In 2018 and 2019, research was conducted on a relay intercropping and double cropping systems to evaluate as a possible alternative to Iowa’s traditional corn-soybean or continuous corn cropping system.
There were 2 sites; one near Kalona and another near Ames, Iowa, to study these possible alternatives. The Kalona site planted cereal rye immediately after corn harvesting. Following cereal rye harvest, soybean was double cropped. In 2018, the cereal rye yield averaged 46.1 bushels per acre and the double crop soybean yield averaged 23.2 bushels per acre. In 2019, the cereal rye yield averaged 30 bushels per acre and the soybean did not reach maturity and were not harvestable. It should be noted, that in 2018 there was an earlier than normal first fall frost.
Near Ames there were 4 treatments were: (1) soybean with winter wheat as a cover crop terminated before planting, (2) winter wheat-soybean relay intercropping with fall strip-tillage after November 1, (3) winter wheat-soybean relay intercropping with no tillage, and (4) soybean double cropped after winter wheat harvest. Wheat yields averaged 57.2 and 30.1 bushels per acre in 2018 and 2019 respectively. The lower yields in 2019 are attributed to cooler, wetter spring conditions. Wheat yields in 2018 were not significantly affected by the strip-tillage treatment, however, in 2019, the wheat yields in the soybean double crop system were significantly higher than either relay intercropping system (Figure 1). Soybean yields averaged 16.3 and 33.0 bushels per acre in 2018 and 2019 respectively. Wet conditions in the fall of 2018 resulted in delayed harvest and pod shattering. In 2019, the higher soybean yield was attributed to the soybean with winter wheat system and the lowest yield was in the double crop soybean system . The fall strip-tillage with the relay intercropping system did provide a higher soybean yield than with no tillage.
In conclusion, soybean and winter wheat (and likely other small grains) can be grown in a relay intercropping or double cropping system in Iowa but with increased production risk. Double cropping soybean following a small grain is very high risk because of much lower soybean yield potential due to early to mid-July planting dates and frost potential prior to reaching physiological maturity. A relay intercropping system reduces some of the risk associated with double cropping, but has some of its own risks. These risks are associated with being able to relay plant soybean into the small grain ahead of the small grain reaching the joint stage; harvesting the small grain before the soybean grow taller than the small grain heads; implement traffic can reduce the small grain harvestable yield; and drought conditions resulting in competition for soil moisture may be limiting for either or both crops. However, it is realistic to use an alternative cropping system to reduce risk of nitrogen and phosphorus losses while potentially increasing overall productivity.
Figure 1. Soybean (yellow) and winter wheat (red) yields under four systems: soybean with winter wheat as a cover crop (S w/ Wcc); winter wheat-soybean relay intercropping with fall strip-tillage (Relay ST); winter wheat-soybean relay intercropping (Relay); and soybean double cropped after winter wheat (Sdc).