A fall-planted cereal rye, wheat, or triticale cover crop can have both positive and negative impacts, for example by tying up nitrogen, reducing soil moisture prior to planting, increasing insect pressure, reducing weed pressure, reducing soil erosion and allelopathy.

Each year we receive questions on termination timing of cereal covers. This question occurs as farmers consider trade-offs between a positive return on investment from the cover crop, by allowing more biomass growth with the potential for yield loss if termination is delayed too long. Information being shared can be confusing, with one source saying to terminate pre-plant while another says to plant green into the cover.

What’s the “right” answer? We don’t know that there is one. That’s because farmers’ goals and level of risk vary. In general, there are less risks to planting green with soybean than corn. This article will share tradeoffs to help you better assess cover crop termination timing for your operation and risk level.

Cereal Rye and Potential Allelopathy

For several years, we’ve heard about the potential allelopathic effect cereal rye can have on corn. To avoid the potential allelopathic effects, it’s often recommended to terminate cereal rye at least two weeks before planting corn. Others report successfully planting corn into green cereal rye, leading to the question: Can allelopathic chemicals from rye affect corn?

We do know studies investigating the effects of allelopathic chemicals from cereal rye on corn germination have mostly been done in laboratory settings. One study found that allelopathic chemical extracts from rye reduced corn root length, but another study found no similar effects. It is hard to say with confidence whether allelopathic effects contribute to slower growth and reduced germination that can sometimes be observed with corn in the field or if it is the result of other environmental factors or simply nitrogen tie-up (which we feel is more likely cause). For one thing, corn has a relatively large seed, making it less likely to be affected by allelopathic chemicals than small-seeded plants. Also, allelopathic chemicals quickly degrade in the soil, so increasing time between cover crop termination and corn planting should decrease the likelihood that allelochemicals would negatively affect corn germination.

Challenges with Terminating Rye Pre-plant

While the potential effects of allelopathy are worth noting, there are challenges with timely termination of rye prior to corn planting. Killing the rye at least 14 days before planting may not allow for much rye growth in early spring or would require delayed corn planting, either way reducing the potential benefits from cover cropping. Weather conditions are not always conducive for effective cereal rye termination. In early spring when temperatures are less than 55° F and cloudy conditions are common, herbicides such as glyphosate can have reduced absorption and translocation, resulting in delayed or partial control of cereal rye.

In contrast, glyphosate applied when weather is favorable can provide very effective termination of cereal rye while also providing early-season weed suppression. An option more growers are considering in 2022 is to use clethodim instead of glyphosate when terminating a small grain prior to corn and/or seed corn planting. Clethodim kills cereals slower than glyphosate allowing for them to stay around longer between the rows for weed and erosion control, if those are part of the farmer's goals.

Benefits to Green Planting

Some 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 and noted the corn also tended to look less yellow or sickly. Two farmers in 2020 also shared the green standing rye held the previous crop residue in place and their corn emergence was more even in those fields compared to planting into terminated cover crop residue. Many have shared the observation of early-season weed suppression.

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. Some farmers have found the need 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 migrating from late-terminated cereal rye to emerging corn plants. We think it’s important 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 cereal 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 while 42% saw a 1-5 bu/ac increase in soybean yields, whereas 59% (83 respondents) saw no yield reduction by planting corn green.

Considerations for Pre-Plant Termination

• Terminating 10-14 days pre-plant can aid with saving soil moisture, particularly in non-irrigated or water allocation situations and is a more conservative (risk averse) approach when rotating to corn. Be aware of any herbicide label restrictions prior to corn planting when using herbicides other than glyphosate for termination.
• Apply nitrogen as a starter as nitrogen tie-up is perhaps a bigger issue than potential allelopathy. This may help the corn not look so sickly as it emerges through the dying rye.
• Be aware that rye death following termination and drying out will depend on environmental conditions each spring. Anticipate it may be difficult to cut through the rye residue when planting and do your best to set equipment accordingly.

Considerations for Planting Green

• Apply nitrogen as a starter with corn when planting green; nitrogen tie-up is perhaps a bigger issue than any potential allelopathy.
• Wait for the corn or soybean seed to begin germination before terminating the cover crop (which may be a few days).
• Waiting to terminate a few days after planting also allows the rye that’s been run over by the planter to regrow and kill better.
• 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, scout for insect pressure, particularly observing if any adult wheat stem maggot flies are present.
• For those who wanted the greatest amount of biomass for weed suppression 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. We realize no one wants an additional pass or expense. Observation and research show that less residual gets down to the soil when cover crops are 12” or taller. How long one waits for the second pass for rye to start dying will depend on the environmental conditions each year. The other option if one chooses to add residual when terminating is to plan on an irrigation or rainfall event shortly after application to help move the residual herbicide down to the soil.
• Please see the following articles for additional information regarding cover crop termination and application of residual herbicide: Final Results from a Multi-state Study on Cover Crop Termination with Herbicides; Managing Residual Herbicides with Cover Crops.
• Risk Management Agency Guidelines allow for planting green and are based on “best management practices” for different zones and for irrigated or non-irrigated ground. You can read more on USDA’s Cover Crops site. This is for crops with a contract change date of June 30, 2019, or later, beginning with the 2020 crop year and including successive crop years

Plan for Options

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 are 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). We realize none of this is easy and we wish you the best with your decisions this year. Please contact the authors if you wish to discuss these tradeoffs in more detail for your specific situation.