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ISSUE 12  JULY 22, 1999

 

PREHARVEST WEED CONTROL IN SMALL GRAINS

    Lack of soil applied herbicides, herbicides applied later than recommended, flushes of weeds
emerging after application, poor weed control from weather, and environmental conditions
promoting excellent weed growth conditions contribute to weedy small grain fields. An excellent
opportunity for weed burn down, perennial weed control and harvest aid is through preharvest
herbicide application. It is important to keep preharvest weed control in small grains in perspective.
The following are some factors to consider before applying a herbicide as a harvest aid:

    1. The expectations for preharvest weed control usually exceed reality - it is not possible to
kill/dry down a 3-foot weed in the same manner as a 3-inch weed. Lower portions of the weed
may not be affected.

    2. It requires time to dry down treated weeds - usually 7-10 days. It may require more time if wet
and/or cool weather conditions occur after treatment. All herbicides labeled for preharvest application
are systemic and slow acting which requires a longer dry down period as compared to contact, fast
acting herbicides.

    3. The intent of a preharvest treatment should be to facilitate harvest and reduce harvest loss.
Preharvest treatments do not decrease yield losses if applied in the prescribed application window.
Realize that yield loss has already occurred from weed competition and weed seed production will
add to next year’s weed problem.

    4. Herbicide drift from preharvest treatments can cause major problems this time of year. Consider
sensitive crops (sugarbeets, potatoes, etc.) and other plants (trees, gardens, etc.) in the general vicinity
of the field receiving treatment.

    The following is a list of herbicides labeled for preharvest treatments in small grains and precautions
on their use.

**NOTE**

    There are no herbicides labeled as a harvest aid for use on oats. Gramoxone Extra (paraquat), Harmony
Extra, Curtail, Express, Peak, Canvas, or Amber are NOT labeled as a harvest aid in small grains. They
are illegal.

2,4-D as a Harvest Aid:
   2,4-D is labeled as a harvest aid in spring wheat, durum, barley, and rye. Labels vary in crop use.
Follow the label.

    If broadleaf weeds are going to interfere with harvest, 2,4-D can be applied at 0.75 to 1.5 lbs/A
(1.5 to 3 pts/A of a 4 lb/gal a.i. product) at the dough stage of spring wheat, barley or rye. Not all 2,4-D
formulations are labeled for preharvest applications.

    Some 2,4-D labels only allow use on wheat, others allow use on wheat and barley and others allow use
on wheat, barley and rye. Choose a brand that is labeled for use on the intended small grain crop. An
ester formulation will give better control and quicker burndown than an amine formulation. If using an
ester formulation, use a low volatile formulation to reduce vapor drift potential. If using an amine, at least
2 pts/A is needed for larger weeds. Do not expect good control on large pigweed or kochia or wild
buckwheat. Large kochia and other weeds with large stems may not burn down and may stay green for
an extended period.

    2,4-D can be tank mixed with Roundup on spring wheat and durum for additional broadleaf control
and grass control. See the following paragraphs for restrictions for Roundup and be sure to always read
the Roundup label.

    2,4-D labels have a grazing restrictions of no dairy and 7 days for meat animals and a 30 day hay
restriction. Do not feed straw to livestock.

Banvel + 2,4-D as a Harvest Aid:
   Banvel is labeled only in North Dakota as a preharvest application in wheat and durum applied alone
or in a tankmix combination with 2,4-D. Apply Banvel at 0.5 pt/A + 2,4-D at 1 to 2 pt/A when wheat is
in the hard dough stage and the green color is gone from the nodes of the stem. Banvel will provide
additional control of wild buckwheat, kochia, common lambsquarters, pigweed spp., sunflower, and
Russian thistle. A waiting period of 10 to 14 days is required before harvest. Do not feed treated straw
to livestock. Caution: Drift to broadleaf crops is especially hazardous at this time.  

