ISSUE 14 August 2, 2001
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. If only dry down of grain straw and grassy weeds is desired then rates of 0.5 to 1 pt/A of Roundup type products are sufficient. Higher rates of 1 qt/A are needed for dry down and control broadleaf weeds, hard-to-control weeds, like kochia and nightshade, and perennial weeds.
5. 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. Most generic brands of glyphosate are 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.
NDSU Extension Weed Specialist