Section 3 - Weed Management

Herbicide Effectiveness Under Weather Stress

Temperatures following herbicide application can influence crop safety and weed control from herbicides. Plants metabolize most herbicides, but metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants. Rapid degradation under warm conditions allows crop plants to escape herbicide injury.

Ideal temperatures for applying most herbicides are between 65 and 85F, and speed of kill may be slow when temperatures remain below 60F. Some herbicides may injure crops if applied above 85F or below 40F. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.

Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, or farmsteads.

Wild oats is actually more susceptible to Puma and Discover during cool rather than warm/hot conditions. Green and yellow foxtail are warm season grasses and may stop growing under cold conditions, resulting in poor control. Grass and broadleaf weeds are controlled more effectively when plants are actively growing. Wild oat control from Puma is significantly reduced by drought stress. Achieve and ALS grass herbicides in wheat generally provide more consistent and greater grass control in warm, dry conditions compared with cool, wet conditions.

Other ACCase herbicides, such as Assure II, Poast, and Select, control grasses better in warm weather when grasses are actively growing. Cool or cold conditions at or following application of ACCase herbicides and significant rainfall shortly after Achieve application may increase injury to wheat.

Cold temperatures, including freezing conditions following application of ALS herbicides, Sencor, and bromoxynil, may increase crop injury of labeled crops with little effect on weed control. Delay applying fenoxaprop, ALS herbicides, and Sencor until daytime temperatures exceed 60 degrees F and after active plant growth resumes.

Basagran, Cobra, Flexstar, Liberty, paraquat, and Ultra Blazer are less likely to cause crop injury when cold temperatures follow application but less weed control may result.

2,4-D, MCPA, dicamba, clopyralid, fluroxypyr, and glyphosate (resistant crops) have adequate crop safety and provide similar weed control across a wide range of temperatures, but weed death is slowed when cold temperatures follow application.

Adjuvants can make a difference in stressed conditions. Superior adjuvants can help overcome leaf barriers like thick cuticle. Most herbicides used are translocated, so movement through the cuticle and then through the plant to growing points are critical for adequate control. MSO type adjuvants do not cause herbicide phytotoxicity in cool weather but rather in very humid and hot weather. MSO type adjuvants usually give greater herbicide enhancement than nonionic surfactants and petroleum oil (COC) adjuvants.

Where product labels permit, addition of crop oil concentrate rather than nonionic surfactant usually results in greater herbicide activity. Some products also allow for N-based spray additives, which tend to improve efficacy of certain products during periods of slight stress.

Ultimately, follow label directions with regard to application: all herbicides have directions for use to maintain crop safety and application effectiveness.

Rain Interference with Herbicide Applications

The minimum interval between application and rain for maximum weed control varies by herbicide; it’s as little as 1 hour for products such as Beyond, Everest, Poast, Puma, Select Max and Starane, but 6-8 hours for products that include Aim, Dicamba, Stinger, and WideMatch. The time interval is 1 hour for 2,4-D ester/MCPA ester, but 4-6 hours for 2,4-D amine/MCPA amine. The 2008 N.D. Weed Control Guide, page 66 (www.ag.ndsu.edu/weeds/w253/w253-3a.htm) lists the minimum interval between herbicide application and rain for maximum herbicide uptake and efficacy. Follow the label for more instruction.

Managing Tank Solution with Delayed Spraying

A spraying delay from rain or other reasons may result in a partial sprayer tank load of unused herbicide solution. Problems may occur as the spray solution sets in the tank waiting for the field to dry enough so the remainder of the spray can be applied. Some herbicides degrade slowly as they set in a water solution, but most formulated herbicides will retain most of their activity over a few days. The exact amount of degradation is affected by water temperature, water pH, and the active ingredient in the tank, so predicting the rate of degradation is not precise.

Some herbicides and herbicide combinations may settle out of suspension with time, so gentle agitation may be needed to prevent the herbicide from forming a layer on the bottom of the tank. Adding fresh water and herbicide to a sprayer with old spray solution is not a good idea if the herbicides in the sprayer are among those which may plug screens and nozzles after setting for a time. Dealing with a partial load of a plugging problem is much better than dealing with a full load.

Finding a way to apply an aged spray solution to a registered crop is always better than dumping the herbicide solution. Most of the benefit of the herbicide will still be realized even if some degradation has occurred, so the total value of the herbicide will not be lost. Removal of the screens at the nozzles and increasing spray pressure will nearly always allow application of an aged spray solution, especially if it was kept in suspension by gentle agitation.

Increased spray pressure will increase application rate, but this will be partially offset by the reduction in activity of the herbicide. Increasing spray pressure from 40 psi to 60 psi will increase spray volume by about 20% and increasing spray pressure from 40 psi to 80 psi will increase spray volume by about 40%. The amount of increase in spray pressure can be adjusted based on estimated herbicide degradation, label limits on herbicide rates and the amount of precipitate in the spray solution.