Ally + 2,4-D as a Harvest Aid:
   Apply Ally at 0.1 oz product/A + 1.5 to 3 pt/A to wheat, durum, and barley in the dough stage and
at least 10 days prior to harvest. For use in wheat/fallow or continuous wheat rotation. Do not use if
crop was treated previously with another sulfonylurea herbicide. For wheat, Ally + 2,4-D can be
tankmixed with Banvel for faster dry down and for weed resistance management. Follow the label for
crop rotation restrictions and refer to the 2,4-D and/or Banvel label for grazing restrictions.

Roundup Ultra and Roundup Ultra RT as a Harvest Aid:
   Roundup and Roundup RT can be applied at 0.5 to 2 pts/A for annual grass and broadleaf weed
control, quackgrass control, and Canada thistle suppression in hard red spring wheat and durum.
Do NOT apply to barley. DO NOT apply more that 2 pts/A of Roundup as a harvest aid. Generic
brands of glyphosate (Glyphos, Jury, Mirage, Rattler, Ruler, Show-Off, Silhouette) ARE NOT
labeled as a harvest aid.

    Ammonium sulfate should be added at 1% to 2% v/v or 8.5 to 17 lbs/100 gallons of water.
Ammonium sulfate increases control of annual and perennial weeds and especially weeds stressed by
dry weather. Ammonium sulfate also eliminates antagonism from ions and carbonates in hard water.

    Application should be made after the hard dough stage (30% or less grain moisture) of the wheat
and at least 7 days prior to harvest. Roundup can be applied by air or ground. Use a spray volume
of 3 to 10 gpa.

    DO NOT apply to wheat grown for seed as a reduction in germination or vigor may occur. Be
aware of the injury potential of Roundup drift on sensitive plants. Roundup or Roundup RT can be
tank mixed with 2,4-D for additional broadleaf control. A new 2(ee) label interpretation has been
granted allowing Roundup RT at 0.75 to 2 pt/A + Banvel at 0.25 to 0.5 pt/A for a preharvest
application to wheat and durum at the hard dough stage and green color gone from stems. A waiting
interval of at least 14 is required before harvest. A surfactant is required and the tankmix can be applied
by ground and air application.

    Control of perennial weeds like quackgrass and Canada thistle has been good with preharvest
applications of Roundup Ultra, and in some cases than post-harvest applications. Post-harvest
application require new regrowth for optimum herbicidal activity. Depending on moisture new plant
growth may or may not occur. Applications made prior to or following fall frost has given good
perennial weed control.

Landmaster BW as a Harvest Aid:
   Landmaster BW (glyphosate + 2,4-D isooctyl ester) can be applied at 3.38 pt/A (54 fl oz/A) to
5.25 pt/A for annual grass and broadleaf weed control, quackgrass control, and Canada thistle
suppression in hard red spring wheat and durum. Do NOT apply to barley. DO NOT apply more
that 5.25 pts/A as a harvest aid. Follow the same guidelines and restrictions as Roundup Ultra
and Roundup Ultra RT.

    Finally, remember that preharvest treatments may not be as effective as you would like them to be.
They may not be able to eliminate all harvest problems. Some fields may need to be swathed in order
to dry them down enough for harvest.

 

EFFECT OF PREHARVEST HERBICIDES ON DURUM QUALITY

    An experiment was conducted at Fargo, ND to evaluate herbicides applied preharvest in durum
wheat. Herbicide labels restrict application of most preharvest herbicides until wheat is in the hard
dough stage, at 30% or less grain moisture, when green color is gone from stem nodes, and at least
7 days prior to harvest. Growers may make applications to allow sufficient time for weed desiccation
from systemic herbicides. Paraquat is used because of quicker activity than systemic preharvest
herbicides and possible interest in registration from the manufacturers. ‘Ben’ durum wheat was planted
April 28, 1998. Plots were kept weed free by applying Achieve + Scoil at 7 oz WDG/A+ 1.5%
v/v + Bronate at 1 pt/A to small weeds. The 50% grain moisture treatments were applied on July 23,
1998 at the soft dough crop wheat stage. The 30% grain moisture and 9days before harvest treatments
were applied on July 29, 1998 at the hard dough wheat stage. Treatments at 3 days before harvest were
applied on August 4, 1998 at the harvest ripe wheat kernel stage. Plots were harvested August 9, 1998.
Durum wheat was milled and processed at the NDSU Durum Wheat Quality and Pasta Processing
Laboratory according to American Association of Cereal Chemists Methods.

