Original Post: Mark Licht , April 2021; Integrated Crop Management News, and Iowa State University Extension and Outreach
With cool weather conditions potentially causing delays in cover crop termination, what options are available?
Cereal rye ahead of soybean is not nearly as problematic. While planting green may not be advised for beginning cover crop users, more experienced cover crop users have planted soybean into green cereal rye.
There are two main considerations. First, make sure the planter is setup properly for the seedbed conditions. This means making sure row unit down pressure is adequate to ensure proper seed placement depth, and the closing wheels are fully closing the seed furrow. Second, is to terminate the cereal rye soon after soybean planting. I am recommending cereal rye be terminated soon after soybean planting this year because, at present, dry conditions are prevailing across much of the state.
For cereal rye ahead of corn, terminating more than 10 to 14 days ahead of corn planting is preferable. There are reports ‘yellow’ cereal rye can cause wrapping in the residue cleaners. Whatever the termination timing, there will likely be increased implications associated with seedling disease, allelopathy, and nitrogen cycling.
Two recent blogs (blog 1, blog 2) from Alison Robertson’s lab tell that temporal and spatial distancing of cereal rye and corn reduced the severity of Pythium seedling infection and mitigated yield loss. Lab studies have also demonstrated that allelochemicals may interact with Pythium to cause more severe seedling disease (Acharya et al. 2021). However, we have a poor understanding of the allelopathic affect in the field. It almost certainly can reduce corn seedling vigor and can be minimized by with more time between cereal rye termination and corn planting. There are also several potential impacts involving nitrogen availability for the corn seedling. We know that cereal rye will take up soil nitrate, and recycle soil nitrogen at some time throughout the growing season. The decomposition of cereal rye immobilizes soil nitrogen in the short-term, and more nitrogen is immobilized if the cereal rye had begun jointing at or before termination.
This year it is likely that corn will be planted green or the termination to planting timeframe will be narrow. Based on our experience, the amount of cereal rye biomass may make a difference. More biomass relates to more severe disease, allelopathy, and nitrogen implications. If the cereal rye is less than 8 inches the risk of nitrogen immobilization and disease infection should be minimal. If the cereal rye is greater than 12 inches consider increasing the corn seeding rate 5 to 10 percent to counteract potential stand loss due to Pythium seedling mortality. All corn seed is treated with fungicides; check your seed label to see what fungicides are on your seed. Mefenoxam, metalaxyl and ethaboxam are fungicides that have excellent efficacy against Pythium. Pyraclostrobin, azoxystrobin and trifloxystrobin also have some activity. Using Priaxor in-furrow could help minimize seedling disease. While Priaxor is labeled for in-furrow applications, there is no public research that we know of that has tested Priaxor under this type of situation. And finally, consider using starter nitrogen and/or applying the remainder of your nitrogen program at an early vegetative stage to ensure sufficient nitrogen supply. Keep in mind that John Sawyer and others have found that corn following cereal rye does not justify a higher nitrogen rate.