—Lancaster Farming, Saturday, October 15,1977
60
Field trip suggested
control analysis
Now would be a good tune to take a walk through your
fields to assess your weed control program’s ef
fectiveness. That’s the consensus of opinion from a recent
survey of weed specialists across the nation conducted by
Stauffer Chemical Company. The specialists surveyed
mcluded Dr. Vivan Jennings of lowa State, Dr. Fred Shfe
of University of Illinois, Dr. Arnold Appleby of Oregon
State, Dr. Nathan Hartwig of Pennsylvania State, and Dr
Douglas Murphy with Stauffer Chemical Company.
One of the major advantages of a fall weed tour, ac
cording to the agronomists, is that you can evaluate the
success of your present program by determining how
much of what species got through and the possible reasons
for this happening. You’ll also know what to expect next
spring because if weeds are surving through until harvest,
they’ll likely have produced seed for an increase in the
infestation. Spotting these in the Fall is easier as the
weeds are more easily identified when full grown and you
can then plan your control program to zero in on present
or potential problems.
In order to adequately assess control efforts, tfte
agronomists stressed the practice of leaving a check strip.
This will give you not only a sampling of your full weed
spectrum, but also act as a control plot to determine ef
fectiveness. Even 95 per cent control of a bad infestation
may leave a lot of weeds, and a check strip is a good way
to pinpoint control success.
Variables Affect Control
Herbicides form the core of most weed control
programs these days, but there are a number of variables
that come into play which can affect chemical per
formance. Taking mto account such factors as weather,
soil texture, organic matter content, pH, fertilization,
planting dates and other cultural practices will aid in
proper evaluation of your fall weed tour information.
If certain weed species have been left partially un
controlled, these variables must be taken into con
sideration before a switch in chemicals or a change in
cropping practices is called for For example, an increase
in rainfall and subsequent soil moisture can present ideal
conditions for wet soil weeds like nutsedge to proliferate
where drier conditions would help greatly to keep it in
check On the other hand, lack of rainfall won’t affect the
activation of soil incorporated herbicides like Sutan + and
Eradicane while surface applied materials like atrazme
need rainfall after application to place the chemical in the
weed germination zone
Soil texture plays a role in chemical performance
making the selection of the proper material impoitant
In sandy soils some chemicals tend to leach below the
weed germination zone while in heavier soils they may be
more effective Conversely chemicals like Sutan + are
more stable in sandy soils than some other herbicides
Organic matter content also is a veiy uii|>oitant con
sideration Some chemicals are adsorbed (become at
tached individual organic and clay soil pai tides and
are prevented from doing their job;, necessitating lughai
for weed
rates tor optimum control The opposite is also true in that
weakly adsorbed herbicides may be over-applied in light
soils causing injury problems. Information concerning
how to adjust rates for soil composition can be found on
the herbicide label
Soil pH has a nronounced effect on tnazme type her
bicides In basic soils with a pH of 7 to 8, there is a super
active condition that occurs with the use of tnazmes
which may cause injury problems. However, in acid soils,
the addition of lime may be needed to enhance tnazme
effectiveness, a r cording to Dr Nathan Hartwig, weed
ipecialr.t at Penn State
Your fertilization can affect weed control in that you are
literally feeding weeds as you fertilize your crops. Any
weeds that do escape herbicide treatments will be that
much more vigorous and competitive with the crop. The
ideal way to deal with this is to eliminate weeds com
pletely. Of course this is not always possible, but the
better your weed control program is, the more fertilizer
there will be available for your crops.
Cultural practices such as tillage methods and planting
dates probably affect your control efforts more than you
realize. Consider this situation as an example: As plan
ting dates are moved up, weeds like foxtails that can
germinate in early May have previously been controlled
through early season tillage. They are being moved into a
more competitive situation with crops like early planted
com because the pre-season tillage takes place before
they germinate. Your weed control program in a case like
this should lean more on chemical than cultural control as
planting dates are moved forward.
A different course of action is required with perennial
weeds like nutsedge and johnsongrass, however. Early
season tillage to chop up the underground reproductive
organs of these weeds is a major part of a successful
control program. Many weed specialists support the view
that planting dates should be delayed if necessary to
assure proper working of the soil, which, when coupled
with chemicals like Eradincane, constitutes an effective
johnsongrass control program on com, for example.
Tillage practices can make or break a perennial weed
program. Weeds like quackgrass and Johnsongrass
produce underground reproductive organs which must be
chopped up to be effectively controlled by Eradicane.
Reduced tillage should not be considered in fields with bad
perennial infestations like these. You should think more
along the lines of rotating to a crop like com where these
weeds can be controlled by tillage and a suitable chemical
like Eradicane.
Weed Spectrum Can Shift
An additional reason to take a weed walk is to determine
whether your weed spectrum is changing and if new
species are becoming established. Continued long term
use of a broadleaf herbicide like atrazine alone, for
example, has allowed fall grasses (fall panicum, wit
chgrass, crabgrass) the elbow room needed to move in
and take over in many areas where broadleaves
previously had been the major problem. A delay in
recognizing such a shift can cost you both in terms of
yields and misdirected herbicide expenses. Weeds don’t
mvate overnight, it usually takes a few years, and if you
can prevent a species from becoming established you are
ahead of the game. As the old adage goes, “an ounce of
prevention is worth a pound of cure.” One way farmers
are putting this to practical use is the widespread move to
broad spectrum tank mixes and sequential chemical
applications like Eradicane followed by atrazine or
dicamba (Banvel) to prevent new population buildups.
What do you look for when taking a weed walk in the
fall’ First try to determine what weeds were not being
controlled and compare their populations to those in the
check plot. If certain species are escaping, the next step is
to determine why. Is it due to the herbicide or cultural
practices that you used or some other variables? Search
for new infestations of perennial weeds. Small patches are
easier to control than widespread infestations.
When trying to assess herbicide performance, look for
any kind of uneven control. If there is streaking, you can
be pretty sure your application is part- of the problem. If
there is control in high ground but not low, possibly the
organic matter content varies enough that herbicide
adsorption is the culprit. A change in chemicals or
cultural practices may be necessary to offset some of
these factors. Rotation of crons and/or herbicides to key
in on tough problem weeds may also help.
Look for new infestations around field edges, you may
be having weed seed supplied free of charge from fence
rows or neighboring fields. If you spread manure, that
could be a source of reinfestation.
Examine problem areas where control was poor or
injury occurred and try to figure out the cause in terms of
the variables outlined above. If non of these quite fit,
contact your county agent, your supplier, or a chemical
company representative, as they may have encountered
some of these problems before The important thing is to
evaluate your weed control program and determine if it is
working as well as you’d like. The benefit of the fall tour is
that you’ll have a handle on the situation rather than
having to rely on trial and error or guesswork next spring.
One tool that could help you further detail your control
measures is a weed map. Though still not widely accepted
by many growers, it is an excellent tool for long-term
strategy evaluations and pesticide decision making Soil
Conservation Service maps with acetate overlays are
highly suitable for plotting weed problems and
movements in relation to some of the factors outlined
above.
What it all comes down to, is that with the narrow mput
to-profit ratios growers are dealing with these days,
anything you can do to increase the efficiency and ac
curacy of your management tools, pays off with interest.
A fall weed analysis of your fields is an important element
of the “total management” approach to farming now so
necessary to increase profit potentials.
What To Look For