    Paraquat was applied at 0.75, 1.25, and 1.5 pt/A at 9 and 3 days before harvest. Roundup Ultra
salt at 2 pt/A, Touchdown + nonionic surfactant at 1.58 pt/A + 0.25% v/v, Landmaster BW at 5.25
pt/A, and Fallow Master at 44 fl oz/A were applied at 50% and 30% grain moisture.

    Grain yield, seed test weight, vitreous kernel content, germination injury, and falling number are
important quality parameters to growers. Test weight, vitreous kernel content, kernel size, and kernel
weight are important quality parameters to durum millers. Protein content, sedimentation rate, falling
number, wet gluten, gluten index, semolina color, and pasta quality are important quality parameters
to pasta processors.

    Grain from treatments applied 9 days before harvest or 30% grain moisture or later generally did not
differ from grain from untreated plots. Parameters indicating durum quality not affected by preharvest
herbicides were: vitreous kernel content (82 to 93%); whole kernel protein (12.3 to 13.4% dry basis);
falling number (390 to 428 seconds - value indicates no sprout damage); grain yield; total germination,
sum of normal and injured seedlings (72 to 82%); semolina extracted from grain (67 to 69%); brightness
of semolina, brightness increases with the value (84 to 85); yellow reading of semolina, yellowness
increases with the value (22 to 23); wet gluten content, a measure of desirable protein in semolina
(26.5 to 29.9%); and ash content (0.87 to 0.91% dry basis). Glyphosate applied alone or in combination
with 2,4-D or dicamba at 50% grain moisture wheat stage reduced durum wheat test weight, 1000 kernel
weight, percent large kernels, percent normal wheat seedlings, but increased percent injured seedlings
and semolina protein. Preharvest herbicides can affect durum wheat quality if applied before the labeled
application window.

Richard Zollinger
NDSU Extension Weed Scientist
rzolling@ndsuext.nodak.edu

 

BURN, CUT, FERMENT: WEED SEEDS STILL SURVIVE

    Weed seed have an amazing ability to survive, no matter what nature or you throw at them. Even
burning fields after weed seed mature will give only erratic control. Field bindweed with 36% germination
before burning can consistantly still have 7% that can germinate, according to very early studies compiled
by the Bureau of Reclamation (Mercer, 1940). Thus, weed species that may have been absent for many
years may suddenly appear after a burn. Likewise, cutting or shredding the weeds may or may not
decrease your weed seed bank on your farm unless you eliminate the weeds before the bud stage.

    Common sowthistle still has 100% germination of seed if the weed is cut in flower. Even Meadow
barley (90%), soft brome (81%), curly dock (88%), shepherdspurse (82%) and common chickweed
(56%) seed are largely viable when the weed is cut with the seed only medium ripe. Some weed seed
will even germinate after being stored in a silo for up to four years while others may lose their germinating
ability in 10 to 20 days depending on silage moisture content, temperature and organic acids present.
Some weed seed may even survive in manure, even during the heating and decomposition processes.
Field bindweed still had 22% viability after stored in manure for two months and velvetleaf had a 2%
viability after one month.

 

WEED CONTROL IS MEASURED IN SHIFTS OR
SIMILAR CYCLES FROM THE PAST?

    Look back over research in weed control and consider how indomitable weeds actually are in farming.

