Cover Crop Burndown Considerations
Posted: April 2, 2013
In last year’s April 10 Field Crop News, we had an article about cover crop management.
The first sentence last year started out “with the warm dry weather we have been experiencing
this spring, cereal grain cover crops and weeds are ahead of normal…… So, this year we start
out “with the delayed winter and cool spring, much of the cereal rye or other small grain cover
crops are slower taking off this spring than in the past”. Wow, what a difference a year makes!
The end result is that don’t get too eager to kill those cover crops this spring until they are
actively growing and hopefully have contributed some benefit to soil quality and the subsequent
cash crop. Here are some reminders:
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Herbicide rate—most glyphosate labels recommend increasing the rate of product as the
cereal grain and weeds mature. This is just as important under cool-cloudy conditions like
we have been experiencing. With Gramoxone, use the 3 pt rate and remember that it is
better on smaller cereals or after they have reached the boot stage—during the period of
rapid culm elongation it is less effective.
Include appropriate adjuvants in the spray tank—include 1 to 2 qt/100 gal nonionic
surfactant (unless fully loaded formulation) plus 8.5 to 17 lb/100 gal AMS or equivalent
with glyphosate. The AMS helps alleviate hard water problems and also can reduce
antagonism if tank-mixing with other herbicides such as 2,4-D. Be sure to add the AMS
first to the spray tank and agitate before adding the glyphosate.
Use a clean water source that does not contain soil or other sediment that can reduce
glyphosate or Gramoxone activity.
Use flat fan nozzle tips that produce a uniform spray pattern and thorough coverage.
Spray in sufficient carrier to achieve good coverage (usually between 10 and 30 GPA).
Make sure the sprayer is accurately calibrated (output, pressure, pattern, speed, etc.) to
deliver the appropriate rate uniformly.
Air temperature before, during, and after application can influence control. Cold nights
(<40 F) will reduce activity, particularly for glyphosate, and especially when followed by
cool (<55 F) cloudy days.
The more time between application and rainfall the better, especially with difficult to
control perennials.
Annual ryegrass (Lolium multiflorum) cover crop control - Historically, annual ryegrass can
be somewhat difficult to control with herbicides in the spring. Some general recommendations
for control of annual ryegrass are provided by a team of agronomists from Illinois, Indiana, and
Oregon State who have a lot of experience with this species. Click here for their complete
management guidelines for ryegrass management in corn and soybean production. For control,
some general take home points:
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Control is best done when the plants are small, 8 inches in height and before the first
node has developed. This is usually late March to early April in the lower Corn Belt and
mid April in the northern Corn Belt – adjust accordingly this year.
While one burndown application may provide control of annual ryegrass, growers should
plan for two.
If annual ryegrass is allowed to reach the flowering stage, the control of the plant is
easier, but some viable seed may develop and may create additional weed management
problems in the future.
For glyphosate, successful control depends on timing, application rate, and weather
conditions. Apply glyphosate at 1.25 to 1.5 lb ae/acre (48 to 64 fl oz of a 3 lb ae gal
product) with AMS and surfactant. Some folks add an acidifier (e.g. citric acid) to
improve performance. Add AMS and other additives to the spray tank, agitate, and then
add the glyphosate. Adding other herbicides to glyphosate and especially triazines or
clay-based materials can antagonize glyphosate activity. Generally, including 2,4-DLVE
in the mixture doesn’t appreciably reduce activity and it can help broaden the weed
control spectrum.
The use of air induction spray tips that produce coarse droplets should be avoided and for
glyphosate, reduce spray application volume to 10 GPA (others recommend a bit higher
carrier volume if the stand is thick and coverage is a concern).
Spray during daylight hours at temperatures about 60F and when annual ryegrass is
actively growing.
If night temperatures drop below 38F, wait three days before spraying and soil
temperatures should be above 45F.
Spray at least 4 hours prior to sunset to allow for maximum translocation.
We still have plenty of time to get our corn planted. The optimum time to plant corn in NE Iowa
is between April 12th and May 2nd. Corn won’t start to germinate until the soil temp is at 500 F
or higher. Some suggestions from Roger Elmore, ISU Extension Corn Specialist – If the sun is
shining, soil temperatures are in the high forties or higher and climbing, checkout the 5 to 7 day
forecast. Plant corn if the forecast calls for more of the same. If the 5 to 7 day forecast is calling
for the weather to settle back into a cold wet period, keep the seed in the bag. Other planting
tips can be found at: http://www.extension.iastate.edu/CropNews/2013/0419elmore.htm
There’s no need to consider changing to an earlier maturity of corn. If in case we do plant most
of our corn in May, hybrids that we are currently planting will adjust to delayed planting dates.
Here are a couple articles to reference:
http://www.extension.iastate.edu/CropNews/2011/0427elmore.htm
http://www.agronext.iastate.edu/corn/production/management/planting/delayed.html