Modified relay intercropping of soybeans into wheat is a very versatile system across many row spacings. Over the past 17 years of intercropping at OSU Unger Farm, three rows spacings have been used. Ten inch spacing was used for the first 15 years, but this period also included three years of 15 inch work. The last two years, twin row wheat has been grown. The variations in row spacing show the flexibility of wheat to respond to various stresses. According to the Ohio Agronomy Guide, as wheat rows increase from 7 inches to 15 inches, yield declines on average by 4 bushels per acre. The complexity of the intercropping system though decreases with wide row wheat.
Some of the basic concepts apply across all three spacings, such as needing to plan for tramlines, where the tractor and planter will run, before it is time to plant the soybeans. In 10 inch row spacing, wheel traffic tramlines for at least the tractor need to be planned for at planting. This will usually mean blocking off rows of wheat so that they do not plant, creating a 20 inch tramline gap for the tractor to follow. In wider wheat rows over 15 inches, it is possible to select tractor tires and align planter tires to fit in the row gaps without eliminating any wheat rows. Without having to plan for tramlines, it simplifies the system a lot. Another benefit to the soybeans of moving to wide row wheat is earlier planting dates of the soybeans. In 10 inch soybeans, our typical planting recommendation was from head emergence through pollen shed. Some producers would plant over a week after pollination was complete. This would lead to a soybean yield below 5 bushels per acre in one out of five years, despite our average of 33 bushels per acre. We didn’t want to plant soybeans any earlier in this system due to low sunlight below the crop canopy causing the soybeans to have very long internodes which could lead to lodging as they stretched to find light. Since going to twin row spacing, we have been looking at earlier soybean planting dates to hopefully increase soybean yields. Our two current wheat growth stages for planting soybeans have been Feeks 10 and Feeks 10.5 (the earliest of planting dates for 10 inch wheat). Over the last two years, our soybeans’ yield has averaged within a bushel for these two planting dates. Averaging 3 maturities together, this average is 41.6 bushels. When comparing maturity groups, we see a range in yield from 36.5 bushels to 46 bushels for a 2.9 and a 3.8 soybean respectively. From these results, there appears to be an advantage to selecting the best yielding, longest maturity soybean for your area to intercrop. Also, work done by Jim Beuerlein in the late 80’s showed that the best intercrop soybean yields occurred with an early May planting date when wheat was between 10 and 20 inches tall and the wheat and soybean row spacing was 14 or 21 inches. This work also showed that the ideal maturity in Wooster was a full season soybean, between a 3.9 and 4.5 maturity, which mirrors our current work.
This past year I have been receiving more calls about pushing the planting date even further ahead than Feeks 10, possibly to Feeks 5. While there seem to be many planting advantages to this idea, since the wheat has not gone through stem elongation and can still be drove over without causing damage, it is not without its own set of challenges. While we plan to look into earlier planting dates this year, a strip plot we planted last year at Feeks 6 showed some of the challenges producers planting this early may face. The challenges we saw matched what producers planting at these early dates have told us. The challenges come mostly at harvest. The soybeans will most likely be as tall as the wheat at this time, which requires special cutter bar covers on your grain table or using a row crop head to harvest the wheat without causing excessive clipping damage to the soybeans. If you are interested in the various MRI trials that have been conducted at OSU Unger Farm, visit the OSU Extension Agronomic Crops Team on-farm research site and search for “Modified Relay Intercropping” https://agcrops.osu.edu/on-farm-research.