There’s no doubt planting soybeans early can increase yield, but we’re still in the learning stages, says Farm Journal Field Agronomist Ken Ferrie. “The purpose of planting earlier is to cause soybean plants to get big enough to start flowering before the summer solstice,” he explains. “That lengthens the period between reproductive stages and increases starch accumulation, producing higher yield.”
With early planting, some principles remain the same as with traditional planting dates, but others might need to change. Keep the following in mind as you select varieties, row spacing and population for early planted soybeans:
1. Choose varieties that reduce weather risk. Adverse weather conditions when plants are at the R1 or R2 growth stage result in flower and pod abortion. At the R3 and R4 growth stages, the result will be two-bean or three-bean pods with flat sections. At R5 and R6, drought will cause smaller beans.
“If early planted soybeans run into stress at any of these stages, they might produce fewer flowers or pods or smaller beans; so you might not see the expected yield benefit,” Ferrie explains.
“You can mitigate against adverse weather by planting multiple maturity ranges, so you never have your entire soybean crop at the same growth stage at the same time,” Ferrie continues. “That will smooth out harvest too — no over-ripe soybeans spilling out of pods and no wasted days waiting for all the soybeans to mature.”
2. Plant later maturities first. If you plant multiple corn hybrids of the same maturity, you can mitigate weather risk by staggering planting dates. That doesn’t work with soybeans because they flower based on the length of night.
“When you plant soybeans early for pre-solstice flowering, night length is the key,” Ferrie says. “A group 4 soybean needs a longer night [more hours of darkness] than a group 2 to begin flowering. Before the solstice, nights get shorter every day; after the solstice, they get longer.
”Based on our observations, we must plant full-season soybeans early enough to reach at least the three-trifoliate stage before the pre-solstice nights get too short. There’s more time to get your short-season varieties planted early because they need fewer hours of darkness to trigger flowering.”
If early planted soybeans don’t get big enough to start flowering before the solstice, they will produce tall plants, Ferrie explains. “The plants will continue to grow, adding vegetative stages until they reach R5 because flowering will be delayed until the nights get long enough after the solstice to trigger the reproductive stage. Tall plants don’t correlate with a yield increase. In two decades of studies, we have seen shoulder-high plants lodge more often and yield less than waist-high plants.”
3. Early planted soybeans might behave differently. With indeterminate soybeans, flowering sets the clock for maturity. “Once plants reach the R1 stage and begin flowering, no matter which side of the solstice, the plants will continue to grow until they reach R5, and then they will stop,” Ferrie says. “An early maturing soybean planted earlier than normal might end up shorter than you expect. If it flowers in early May, it might stretch the interval between reproductive stages from the normal five to seven days to 10 days or more, but it still will stop growing when it reaches the R5 stage.”
In Ferrie’s 2019 studies, a group 2.2 variety was 1½' shorter than a group 4.1 variety when both were planted on April 23. When both were planted on June 3, the two varieties were much closer in size at harvest.
When planted on April 23, both varieties endured slow growing conditions — only 17 growing degree units during that month. “The group 4.1 plants did not get large enough to trigger flowering before the nights got too short prior to the solstice,” Ferrie says.
“But the group 2.2 plants did. Both plants reached the V3 stage at the same time. However, because the group 2.2 plants flowered early, they reached the R5 stage sooner and stopped growing. The group 4.1 variety flowered later, after the solstice, and continued growing later into the season. Once soybeans start flowering, they will finish vegetative growth in 30 to 50 days.”
4. We’re still learning about weed control with early planting. With normal planting dates, narrow rows and higher populations improve weed control. They produce a faster crop canopy, shading out weeds just as most herbicides begin to run out of steam. “In our studies, wide rows and lower populations resulted in more weed escapes,” Ferrie says.
“Our studies have shown with normal planting dates, narrow rows close their canopy seven to 10 days sooner than wide rows,” he explains. “In our early planting studies, wide rows closed their canopy about the same time as narrow rows planted later.”
But, along with weed control benefits, early canopy closure might have a detrimental effect on yield.
“The fact early planted soybeans set pods early but take more days to close the canopy might be the reason early planted soybeans hang onto more of the lower pods, which add to yield,” Ferrie says. “These are the first pods to develop on the plant. If they get far enough along before the rows close and the lower trifoliates fall off, the plant will maintain those pods. But if they are ½" or less when the trifoliates drop off, those pods usually abort.
“With soybeans’ amazing knack to branch and set pods based on their sunlight-capturing ability, we must go back to the drawing board with row spacing and population recommendations for early planted soybeans,” Ferrie concludes. “We will learn more as we continue our early planting studies.”
High-Yield Soybeans Series
Adjusting practices to grow higher soybean yields isn’t simple. There are technology and management practices to master. To help you put everything together, visit AgWeb.com/high-yield-soybeans