 variety selection criteria, but it should not be the only criteria. Failing to select high-yield varieties will not end with record yields at harvest. To that point, yield performance must be looked at from many locations and, preferably, over a couple years to ensure genetics that are robust enough to perform in variable weather conditions.
Planting a full-season soybean variety pays dividends more frequently than not. In Iowa that may mean a 4.0 maturity group soybean in southern Iowa or a 3.0 maturity group soybean in northern Iowa.
Choosing bean varieties resistant to soybean cyst nematode and those with resistance or tolerance to sudden death syndrome, phytophthora and iron deficiency chlorosis will be essential to achieving the yield potential available. SCN is often cited as the biggest yield robber in Iowa. Some years when weather and soil conditions are conducive to SDS development, this disease claims even more yield than SCN.
• Fertilizer program. Soil fertility can be a big decision with costly implications. It’s still customary in Iowa to apply a two-year phosphorus and potassium rate prior to corn in a corn-soybean rotation. Why should soybeans play second fiddle to corn? With two-year application rates, the corn has first opportunity to use those nutrients. This places corn in a non-limiting environment. It doesn’t sound bad, and it even looks good from an application cost efficiency standpoint.
But in years with perfect corn-growing conditions where yields exceed expectations, the question arises as to whether there is enough phosphorus and potassium left in the soil to provide a non-limiting environment for soybean production? To complicate that issue, soybeans have a higher potassium requirement than corn. Where possible, you should use a sound two- or three-year soil sampling program and make single-year phosphorus and potassium applications, based on soil tests.
• Inoculants, foliar fertilizer. Both of these management practices are probably better discussed in a separate article. In short, data presently available from Antonio Mallarino, ISU Extension soil fertility specialist, suggest soybean response to micronutrient applications do not pay for the product being used.
What about inoculants? Extension specialists still rely on the recommendation that inoculants are best used when a field has been out of soybean production for five or more years, except if there are extremely dry conditions, or if flooded conditions have occurred.
• Early planting. Planting in late April or early May in Iowa usually maximizes soybean yield. Research conducted by ISU shows the best time to plant soybeans in Iowa is the last week of April in the southern two-thirds of the state and first week of May in the northern third, if soil conditions are suitable.
Research from Iowa and neighboring states has suggested a yield loss of 0.2 to 0.7 bushel per acre per day when planting is delayed past May 10. Greater penalties occur the later the crop is planted. But earlier planting dates don’t guarantee higher yields. Fluctuations in rainfall and temperature can cause yield variations year to year.
Capturing sunlight should be a high priority for higher yields. Planting soybeans between April 25 and May 15 is a first step to eliminating sunlight hitting the soil.
• Narrower rows. The next step is to move to narrower row spacing. Early vegetative growth in 15-inch or narrower rows will result in more sunlight being captured by soybean leaves and less reaching the soil surface with the added benefit of soybean plants being more competitive against weed populations.
Moving from a 30-inch to 15-inch row spacing can result, on average, in a yield increase of 3.7 bushels per acre. Generally, a 15-inch row spacing when combined with earlier planting dates results in greater yield response than 30-inch rows. We see bigger yield responses in narrow rows with early planting dates and smaller yield responses with later planting dates.
• Plant population. Seeding rate and plant stand get a lot of attention. For the last five to 10 years the recommendation from ISU Extension has been to plant between 125,000 and 140,000 seeds per acre with the intent of ensuring a final stand of 100,000 plants per acre or more.
This recommendation has been reinforced by a project funded by the United Soybean Board across soybean-producing states, from Minnesota and Wisconsin down to Louisiana. That study confirmed higher seeding rates rarely provide yield benefits to cover the cost of more seed.
• Pest management. Whether it is weed, insect or disease control, managing pests is highly important to protecting soybean yield potential. Weeds rob soybeans of nutrients, water and sunlight. Bigger weeds compete at a greater level than smaller weeds. This makes it even more important to make herbicide applications timely, as well as use products that provide some residual activity.
Foliar insecticides and fungicides should be used prudently to maintain product effectiveness. Scouting fields to ensure damaging levels of insects or disease severity are present will provide the best indication of whether a product will have a large, small or no yield response.
Growing soybeans hasn’t gotten easier in recent years. It is still important to look at best management practices that are known to result in positive yield responses and figure out how to tweak soybean production systems to reach higher yield levels.
Licht is an ISU Extension field agronomist at Nevada. Email [email protected].