Considerations For Fall-Seeded Small Grains
URBANA, ILL.
The dry conditions of the 2012 growing season
did little to improve the performance of
many herbicides used in corn and soybean.
Of perhaps comparable consequence is that the
degradation of many herbicides has also been
reduced by the exceptionally dry soil conditions.
Concerns about herbicide carryover were discussed,
and recent results from greenhouse
bioassays have sustained some of these concerns.
About two weeks ago we collected soil at Urbana,
Brownstown, and Perry from plots used
for weed control experiments this year to initiate
greenhouse bioassays. Our goal for the assays
was to improve our understanding of how the
dry conditions have impacted herbicide degradation
in soil. Using our research plots allowed
us to sample from areas where we knew what
herbicides had been applied, when, and at what
rates as well as the precipitation amounts received
before and after application. Soil samples
were also collected from nontreated control
plots.
The soil collections were placed in greenhouse
pots; half the pots were sub- irrigated for 7 days
before planting corn, soybean or wheat, while
the other pots were sub-irrigated only after
planting.
Crop species emerged within 7 days after
planting. Wheat growing in soil collected at
Perry began to demonstrate symptoms of injury
to some herbicides within a few days after emergence,
suggesting that residues remained high
enough to injure the crop. Injury symptoms
have not developed on emerged wheat growing
in soil collected from Urbana and Brownstown.
Based on these early observations, we offer the
following comments:
• Bioassays conducted with field-collected soil
provide an estimate of whether herbicide
residues are high enough to injury rotational
crops, but they do not quantify the amount of
herbicide remaining in the soil. We recognize
that indoor bioassays are not infallible, so we
implemented field bioassays in the same plots
from which we extracted the soil collec- tions
used in the greenhouse bioassays. We’ll apprise
you of the results as they become available.
• Keep in mind that all of the herbicides included
in the greenhouse bioassay have rotation
intervals for wheat, and these intervals were not
satisfied at the time the soil was collected from
the fields. If you plan to sow wheat in fields
where at least one herbicide has been applied
in 2012, be sure to check the label of each herbicide
applied for rotational intervals. Most intervals
are based solely on time, but for some
herbicides they are lengthened based on other
factors, such as soil pH values or droughty conditions.
• The recent rainfall across much of Illinois
will help promote herbicide degradation, but it’s
unlikely to have completely eliminated the risk
of herbicide carryover in all areas of the state.
Even with the recently elevated soil moisture,
herbicide degradation simply takes time.
In fields where there is no risk of herbicide
carryover, be sure to control any existing weed
vegetation before planting wheat. Products such
as glyphosate, Clarity, Gramoxone, and
Sharpen can be applied before planting wheat
(but be sure to wait the required time after applying
Clarity – 15 days at 8 fluid ounces and
30 days at 16 fluid ounces). Δ
DR. AARON HAGER: Extension Specialist,
Weed Science/IPM, University of Illinois