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.
Crabgrass Preventer: Warm season annual grasses such as crabgrass and foxtail germinate when soil temperature at the 0-2” depth is consistently between 60-70F. Thus, we often say that reasonably, crabgrass preventer can be applied when soil temps at the 0-2” depth are consistently around 55F. Our CropWatch soil temperatures are measured at a 0-4” depth (https://cropwatch.unl.edu/soiltemperature). Based on them, it may be time to apply or at least getting close in the York area. If you’d like to determine the soil temp of your site locally, you can use a temperature probe or a meat thermometer (that you won’t use for cooking). Make a mark at 2” from the base and it will give you an idea. Remember to blow off or sweep lawn clippings and fertilizer from the sidewalks back onto lawns!
Moths: Our Extension entomologists are also starting to see black cutworm, variegated cutworm, and true armyworm moths in pheromone traps throughout Nebraska. You can see pictures and the counts (which will be updated) in CropWatch at https://go.unl.edu/jdd3.
Planting Green: Been receiving a number of questions throughout the state on this. We wrote a planting green article for CropWatch this week https://go.unl.edu/ysyi. We have minimal research but in the article, we explain more regarding herbicide considerations, what the research shows regarding allelopathy, and considerations based on growers’ and our observations and experiences. We haven’t found any wheat stem maggot flies in rye yet in Clay, York, or Seward counties. The flies we’re getting questions on are small brown flies and also seed corn maggot flies. Having an insecticide seed treatment on corn and beans will help against seed corn maggots. More info. from Iowa State: https://crops.extension.iastate.edu/cropnews/2020/04/seedcorn-maggots-flying-iowa
Seed Treatments for SDS: Continuing from last week, the summary is that ILeVO is an effective seed treatment in fields with high sudden death syndrome (SDS) pressure. However, not all areas of the field have the same amount of pressure, making it difficult to justify the cost field-wide. Three Nebraska on-farm research studies were conducted in soybeans in 2017 with a multi-hybrid planter. Soybeans with a farmer’s choice base seed treatment (check) were compared to the base seed treatment plus ILeVO. The goal was to look at site-specific application of ILeVO to reduce input costs while still effectively managing sudden death syndrome (SDS) pressure. Management zones were created using historical yield maps to show which zones were conducive to SDS pressure (SDS zone) and which weren’t (standard zone). Check strips and ILeVO treated strips were compared in both zones. Two of the locations showed no difference between the base treatment and base treatment + ILeVO in the SDS or the standard zones. At one location, SDS was not present in the field. In the other, the ILeVO treatment had significantly lower disease levels than the standard treatment and overall disease incidence was considered low. At the third site, the standard + ILeVO treatment yielded higher than the standard treated seed in the SDS zone. There was no difference in treatments in the standard zone. The SDS zone was around 50 acres and along a creek that ran through the field (Figure 1). Additional ILeVO studies were conducted in 2015-2016 via on-farm research in Dodge, Clay, and Nemaha Counties where an untreated check, base seed treatment, and base + ILeVO were compared. SDS incidence ratings were taken in addition to soybean cyst nematode samples. In two of the six fields, there was a yield difference between the base + ILeVO and other treatments, even though disease incidence was low. Two sites also had a significant decrease in SDS pressure with the ILeVO treatment, but it didn’t correlate in increased yield. These studies found ILeVO to be effective in reducing SDS pressure, but yield response and profitability depends on disease development and how widespread in the field. SDS pressure was found to be higher in frequently ponded soils or areas of the field with creeks or intermittent streams. We have no on-farm research data on Saltro although we have a York Co. study on it this year.