Five Factors Can Impact Soybean Yield

By Karen Simon

At this time of year, as soybean growers sit down at their desks to plan their strategy for the 2010 growing season, every grower’s goal is to maximize yield.

Some growers say soybean yield capabilities have lagged behind the advancements achieved in developing high-yielding corn hybrids. In fact, soybean yields have increased nearly 32 percent nationwide in the last 20 years. Recent research has shown that management practices have as much, if not more, impact on yield than the actual variety.

“I’m not a big believer in the idea that we’ve hit a yield barrier,” says Vince Davis, soybean extension specialist at the University of Illinois, Urbana-Champaign. “While there might be a physiological limit to yield, we’re not there. Yield contests demonstrate that time and time again.”

Palle Pedersen, soybean extension agronomist at Iowa State University, agrees, explaining that there are five factors that contribute to maximizing soybean yield: water, nutrients, temperature, time and light.

“We have advantages and disadvantages for growing soybeans in Iowa,” says Pedersen. “Time is our disadvantage – because of our short growing season we must select varieties that fill as much of the growing season as possible. 2009 was a perfect example. In many parts of the state full-season varieties out-yielded the short-season varieties significantly. It’s tempting to plant short-season varieties, but you could be giving up yield.”

One of Pedersen’s projects funded by the soybean checkoff demonstrated that not maximizing the full growing season can cause a 5 to 9 percent yield reduction.

Davis says the past couple years, planting date has been a huge factor in Illinois, mostly driven by wet spring weather. “More than half the state isn’t getting planted until June,” he says. “After mid May we’re losing significant soybean yield. It’s difficult competing against corn for planter time, but we should be able to come up with a more efficient method to get planting accomplished in a more timely manner.

“Next year, our big challenge will be compaction,” Davis continues. “Most of the corn was mudded out this fall, causing significant compaction almost statewide. We won’t have the conditions to alleviate this until next fall at the earliest, and this can negatively impact soybean yield.”

Water is never a big issue for most locations in the Midwest, but Pedersen suggested that one of the best investments a grower can make is tiling the field. “Soybeans don’t like wet feet, and often we have too much water in the spring in Iowa,” he says. “Adequate drainage helps soybeans develop a taproot system that’s critical for maximum yield.”

Nutrients are also a very important factor when maximizing yield. “Plants need nutrients just like we need food,” says Pedersen. “If we reduce nutrients, it will hurt yield.”

There is also evidence that nutrient deficient fields cause other problems, like increased aphid populations.

“As farmers mine their soils they are relying on residual soil fertility – putting down fertilizer ahead of corn, not soybeans – which causes them to fall further behind when it comes to fertility on soybean fields,” says Davis.

Pedersen says that as long as enough fertilizer is applied prior to corn, it should be OK, but he urges growers to think about the nutrients that are being removed and look at soil sample analysis data in detail.

Temperature is another factor that impacts yield. However, this is not one growers can control. This year illustrated how important warm temperatures are to creating high yielding soybeans. There were many cool nights in July in Iowa and Illinois, which reduced pod set.

Light interception is also critical for soybean production. It has been demonstrated that there is a correlation between seasonal photosynthetic rate, biomass accumulation and yield. In other words, intercepting more light helps build more biomass and will increase yield. Planting early helps soybean plants get a head start by avoiding early stresses caused by disease or insect pressure or weed competition. Planting in narrow rows also helps close the canopy early to aid in light interception and building biomass during flowering and pod set, which is critical to yield.

“While these five variables are important, it all comes back to genetics,” says Pedersen. “Plant the right genetics early and in narrow row spacing gives the plant the right start and the foundation to battle potential street, as well as maximizing yield.”

According to Davis, the biggest mistake a grower can make is not investing the time to evaluate multiple sources of information and match needed traits to specific fields or conditions. “Fields were highly variable this year. The wet weather brought attention to the poor areas,” he says. “We need to think about what we can do to fix those areas prior to planting next year’s crop.”

High Yield Fields Demonstrate Possibilities

Cliff Mulder, a soybean grower from near Pella, Iowa, has had one of Palle Pedersen’s research studies on his farm for the past five years. He has been able to apply what he’s learned over the years, making changes to how he grows soybeans. He estimates he’s been able to increase his yields an average of 8 bu/acre.

The high-yield soybean research plots on Mulder’s farm averaged 104 bu/acre in 2009, but he stresses that this field had everything going for it, including a low soybean cyst nematode count. It had been planted to alfalfa in previous years.

“Whole field averages of 100 bu/acre may not be attainable, but 80 bu/acre yields, when looking at state averages are attainable, and that extra 30-40 bu/acre is all profit,” says Mulder. He adds that part of this yield increase can be attributed to better varieties, but management practices are also a factor.

What has he learned as a result of soybean checkoff funded research on his farm?

• Seed treatments (insecticide plus fungicide) do pay.
• Narrow rows (15 inch) increased yield.
• Adequate fertility is very important.
• High yields are achievable in no-till fields.
• Anything over 100,000 final planting population didn’t increase yield, so easy does it on the seed.
• Full-season varieties out-yielded mid-season varieties.

Finally, Mulder found yield is highly impacted by planting date. Planting the last week of April or the first week of May often provides the highest yield. Waiting until mid May will often reduce yield by 10 to 20 percent. “Plant early,” he says. “For years we planted corn and then soybeans, but we may want to rethink that.”

*Content Funded through the Soybean Checkoff