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Herbicides and Cover Crops


Hi I’m Daniel Smith, Southwest Regional
agronomist for the Nutrient Pest Management Program in the University
Wisconsin Madison and Extension. Today I’m going to talk to you about
herbicides and cover crops. First thing to keep in mind is what is a cover crop
versus a forage crop. A cover crops when no biomass is harvested and uses the
plant of your soil conservation purposes. A forage crop is where we’re watching
the harvest that biomass through either grazing or mechanical harvest. The cover
crops can certainly be utilized as forage but there’s a few things that we
should think about before we plant them to ensure success. Most pesticide labels
may or may not have cover crop data but they usually have forage crop data that
we should reference for rotational restrictions, and we should plan our
pesticide applications and cover cropping around these rotation
restrictions if we intend to harvest these for feed. So we look at an example
look at several herbicide labels where we’re looking at winter rye and looking
at these rotational restrictions. A rotational restriction on a herbicide
label tells us how long we have to wait between herbicide application and when
we can plant a crop that’s going to be harvested, that potentially may have some
negative impacts from these herbicides being applied, and thus we want to limit
the exposure that food crop to those herbicide residues. For herbicide A
here we look at the herbicide label and says do not rotate to food or feed crops
other than those listed below for all other crops not listed wait at least 12
months following the last application. Barley, oats, rye, and wheat, may be planted
four and a half months following treatment. We look at herbicide B and it
has similar language except for the rotational use for this herbicide is 11
to 18 months depending on the herbicide rate. This is something we should be
thinking about when we’re planning our herbicide applications this year is, well
am I gonna plant a cover crop in the fall that I’m then gonna want to graze
or harvest for forage value the next spring, if so I needed to examine that
rotational restriction to make sure that I’m not gonna have a problem when it
comes to next spring when I want to harvest it and there may still be a
herbicide rotational restriction on that field. We look at another example in this
particular herbicide has cover crops right on the label. And the language is
fairly clear and it says cover crops are not grazed by livestock nor harvested
for food. And these cover crops are planted for mostly soil and ecology
based reasons out in the field. We looked at using these herbicides and then
planting a cover crop and it says on the label that significant injury may occur
if we’re not following these full setback periods. Often on these labels
AHS included some language about doing a field or a small-scale bio assay. That
way we’re testing for these residuals and seeing if cover crop seed
will germinate and survive any potential residue that’s out in the field, thus
saving us from seeding in an entire field and having a failure, or having an issue
down the road where we really can’t harvest this biomass from this field
because we do have a rotational restrictions still in place. We look at an
example through pictures to better illustrate this we have herbicide A here
on the top and herbicide B. We’re applying both these herbicides on May
1st and you can see I’m planting green into a cover crop.
Four and a half months go by and it’s September 15th and it’s time to harvest
our silage corn so we have our field of silage corn harvested and I want to
plant some winter rye out there for a cover crop. I want to hold the soil in
place, maybe I’ll go ahead and raise that cover
crop but I should look at my herbicide label and just see if I can legally do
that or not. When I look at my herbicide label herbicideA is gonna allow me to
do it because there’s been four and a half months, herbicide B however though
has an 11 month rotational restriction. So it’d be illegal to plant my forage
crop on April 1st the following year and I’d have no rotational restriction.
