ISSUE 13 August 13, 2009
LATE-SEASON SCOUTING – WHY ARE WEEDS PRESENT IN A
FIELD??
The presence of weeds late into the growing season can be caused by many
things. The following list represents potential causes of weeds being present
late in the growing season:
Sprayer skips. Caused by skips between sprayer passes or near
end-rows/head-lands and from plugged nozzles. In these skipped areas any weed
species capable to be in the field may be present. All individual plants in
the skipped area should look normal and healthy, unless some plants received a
small dosage of herbicide. These areas generally are small and infrequent.
Selection of an incorrect herbicide. Some herbicides are extremely
selective. If an herbicide is not effective on a particular weed species, then
it will survive the herbicide application. The level of susceptibility will
determine the length of time of injury symptoms. Plants throughout the field
will survive an application of an ineffective herbicide. There may be
differences in response between plants within the field, but it should be
minimal.
Poor environment at time of postemergence herbicide application. Weed
species may survive an herbicide application if temperatures are too cool
(most likely) or too hot (usually not, especially glyphosate, but possible
under extreme temperatures), relative humidity is very low, soil moisture is
excessive or lacking, poor fertility, or plants are under stress for other
reasons. Those species present in the field that are the most difficult to
control with the herbicide being applied are the ones most likely to be
present. All species may be present if conditions were really poor at the time
of application. A high percentage (40 to 100%) of plants within the population
of the most difficult to control species are likely to survive when an
application occurs under poor environmental conditions. Individual plant
response can differ between plants within the field, but likely will be fairly
similar. Plant survival should occur throughout the field or in the areas of
the field where the poor environment occurs.
Too short of rain-free period. All postemergence herbicides require some
minimum period of time between application and rainfall to achieve maximum
weed control. Rain-free periods for herbicides range from 0.5 to 8 hours after
application. If this time period is too short, then plants can survive the
herbicide application. Those species most difficult to control with the
herbicide will be the ones most likely to survive. A high percentage (40 to
100%) of individual plants within the population of the most difficult to
control species are likely to survive with a short rain-free period. The most
surviving plants will be in the part of the field with the shortest rain-free
period. There may be differences in response between individual plants within
the field, but it is likely to be minimal. Lambsquarters is most likely to
survive glyphosate if rainfall is received too soon after application. The
greater (> 6 hours) the rainfree period, the better.
Dust/soil on plant leaves. All postemergence herbicides may be affected by
too much soil on plant leaves. However, glyphosate, paraquat, and
Ignite/Liberty are most effected my just small amounts of dust. The most
difficult to control species with the herbicide being applied will be the most
likely to survive the application. The highest dust areas of the field,
typically the area near the sprayer tracks, will have the greatest number of
surviving plants. A wide range in individual plant response to the herbicide
is likely.
Late-season weed emergence. Two major causes of late-season weed emergence
include a reduced crop canopy and the application of an herbicide prior to
crop canopy closure having limited or no residual herbicidal activity. The
greater the reduction in crop canopy, the more likely weeds are to emerge
later into the season, especially pigweed species, waterhemp, giant ragweed,
and grass species. The greater the sunlight reaching the soil surface, the
greater the weed emergence compared to a dense crop canopy that reduces
sunlight exposure to the soil surface. A reduced crop canopy may be caused by
reduced crop plant population or reduced crop growth due to excessive water,
lack of water, disease, insects, crop injury (herbicide, hail, other), or
early-season weed competition. Individual plants should look normal and
usually are not limited in their growth. Multiple species are likely to be
present. Late-season emerging species such as pigweed, waterhemp, giant
ragweed, sunflower, common cocklebur, annual grasses, and nightshade are most
likely to be present. Weeds will be found in patches were the crop canopy is
reduced most or throughout the field in the case of an herbicide application
applied prior to crop canopy closure.
Herbicide resistance. Individual plants within a population/field may
survive the herbicide application. If resistance is caught early enough, few
plants may be found scattered throughout the field or found in higher
densities in small patches. In subsequent seasons, if the same herbicide is
applied, the number of resistant plants will increase in the population and
will become more numerous and scattered throughout the field. Plants having a
high level of resistance to ACCase-, ALS-, and/or photosynthetic-inhibiting
herbicides are likely to have a high frequency of plants appearing normal.
Therefore, individual plants are either dead or normal in growth with few
individuals having a range in response to the herbicide. Plants having a low
level of resistance to glyphosate, PPO-inhibiting herbicides, or plant growth
regulating herbicides are likely to have a complete continuum of herbicidal
response. Therefore, plants will be dead, have normal growth but ALWAYS
be stunted, and have all types of responses between these two extremes.
Another type of response indicating low-level resistance is when individual
plants are dead, have a dead main meristem with axillary growth, or have a
main meristem with perfectly normal growth. The lower the level of resistance
the greater the herbicidal activity and the greater the response differences.
The pictures below demonstrate this type of response 10 to 21 days after
application for common ragweed (first picture) and beyond 28 days after a
late-season application to waterhemp (second picture). Usually (if resistance
is caught early enough) the majority of individual plants will be controlled
by the herbicide application with few plants surviving.
HAND-WEEDING – WHY IS IT IMPORTANT?
As I drive around Minnesota and North Dakota, I see weeds emerging above crop
canopies. If these weeds are due to herbicide resistance, then the best
management strategy is to remove the plants by hand, unless the weeds are too
frequent. A plant that survives an herbicide application, but is removed prior
to seed production, can not add to the frequency of resistance within the field.
Allowing resistant plants to produce seed only increases the frequency of
resistant plants within a population. Species of most concern are Roundup Ready
canola, bolting Roundup Ready sugarbeet, waterhemp, giant ragweed, common
ragweed, lambsquarters, biennial wormwood, and kochia. An increase in resistant
plants will cause a decrease in profit at some time due to increased input costs
and crop yield loss. Growers that planted Roundup Ready sugarbeets are
contractually bound to remove bolting Roundup Ready sugarbeet plants from their
fields. Bolting Roundup Ready sugarbeets are currently visible from the road in
some Roundup Ready sugarbeet fields.
LATE-SEASON GLYPHOSATE APPLICATIONS IN ROUNDUP READY
SUGARBEET
Glyphosate can be applied to Roundup Ready sugarbeet up to 30 days prior to
harvest. Therefore, glyphosate should not be applied in any fields that will be
harvested in the first two weeks of prepile. There is certainly still time to
apply glyphosate to sugarbeets that will not be harvested until October. The
best time to apply the glyphosate is when 25 to 50% of the most frequent weed
species in the field begins to flower. Applications of glyphosate at this time
may not completely kill plants, but will negatively affect flower development
and drastically reduce seed production. Seed production can even be reduced on
glyphosate-resistant plants with a late-season application. If plants are
present because of germination after the last glyphosate application, then they
are more likely to be completely controlled with this application compared to
resistant plants. Determining whether to spray is difficult, because late-season
weeds will most likely not reduce sugarbeet yield. However, a significant
reduction in weed seed production will make weed control easier in the future,
especially if the plants are glyphosate-resistant.
Glyphosate can be mixed with fungicides with no apparent injury to the
sugarbeet or loss of weed control or disease control. However, Monsanto will not
stand behind any such treatments if problems arise. Extensive research has not
been conducted to be 100% confident that no problems will arise from such
mixtures. Also, mixing glyphosate with a fungicide will increase the potential
for drift since the fungicide should be applied with small spray droplets.
Jeff Stachler
NDSU and U. of MN
jeff.stachler@ndsu.edu