    Comparing many of the current problem weeds with those listed from an article written in 1983 (Weed
Control, Dryland Agriculture Agronomy Monograph No. 23) by Dr. A.F. Wiese, many of the same
problems exist along with some of the lessons learned then and now. On perennial weeds, "dryland
agriculture is plagued with many perennial weeds, primarily because frequent tillage is not necessary to
control annual weeds. Infrequent tillage allows perennial weeds to store reserves in roots that penetrate
up to 7 meters in deep fertile soils (Green, Davis, Wiese, 1965; Frazier, 1943)." One of the perennial
grasses that affect dryland farmers in the North is quackgrass.

    "Field bindweed is the most widespread broadleaved perennial weed in the western USA and is a
serious problem in both dryland and irrigated crops. In the Northern Great Plains and Canadian Prairies,
Canada thistle infests almost every field and is the most widespread broadleaved perennial weed in the
region. Leafy spurge is particularly troublesome in the Dakotas and central Canada and is found in
458 counties in 26 states from coast to coast in the USA (Dunn, 1979). Perennial sowthistle also plagues
northern growers. In general, perennial weeds reduce yields about 30 to 50%, but variations are large
among weeds and crops." In a 12-year study at Hays, Kansas, land infested with field bindweed had an
average reduction of 35%. "Presently, the most effective method of controlling large areas of broadleaved
perennial weeds is with a combination of competitive crops, cultivation, and herbicides. Specific studies
have been reported for field bindweed (Phillips and Timmons, 1954; Derscheid, Stritzke, Wright, 1970;
Wiese and Rea, 1959). Usually three to five years were required for complete kill of the weed. Using
an herbicide increased speed of weed kill and decreased the number of tillage operations compared to
intensive tillage alone."

    Weed competition can be specific to each crop also, such as in corn. "Uncontrolled growth of smooth
pigweed in corn reduced grain yield 39% compared with weed-free corn. Total dry matter yield of crop
and weed was about equal, regardless of weed population (Moclani, Knake, Slife, 1964). One of the
most feared weeds in corn fields is giant foxtail. In a three-year study˙grain yield reduction was 25% for
the heaviest (giant foxtail) population (Staniforth, 1961). Foxtail seeded with the crop reduced corn yields
13%. Seeding the weed three weeks after planting did not decrease corn grain yield, but weeds made
seed (Knake and Slife, 1965). Quackgrass competition drastically reduced corn height, yield, and
delayed maturity (Bandeen and Buchholtz, 1967). Unfertilized corn yields were reduced 84% by
quackgrass. An economic evaluation of preemergence herbicides applied in bands over corn rows
showed that over a 13-year period the net cost of herbicide was less than the value of corn lost through
failure to control weeds in the row (Staniforth and Lovely, 1964)." Postemergence herbicides also are of
much value in controlling weeds and increasing corn yields.

    "In the northern spring wheat-growing areas of the USA and Canada, wild mustard, lambsquarters,
common ragweed, red sorrel, common chickweed, wild garlic, henbit, wild buckwheat, and Tartary
buckwheat are problems in spring wheat fields (Nalewaja and Arnold, 1970). According to Nalewaja
and Arnold (1970), weeds vary in their ability to compete with wheat. Wild oat and mustard were
much more serious competitors than yellow foxtail and wild buckwheat. "

    Sixteen years of weed research since the information recorded in the agronomy monograph has
revealed new chemical compounds and variations on spraying techniques. However, many of the problems
prevalent then are still present. Weeds are indeed survivalists. "The science of weed control has indeed
advanced more since 1942 (the first use of 2,4-D) than in the previous million years. But chemical weed
control is still in its infancy when compared to plant breeding or soil science, for example (Klingman and
Ashton, 1975, Weed Science Principles and Practices)."

Denise A. McWilliams
Extension Crop Production Specialist
dmcwilli@ndsuext.nodak.edu


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