I could still go ahead and plant my cover crop I just not have any forage
harvest as an option, and any herbicide carryover from that herbicide onto that cover crop
would be the responsibility of the producer and clearly if we had some
herbicide carryover that may injure the cover crop that may reduce the stand and
may result in the stand failure so that’s a pretty significant risk but as
far as being able to plant a forest crop we’ve got to wake those 11 months and
then clearly the next spring herbicide a after four and a half months in planting
that cover crop we could harvest that for forage however
then the other application herbicide B we’d have to just terminate that cover
crop and we could plant into it still have great weed suppression benefits if
it still have some soil conservation benefits obviously but we would not be
able to use that for forage we look at herbicide persistence and carryover this
is when we see reduced stands of cover crops out in the field and they may not
be noticeable when we drive by in the pickup truck and look at that field we
may not see a decrease standard cover crops well when we go out and Scout we
may find areas of field we might find a whole field that has less cover crop
than a neighboring field that we plant in the same day that’s when we start to
think about gosh maybe there’s some herbicide carryover issues going on out
here so we have two pictures here of a radish planting we have a non treated
plot nice stand of radish and then we have some herbicide carryover a severe
stand reduction of the radish you can see some injury occurred on that radish
and so how does this happen this Campins when we apply a residual herbicide and
it sticks around for a little bit longer than intended sometimes it doesn’t stick
around for longer than tended it’s just that length of the residual is that long
that we have the enough herbicide out and so that’s gonna negatively impact a
cover crop and in that particular instance that would be maybe an
interceding pass where there’s only 20 or 30 days between application and
establishment and this is where looking at our herbicide labels and doing some
label researches very important if we’re gonna use cover crops and residual
herbicides in our crop rotation we look at the properties that influenced
herbicide carryover and persistence we think about the chemical properties of
the herbicide one particularly the half-life of the herbicide the rate of
the herbicide application the soil pH heavy soils and high pH equals longer
residual that herbicide often the organic matter content of the soil the
amount of plant surface plant residue and tillage so the tillage is going to
dilute through these residues we think about temperature and rainfall less
rainfall equals longer residual of these herbicides in most cases and then the
microbes in the soil and how do they’re going to break down this herbicide
so we look at a example from the farm landscape of herbicide carryover this
fields in southern Wisconsin and we dig into scouting
nice standard cover drops a little bit closer we find that something’s missing
or stunted so look at this photo on the tailgate and we see that there’s a nice
healthy stand of peas that we dug up from along the edge of the field but
when we move into the field farther we get this stunted plant and we see a lot
of the people a ssin and actually been reduced throughout that field and we
start to look at that pea plant a little bit closer and we see some stunting we
see the nodes and are a little bit closer together
we see some clear herbicide injury and this was from a herbicide that was
applied about three months prior to cover crop establishment had some
rotational restrictions on it and then our weather conditions were not were
favorable for some herbicide carryover to take place looking a little a few
more examples of this and we look at some research that was done several
years ago at the University of Scott’s ins Madison’s agricultural and life
sciences research station in Arlington and we see in our non-treated plots we
have this nice nice healthy stand of annual rye grass with a bill sixty six
percent cover is the number in the white box so this green cover about sixty six
percent of the soil is covered with this cover crop we start to compare that to
other herbicide treatments we can see that’s the crease significantly
depending on what herbicides used so we’re seeing some herbicide carryover in
these plots moving on we can think about what to you in the spring with our cover
crops and we can think about our cover crops to come in spring weeds if they’re
not managed properly so our first slide here shows a photo of a nice healthy
stand of winter rye you know right grass clearly a very healthy cover crop
providing some great benefits and we look at the slide on the right is a
nicely terminated cover crop that we can plant back into so we often get this
question well is it possible to have herbicide resistant cover crops and we
look at some of the herbicide resistant cases from across the world we find out
that italian ryegrass better known as at annual ryegrass shows resistance to five
herbicide side of action groups in 14 states in the US and some countries
across the world so when we think about this this is also where some of our
inner eye grass seed is produced so we should be cautious when using any rye
grass and always have a really good termination plan frame your eye grass
prevent it from going to seed oftentimes I’m determining in your eye grass it’s
two to three weeks before it even thinks about producing seed so we shouldn’t
have a huge extreme issue of an escape of an ryegrass
but something to keep in mind in a management plan we think about seed
production and a little bit more in depth we think about how quickly these
cover crops can flower and produce seed in the spring in certain cover crops can
overwinter from these seeds such as rye buckwheat and hairy vetch they all
overwinter and then they produce seed the next year so this first photo here
is my hand and that’s buckwheat seed and that’s about 60 days after buckwheat was
planted in a research trial at the Lancaster Agricultural Research Station
moving on to the next photo that’s winter rye and it’s near mid-june and
it’s starting to think about producing valuable seat as well as is the the next
slide on the back of a tailgate is a narai grass so that’s producing viable
seed and near the end of May and then finally the last photos from that
southern Wisconsin field again and this is one that we often don’t see go to
seed in Wisconsin that’s radish radish ex who produced seed in this particular
field and could become a weed if we didn’t think about it for the next
growing season we think about our cover crops that overwinter versus the ones
