A seven-year University of Arkansas rotation trial demonstrates how farmers can increase soybean yields by incorporating corn or grain sorghum into that rotation.
During the University of Arkansas' online corn field day, Jason Kelley, Extension wheat and feed grains agronomist for the University of Arkansas System Division of Agriculture, spoke on a crop rotation project that has been in the making since 2013.
"This unique project, funded by the Arkansas Corn and Grain Sorghum Promotion Board, the Arkansas Soybean Promotion Board, and the Arkansas Wheat Promotion Board, started in 2013," Kelley said. "If you think back over the last 30 years or so, a lot of things have changed with crop rotation. Thinking about soybean rotations, we had a lot of wheat and double-crop soybeans 30 years ago."
Rotation for better yields
Today, farmers typically plant earlier-maturing soybeans as soon as possible in April.
"We went from a double-crop system to an early-planted soybean system with very little wheat," Kelley said. "At the same time, our corn acres have dramatically increased. We went from 100,000 acres roughly 30 years ago, all the way up to 650,000 this year in Arkansas. Some years, we even have more than that."
A lot of growers also incorporate corn into their rotation system.
"The project aimed to verify whether we were making the right decision by having an early-planted soybean system and including corn," he said. "We also wanted to know how this translated yield-wise and economically."
The project studied eight rotations. One rotation is a continuous, early-planted soybean rotation, which is furrow-irrigated and planted in mid-April.
"We also have some plots with continuous corn for seven years now, and another plot is a rotation of soybean one year, corn the next," Kelley said. "There are also plots with wheat followed by double-crop soybeans, and we have grain sorghum followed by soybeans in other rotations as well as some other plots with combinations of wheat in the rotation.
"The highlights of this project include the yield and economic response of including corn or grain sorghum on early-planted soybeans because it's pretty dramatic."
The continuous soybean plots averaged about 54 bushels. However, for six years, the yields went up 6 to 8 bushels on average each year for the early-planted soybeans followed by corn or grain sorghum.
"That is pretty dramatic when you think 6 to 8 bushels with soybeans being $8 to $10 a bushel," he said. "That's a nice amount of money to gain just from crop rotation."
50/50 rotation mix
Looking at the economics over six years out of the eight rotations, the one on the top of the list is corn followed by early-planted soybeans, a 50/50 mix.
"Corn one year with early-planted soybeans the next year is economically the highest return rotation," Kelley said. "Whereas, the rotations with wheat or grain sorghum, the economics at $3 a bushel for grain sorghum weren't promising economically. However, these results could be a matter of bad yields or prices in a particular year.
"Also, we may have had good yields, but if we don't have good prices along with it, the economic return may not be as great as we would like."
After harvest each year, the team takes soil samples out of every plot in the fields to track nutritional changes over time, such as phosphorus and potassium.
"We follow the current University of Arkansas recommendations for fertilizer, and regardless of the rotation, we did not see a big change one way or another as far as increasing or decreasing the levels of phosphorus and potassium," he said.
"The one exception is zinc. In corn, we're putting 10 pounds of zinc per acre each year corn was planted. The zinc levels were very low at the beginning of the trial, with the soil pH around 7, so we needed zinc, and over time with zinc sulfate applications, those levels have increased to optimum levels since we first started this trial."
Soybean cyst nematode, pigweed
The team also monitored soybean cyst nematode during the duration of the trial.
"We have a lot of nematodes in the trial field," Kelley said. "Other areas of the state have root-knot nematodes, but we only have the soybean cyst nematode. When we started this trial, we had levels that were high in most plots. By imposing these rotations over six years, that has changed."
Where corn or grain sorghum was incorporated, the soybean cyst nematode numbers went down considerably, which has most likely helped in increasing the soybean yields.
"One other aspect of rotations is weed control," he said. "Across all the plots in the trial this year, we had very few weeds, but pigweed is our main nemesis."
Over time, the pigweed numbers have gradually been reduced thanks to corn rotation.
"We have a lot of options in-season to control pigweed in corn, especially if we use a two-pass program with some herbicide applied at planting and a post-emergence later," Kelley said. "If we do that, we start clean, and we often have season-long control.
"I see a lot of fields at the end of the year with pigweeds that go to seed, but in this situation, as soon as we harvest the corn, we either spray Gramoxone to kill any escape pigweeds, or we use tillage."
Using these methods, their pigweed numbers went down. However, in the continuous soybean plots as a part of the trial, there are significantly more pigweeds in those plots versus ones that have been rotated.
"Added weed control is another advantage to rotations," Kelley said. "However, we haven't seen a difference in disease pressure between the rotated plots and non-rotated. I can't say whether the rotation is helping or hurting us there.
"Also keep in mind, we can increase yields, but that doesn't necessarily mean we increase profitability or overall economic returns, which is dictated by grain prices. If we have a good grain price in combination with a yield increase, that is ultimately what we like to see."