Extension Update by Jenny Rees

Cover crop termination tradeoffs: The spring rains and warmer soil temperatures are allowing rye and wheat cover and grain crops to really take off. Each year I receive questions on termination timing of these cereal covers. This question occurs as farmers consider the cost of the cover crop, their goals, and gaining more biomass growth for their investment. I agree that information being shared is confusing. One source says to terminate the cover crop pre-plant while another says to plant green into the cover. What’s the ‘right’ answer? I don’t know that there is one. That’s because farmers’ goals and level of risk vary. I am a fan of ‘planting green’ because of farmers’ and my observations; however, it doesn’t fit all situations. In general, there are less risks to planting green with soybean than corn. This column will share tradeoffs to help you better assess for your operation and risk level.

A rye cover crop can impact corn and soybean in several ways, for example by tying up N, by reducing soil moisture prior to planting, by increasing insect pressure, by reducing weed pressure, by reducing soil erosion, and by allelopathy. We often hear about terminating a rye cover crop 14 days prior to planting corn to reduce potential for allelopathy. Studies investigating whether rye cover crops impact corn germination have mostly been done in laboratory settings with mixed results. It is hard to say whether allelopathic effects contribute to slower growth and reduced germination that can sometimes be observed in corn in the field.

While the potential effects of allelopathy are worth noting, there’s challenges with terminating rye prior to corn planting. Killing the rye at least 14 days before planting may not allow for much rye growth or results in delayed planting, either way reducing the potential benefits from cover cropping. Weather conditions have not always been conducive for effective cereal rye termination. Farmers have shared the difficulty of planting through the partially decomposed ‘mushy’ cover crop. Farmers also noticed corn planted into these conditions often came up slow and had a yellow, sickly look to it for a time. Farmers that switched to planting green, say it was easier to plant compared with planting into the decomposing-dying cover. They noted the corn also tended to look less yellow or sickly. Two farmers in 2020 also shared the green standing rye held the previous residue in place and their corn emergence was more even in those fields.

Risks to planting green: In spite of these observations, planting green is not for everyone and one needs to assess the risk of doing so. Cover crops use moisture and can dry out the seed bed. Some farmers in non-irrigated situations have planted corn/soybeans into dry seedbeds when planting green and hoped for rains. Last year, some farmers had to run pivots to get moisture into the seedbed. Thus, there’s greater risk for farmers with non-irrigated land and those in water allocation situations. Another risk is the potential for increased insects. In 2017, wheat stem maggot was observed. I think one needs to have insects in the back of one’s mind when planting green. Research from Penn State and Wisconsin showed no yield difference when soybean was planted green vs. planted into pre-plant terminated rye or triticale. Research from Penn State showed yield loss 50% of the time when corn was planted green vs. into pre-plant terminated rye or triticale. A 2020 survey of Nebraska and Wisconsin farmers who planted green showed 42% (77 respondents) saw no yield increase and 42% saw a 1-5 bu/ac increase in soybean yields. 59% (83 respondents) saw no yield reduction by planting corn green. Our 2021 survey can be found here: https://ssp.qualtrics.com/jfe/form/SV_3XeaLgSdlxnXo1M.

Considerations for Planting Green: To minimize these risks consider the following. Apply nitrogen as a starter with corn when planting green; we think nitrogen tie-up is perhaps a bigger issue than any potential allelopathy. Wait for the corn or soybean seed to germinate before terminating the cover crop. If irrigation is available, have the irrigation system ready to go prior to planting in the event you need to add some moisture into a dry seedbed. Upon planting the field, observe if any adult wheat stem maggot flies are present. If they are, consider adding a cheap insecticide in with the herbicide during termination. For those who wanted the greatest amount of biomass for weed control in soybean, termination of rye occurred closer to heading. For those who plan to roller-crimp rye for weed control, termination occurs at boot stage to heading. When terminating a rye cover crop, if the cover is 12” or more and you’re planning on a residual herbicide, consider waiting on the residual as a second pass after the rye starts dying. I realize no one wants an additional pass or expense. Observation and now research shows that less residual gets down to the soil when cover crops are at least 12” tall. How long one waits for the second pass for rye to start dying will depend on the environmental conditions each year.

With the way things are growing this year, it may be wise to have a Plan A and Plan B in mind if you plan on planting green but the cover crop is getting taller than you are comfortable with, especially for corn. For example, Plan A for a non-irrigated situation may be that you’re planning on planting green unless the cover is X inches tall by a certain date (ex. April 10-15) upon which you will choose to terminate pre-plant instead (Plan B). I realize none of this is easy. Feel free to call if you’d prefer to talk through it for your specific situation.