that winter kill and we find some variability in a new i graph so we had
did some field studies that were in 2013 2014 three out of our four varieties
survive the winter however in 2014 2015 all of our winter rot are all varying
the ryegrass rather winter killed we look at the chart on the right hand of
the screen and we see some our most common cover crops starred we see in
your eye grass may overwinter Oates’s always been over what is always going to
winter kill winter Rye is always going to overwinter radish will always winter
kill crimson clover may or may not winter kill depends on where you’re at
in the state of Wisconsin and depends on what kind of winter we’ve had and then
red clover a little typically always overwinter will determine ate these
cover crops once they weave over wintered obviously if we’re planting
some of these grass species that winter kill we’re gonna have a nice seed bed to
plant into that mulch from the the terminated
Covercraft via winterkill but if we need to do some termination order some of our
two other tools and crimping can be a great tool to have in the toolbox
especially we went to ride it crimp the winter rye and plant soybeans back into
has been shown in Wisconsin to produce a great soybean yield and provide great
weed control we think about mowing summer cover crops so you can get away
with mowing summer grass species however be mindful that some of these cover
crops will quickly regrow such as annual rye grass will regrow very similarly to
the grass in your lawn winter rye if we mow it towards the end of the vegetative
and into the reproductive stages of life we can expect very little regrowth to
occur and we can be fine with planning just a second herbicide pass to clean up
weeds in any escape dry plants that survived the death mowing pass and then
we can think about doing some tillage however I’m not gonna recommend till it
passes unless we need to for our crop rotation or a cropping system because
tillage is gonna really undo some of those benefits of the cover crop that
we’ve seen such as reducing soil erosion building soil health
that type of thing finally we have herbicides in the toolbox we can think
about using glyphosate burn down past but we want to have good weather
conditions a termination timing Brassica termination we’re not gonna really have
to think about terminating these brassicas in Wisconsin because they’re
gonna winter kill the two photos show two extremes we have one picture here
that shows the the Brad D interceded radish nebraska species that is really
not going to produce a whole lot of biomass in the fall and it really is not
gonna provide a lot of cover crop benefit so we should think about maybe
the timing of when we planted that to improve the benefits but again that
green biomass we’ve seen in the fall isn’t gonna be there in the spring we
look at the photo on the right again we can think if we planted monoculture
brassicas we’re gonna have a field that is the soil is been loosened and is very
prone to erosion so we should think about planting these species with a
grass cover crop that again is going to need to be terminated in the spring
finally the legumes are going to require some termination depending on what
species you plant and again we covered a few
common ones are gonna winterkill at crimson clover peas but red clover and
vetch they’re gonna need to have a termination pass down in the spring a
few of these are good candidates for crimping you can see that on the slide
mowing may be an option for a couple of these if we’re going to terminate them
in the same year that they’re planted but the species that overwinter are not
going to be species where we mo and then tillage of course is always an option
the toolbox but again I’m going to advise against it unless we absolutely
have to for a cropping rotation we think about terminating the legumes I’m gonna
recommend a application of a glyphosate product coupled with a growth regulator
herbicide and these will provide adequate control of these cover crops
and any weed species that are out in that field how you now underneath that
cover crop residue we think about work to do if we’re gonna do a forage harvest
of some of these cover crops so we have winter rye and a new rye grass we’re
gonna harvest these species around boot stage for the winter rye and what’s that
gonna look like if we allow those to regrow so the very front of this photo
shows winter rye two weeks after harvest and we do get a little bit of regrowth
but you know we’re talking about five to six inches of regrowth we can easily
plant into that and clean that up with a post emergent application any ryegrass
however though quickly regrow produced quite a bit of biomass but after harvest
and that would be a clear weed and provides some pretty strong competition
to whatever trap we planted back into that we think about harvesting that
winter rye ryegrass reforged value and then spraying it the same day so this
research showed that we could go out we could harvest our winter rye and a new
ryegrass take that biomass off the plot store it
for feed value and then we go ahead and spray it the very same day and we could
have excellent control after two weeks with just a burn down application of
life assay and then finally we look at glyphosate alone on these cover crops on
this particular instance we’re leaving that cover crop residue on the on the
soil we want to go out and we want to spray those cover crops off we could go
out and plant into residue I also want to encourage
producers to explore their options when it comes to managing this cover crop
residue by ensuring that they have proper plants planter setups their plant
into that residue and consider modifying their equipment for optimum success in
these residues high residue systems cover crops and crop insurance we want
to always ask our our crop insurance agent and check our policies to avoid
future problems with our crop insurance and and cover crop termination can be
quite simple for crop insurance reasons however there are some some reasons why
we should investigate a little bit further and talk to our agent to ensure
that there’s no problems down the road I also want to encourage producers to
terminate their cover crops under good weather conditions so when the cover
crops actively growing when the daytime temperatures are above 50 degrees and
the nighttime temperatures are below 40 are above 40 degrees rather when the
cover crops actively growing ensure that the sprayer is set up for an optimum
application turn series success if we’re going to use a herbicide terminate your
cover crop with that I’d welcome any questions and emails at the phone and
email address on the screen as well as reaching out to your local Extension
educator for more information

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