Using Herbicides at Reduced Rates

Ideally, control of target weeds at the lowest herbicide rate provide the greatest return over herbicide and application costs. This “best” herbicide rate will be different for every combination of herbicide-weed-environment-adjuvant combination. Sometimes, the “best” rate will be lower than the lowest rate on the herbicide label. Following are factors considered by companies when they write a label.

Weed Size and Crop Size. Companies make an assumption of weed and crop size at herbicide application. Small weeds are more susceptible to herbicides than large weeds, but small crop plants may also be more susceptible. Reduced herbicide rates may be used if herbicides are applied to weeds smaller than listed on label. The crop will also probably be smaller, so knowledge of crop safety also is needed.

Environment. Companies write labels that cover most environments in which herbicides are used. Environment has a large influence on the efficacy of herbicides. Herbicide rates may be reduced under ideal environmental but special knowledge is needed on the environment-herbicide interaction.

Adjuvants. Most POST herbicides require addition of adjuvants such as surfactants, crop oils, methylated seed oils, or fertilizer. See section on spray adjuvants (A5) for more information. Adjuvant information is fairly general on pesticide labels to address adequate weed control under most situations. Herbicide rates sometimes can be reduced by using adjuvants that are highly effective with a specific herbicide but additional knowledge is needed. The herbicide-adjuvant combination must be safe on the crop as well as provide good weed control.

Method of Application. Special knowledge of the best application method for a specific herbicide and situation may allow a reduction in herbicide rate.

Weed Species. Labels sometimes list weed species separately on the label with different rates for different weeds. Herbicide rates may be reduced when highly susceptible weed species are present.

Performance Complaints. Using reduced rates may result in poor weed control. User assumes all risk and liability of unacceptable weed control when less than labeled rates are used

Are Low Rates Legal? A herbicide user can legally choose a rate lower than listed on the herbicide label unless the label specifically prohibits low rates. However, the company has no obligation to support herbicide efficacy 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 rate of herbicide that is less than the labeled rate.

Evaluate Late-Season Glyphosate Treatments

Weeds that poke heads through the soybean canopy later in the growing season may tempt growers to load up the sprayer for one last trip across the field. However, potential value of these late season treatments should be evaluated prior to spraying. Realize that impact on yields has already occurred, thus little yield benefit is likely to be achieved. If the weeds have not initiated seed set at the time of application, it should be possible to reduce seed production. However, if the fruiting structure is visible, it is unlikely that killing the weeds at this late date will influence seed production or viability of the seed. Many people think that late season treatments will reduce the viability of seeds that are produced, but research has consistently shown that seeds that have been initiated at the time of application are unlikely to be greatly influenced. The other possible benefit of the late season treatments is harvesting efficiency, and there may be situations where this may make the treatment worthwhile.

Seven Tips to Avoid Herbicide Crop Injury

These days, we need to be careful and clean herbicide residues out of the sprayer even when switching between different corn or soybean types. For example, a conventional corn hybrid could be damaged by a sprayer contaminated with glyphosate after spraying Roundup Ready corn. Some of the most serious contamination problems are glyphosate injury to corn and dicamba injury to soybeans. Both of these crops are very sensitive to these herbicides.

Severity of crop injury will vary by herbicide and also depends on factors such as weather conditions, crop growth stage, and adjuvants. Some guidelines to consider regarding tank contamination:

1. Once injury occurs, there is no fix.

2. Postemergence-applied herbicides are more likely to injure crops than herbicides applied preemergence because the herbicide is applied directly to the leaves rather than being diluted in the soil. Also, several preemergence herbicides have little or no foliar activity. Therefore, be especially careful to clean postemergence herbicide residues from spray equipment.

3. Systemic herbicides like glyphosate, dicamba and other growth regulators, ALS inhibitors (Accent, Raptor, etc) and ACCase inhibitors (Assure, Poast, etc.) are a greater concern than contact herbicides, because systemic herbicides damage the growing point. Contact (non-mobile) herbicides only damage sprayed leaves. Relatively high concentrations of contact herbicide residues are required to cause long-term damage in most cases. However, low concentrations of systemic herbicides can cause serious damage.

4. Just spraying until a tank is empty does not mean that all the herbicide is removed from the spraying system (sump, filters, pump, lines, etc.). Proper cleaning and rinsing is needed to remove the remaining spray solution or herbicide residues.

5. Clean spray equipment as soon as possible after use. Dried residues are more difficult to clean and remove.

6. Follow the label’s directions for the best cleaning agent to use. On several labels, you will note that the cleaning procedures recommend that the cleaning solution stand in the sprayer for several hours to overnight. Cleaning a spray tank is not a job that should be rushed, especially with certain herbicides that are highly active on sensitive crops.

7. Never add chlorine bleach to ammonia or liquid fertilizers that contain ammonia, because toxic chlorine gas can be formed.

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