Controlling Weeds After Harvest
URBANA, ILL.
Have you struggled with winter annual
weeds getting in the way of spring planting?
Many farmers are fighting off these
weeds with fall herbicide applications. Applying
a herbicide now may help you save time next
spring, said Aaron Hager, University of Illinois
Extension weed specialist.
Before making a fall herbicide application,
Hager offers a few reminders:
1. Scout fields before making any application
to determine what weeds are present
and if their densities are high
enough to warrant treatment.
2. Many herbicides used prior to
or after crop planting/emergence
can be applied in the fall, but not all
herbicides are labeled for fall application.
Atrazine, for example, is
widely used before and after corn
emergence, but is not labeled for fall
application. Check the label to determine
if fall application is allowed.
3. Some herbicides approved for
fall application have application
timing restrictions on their labels. If
you are considering applying a
treatment (such as glyphosate) that
does not possess much soil-residual
activity, the application should be
timed to occur after the majority of
winter annual species have
emerged. Instead of applying such a
treatment in early-October, a midto
late October application timing
might provide better results. If your
fall application will include a herbicide
with soil-residual activity, then
the application could be made sooner,
but check the product label.
4. Combinations of one or more herbicides
can broaden the weed control spectrum.
This can be very important if
winter annuals have already emerged before the
application is made. Combining 2,4-D and/or
glyphosate with soil-residual products can improve
control of emerged species and help control
biennial or perennial species. Include the
appropriate spray additives with all applications.
5. Location in the state can influence fall herbicide
applications. Fall herbicide applications
seem to “fit” better in areas of central and
southern Illinois. Labels may indicate fall applications
that can be made only in certain geographical
regions of the state.
6. Fall applications that include soil-residual
herbicides may not always result in a clean field
by planting time next spring. Delays in spring
fieldwork may allow the fields to green up before
the crop can be planted. Occasionally, if the
suite of winter annual weed species is adequately
controlled, the emergence of summer
annual weed species may occur sooner than if
winter annuals were still present.
7. Do not utilize a fall herbicide application as
an avenue to provide residual control of summer
annual weed species. Control of summer
annual species, such as waterhemp, is often improved
when applications of soil-residual herbicides
are made closer to planting compared with
several weeks (or months) prior to planting. If a
soil-residual herbicide will be part of a fall herbicide
application, select an application rate
that will provide control of winter annuals
throughout the remainder of 2011, and do not
increase the application rate in hopes of obtaining
control of summer annual species next
spring.
8. With the increasing prevalence of horseweed,
including glyphosate-resistant populations,
fall herbicide applications may prove
more efficacious than spring applications.
Glyphosate alone may not provide adequate
control when applied in either fall or spring, but
a fall application timing provides an opportunity
to utilize higher application rates of products
(such as 2,4-D) than are feasible to use in
spring.
Apart from winter annual weed species, fall
months may offer a good opportunity to apply
herbicides for improved control of certain biennial
and perennial weed species. These species
often become established in reduced or notillage
fields and can be difficult to control with
herbicides once the populations are established. Δ
Winter annual weed species, such as common chickweed, can form dense
populations prior to spring planting. Scout and identify which species have
emerged before making a fall herbicide application.