ISSUE 14 August 3, 2000
WEED CONTROL IN PASTURE, RANGELAND, AND CRP
Growers continually ask how to control Canada thistle, perennial sowthistle, and other annual and noxious weed in pasture, rangeland, and CRP WITHOUT killing desirable legumes, such as, alfalfa, sweet clover. Unfortunately, most herbicides that are most effective in controlling annual and perennial broadleaf weeds also kill legumes. These herbicides include Tordon, 2,4-D, Banvel/Clarity, Curtail, and spot treatment with Roundup. On page 48 of the 2000 ND Weed Control Guide lists Plateau for annual and perennial weed control in pasture, rangeland, and CRP. Plateau is in the same chemistry as Pursuit and Raptor and is safe on legumes, including alfalfa and sweet clover.
The Plateau label lists control of many annual broadleaf and grass weeds at 4 to 6 fl oz/A and many perennial broadleaf weeds at 8 to 12 fl oz/A (See label). The label gives only suppression of Canada thistle a the highest rate. 1998 research by Dr. Rod Lym show May application at 12 fl oz/A + MSO adjuvant gave 80% control when evaluated July, 70% control in August, and 6% control in October. Fall applications did not provide greater than 58% control at any evaluations the next year (See 1998 ND Weed Control Research, pink section, page 13). In other words, spring applications only give 3 months of control and fall applications do not work. The other factor in the equation is the price of Plateau. Plateau at 12 fl oz/A is $27.00/A and the MSO adjuvant is $2.50/A. The total cost is approximately $30.00/A. One would seriously have to justify 3 months of weed control for $30.00/A.
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 years 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.
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 providing greater control 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.
NDSU Extension Weed Specialist
BETWEEN-ROW MOWING POTENTIAL FOR WEED CONTROL?
Recent research out of Missouri has shown that alternative weed
control can be used in soybeans. In the current issue of Weed Science (48:487-500), a new,
between-row mowing system was used to control weeds. The system was designed to reduce
herbicide use. Using band-applied herbicides over crop rows and two or more between-row
mowings was tested on soybean over six years. By mowing the weeds close to the soil
surface two or more times during the growing season, annual grass and broadleaf weeds were
killed or suppressed. The key weeds that were controlled with the system and monitored
were common ragweed, giant foxtail and waterhemp when the mowing was properly timed. Later
shading by the crop contributed to further weed suppression in this system. The yield of
soybeans across the treated plots was not significantly different than the yields in the
weed-free plots. The herbicide rates that were cut across the field were only reduced 50%
due to only 50% of the field area being sprayed due to banding (so a full rate over the
top of the rows). Since many farmers who
cultivate often also band apply herbicides, this study confirmed that alternative means of weed control can be useful and that timing of any control measure is very important. And, while this experiment reduced herbicide use 50% by band applications, reductions up to 60% should be possible where herbicides and mowing/cultivating are properly timed. So, now if only those solar-powered mowing units (suggested as the operator-free lawnmowers of the future) that can be programmed to go down
rows, tram-lined down the field or structured on GPS grids could be put in place and they might be equipped with micro-rate tanks for side squirts of various herbicides within the row during the season, we might have a modern, good system in place! Until then, look at the weed-free field of soybeans off the soybean cam now on the Internet at:
CONTROL WEEDS EARLY BEFORE BEGINNING SEED SET
In a study established down in Louisiana, researchers tested if weed seed production and seedling emergence was affected by late-season glyphosate (Roundup) applications. Monitoring common cocklebur, hemp sesbania and sicklepod weed species, Roundup was applied at initial weed seed set, mid-seed fill and at weed physiological maturity using three rates of herbicide. Common cocklebur bur weight and burs per plant were reduced by 69-70% with applications at the initial fruit set. With this early control, the resulting weed seedling emergence was only 3%. Roundup applied at the initial seed set of hemp sesbania reduced resulting weed seed weight by 73%, the seed per plant by 86% and the weed seed viability by 94%. The sicklepod treated at initial weed seed set showed a weed seed weight reduction of 46%, a total weed seed number reduction of 83% and a tested weed seed viability reduction of 66%. In the field experiments also run with the previous greenhouse results, common cocklebur and hemp sesbania seed production was only reduced with the very early, initial seed set herbicide treatment. Unfortunately, the sicklepod, a more wily weed, showed inconsistent responses in the field with no measurable reductions in seed per plant or seedling emergence in the first year of the treatments. The second year of the study, the sicklepod did show initial seed set treatments could reduce the seed per plant by 88% and seedling emergence by 72% in the early treatment. Results indicate that even low recommended rates of Roundup at initial weed seed set can reduce seed production and subsequent weed seedling emergence, however, later applied treatments often have little effect on seed viability.
Extension Crop Production Specialist