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ISSUE 7  June 18, 1998

 

HOW TO MAKE REDUCED HERBICIDE RATES WORK

    All users of herbicides want to apply the lowest herbicide rate that will control the target weeds. Or, from a different viewpoint, all users of herbicides want to apply the rate of herbicide that will provide the greatest return over the cost of the herbicide plus application. This "best" herbicide rate will be different for every combination of herbicide-environment-weed species-crop-weed growth stage-crop growth stage-adjuvant-weed density-tolerance for risk by user. Sometimes, the "best" herbicide rate will be lower than the lowest rate on the herbicide label. To understand why this is true, we need to understand some of the assumptions and considerations of the companies when they write a label.

    Weed Size and Crop Size. Companies must make an assumption about the normal size of weeds and crops when the herbicide will be applied. Generally, small weeds are more susceptible to herbicides than large weeds but small crop plants also may be more susceptible. Herbicide rate reduction may be possible if the herbicide is applied to weeds that are smaller than the weed size on the herbicide label. The crop also may be smaller than the suggested crop stage at application so knowledge of safety to the crop would be needed. So, herbicide rates can sometimes be reduced by early application but special knowledge is needed.

    Environment. Companies can not write a label that anticipates all environments in which a herbicide may be used. Environment has a large influence on the efficacy of herbicides. So, herbicide rate reduction may be possible if the specific environment will favor high phytotoxicity from the herbicide but special knowledge is needed on the environment-herbicide interaction.

    Adjuvants. The efficacy of some postemergence herbicides is enhanced by adjuvants such as surfactants, crop oils, methylated seed oils or fertilizer. Adjuvants vary considerably in ability to enhance herbicide efficacy even among a class of adjuvants. For example, some surfactants may double percent weed control from a herbicide while other surfactants may have little effect. The number of adjuvants presently sold is so large that testing all adjuvants with all herbicides is not possible. Companies must set label rates for herbicides by assuming that adjuvants will or will not be used and that adjuvants used will be similar to those tested with the herbicides. Herbicide rates can sometimes be reduced by using an adjuvant that is highly effective with the specific herbicide but special knowledge is needed. The herbicide-adjuvant combination must be safe on the crop as well as provide good weed control.

    Method of Application. Sprayer pressure-volume-speed-nozzle type-delivery system will influence herbicide efficacy. Companies must assume a average or common application method when writing a herbicide label. Special knowledge of the best application method for a specific herbicide and situation may allow a reduction in herbicide rate.

    Weed Species. Weeds may vary considerably in susceptibility to a herbicide. Companies sometimes list weed species separately on the label with different rates for different weeds. Some labels do not vary rates by weed species. Herbicide rates can often be reduced if the field is infested only with weed species that are highly susceptible to the herbicide. Special knowledge of weed response to the herbicides is needed.

    Performance Complaints. Chemical companies recommend product use to obtain adequate performance across most all conditions and uses. However, product complaints are seldom avoided. Herbicide rates on labels are often set to reduce the risk of less than adequate weed control and to reduce numbers of complaints. A herbicide user who is willing to accept a higher risk of poor weed control can reduce herbicide rates below the labeled rate. This should only be done when conditions favor high efficacy from the herbicide and susceptible weeds are present.

    Production Cost of the Product. Companies rarely consider the production cost of the product when they set the labeled rate of a herbicide. Companies that sell herbicides are not selling a product, they are selling a service. The companies would prefer to recommend the least amount of herbicide to control the weeds because the value of the service (the weed control) would be the same regardless of rate of herbicide. So, companies do not deliberately set rates higher than necessary but refinements in rates are possible with special knowledge about specific situation.

    Are Low Rates Legal? A herbicide user can legally choose a rate lower than listed on the herbicide label. However, the company has no obligation to support herbicides when the application rate was less than labeled rates. Herbicide users should not expect a company representative to provide any comfort or assistance if weed control is less than expected from a low rate of herbicide.

Alan Dexter
Extension Sugarbeet Specialist

 

FOXTAIL (pigeongrass) CONTROL IN SMALL GRAINS

    Making a decision on whether to control foxtail in small grains is not always easy. Research from North Dakota State University and in Canada has shown that infestations of foxtail often will not decrease wheat and barley yields, however; heavy foxtail infestations can cause harvest problems (especially when straight combining) and can cause dockage when the grain is delivered to the elevator.

    These are some situations when cost of a herbicide treatment for foxtail control is not justified. They would include:

    1.  When foxtail infestations are light - Less than 30 plants/ft2.

    2.  When the foxtail emerges after the crop is in the 3 to 4-leaf stage. This is especially true for barley. Once the small grain is in the 3 to 4-leaf stage, it can usually out compete foxtail, there by making a herbicide treatment unnecessary. However, if the foxtail population is heavy (100 plants/ft2 or more) control is probably needed.

    Another factor that complicates this situation is moisture stress. Weeds will generally cause greater yield losses under drought conditions, therefore foxtail control would be more important in droughty fields.

    Making the decision on whether to apply a herbicide for foxtail control is more complicated when the foxtail is emerging with or shortly after the small grain; as is the case in many fields this year. Some of the options to consider for foxtail control are:

1.  If the foxtail infestation is heavy, and just emerging with the crop, consider harrowing or rotary hoeing AS SOON AS POSSIBLE. Once foxtail is in the 2 to 3-leaf stage, harrowing or rotary hoeing will not give effective foxtail control. Small grains can be harrowed or rotary hoed until the 3-4 leaf stage with little effect on yield.

2.  If a harrow or rotary hoe are not an option, then consider a herbicide. Hoelon needs to be applied to small foxtail (1 to 3-leaf), therefore the decision to apply Hoelon needs to be made early in the season. If wild oats are not present, then Stampede CM can also be considered.

3.  If the foxtail infestation is light to moderate, then it may be possible to wait and see if the crop will be able to out compete foxtail. If foxtail is still a problem by the time the small grain is in the 5 to 6-leaf stage, then herbicides can be used for control.

    It is important to consider all methods of foxtail control.

 

WILD OAT CONTROL IN SMALL GRAINS

    Several cultural approaches are available for wild oat control. They include: delayed small grain seeding, post seeding cultivation and competitive crops.

    The most practical cultural method of wild oat control is delayed small grain seeding. Delayed seeding involves as early soil cultivation to stimulate wild oat germination with one or two subsequent cultivations to control emerged wild oat prior to crop seeding. Delayed seeding, though effective in controlling wild oat, has been shown to cause up to a 40% wheat yield reduction when compared to early seeding.

    Planting competitive crops is another effective cultural control method for wild oat. Barley and rye are more competitive with wild oat than spring wheat. Also, warm season row crops, such as sunflowers, soybeans, and corn should be considered in fields with heavy wild oat infestations.

    Many growers would like to eradicate wild oat from their fields. Research has shown that wild oat eradication may not be practical or economically sound. Therefore, a combination of cultural and chemical control methods should be used to manage wild oat populations and prevent unacceptable yield losses.

    Good wild oat control with any herbicide requires proper timing of applications. Postemergence wild oat herbicides require application to wild oats and crops at precise leaf stages. Leaf number on wild oats is determined by counting the leaves on the main stem and disregarding the tillers. The youngest leaf is counted as a full leaf only when another leaf becomes visible. Lower leaves which may have died from various stresses, such as frost, should also be counted in the total leaf number. An accurate leaf count is important for optimum wild oat control.

    Climatic conditions must also be considered when choosing a wild oat herbicide. For example, some wild oat herbicides work better under dry conditions than others.

    There are a number of tradeoffs for the advantages any one postemergence wild oat herbicide might offer. Early wild oat control can mean better yields because the weed has less time to compete with the crop. However, when a herbicide treatment is applied early, odds are greater that a late flush of wild oats will require a second application, or that some wild oats might escape treatment. Any uncontrolled wild oats can reduce yields, and will produce seed that contribute to next year's wild oat problem. In general, under heavy wild oat pressure (over 30 plants/square foot) research has shown that a herbicide treatment should be applied as soon as possible to prevent high yield losses.

    Below is a table showing wheat yield reduction from foxtail and wild oats. Information is compiled from research performed at NDSU.

GRASS WEED COMPETITION
    (% Wheat Yield Reduction)

# Weeds/square yard

Foxtail

Wild oats

10

50

75

100

150

0

4-5%

6-7%

8-9%

15%

8-9%

18%

25%

34%

40%

Richard Zollinger
NDSU Extension Weed Specialist


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