ProCrop

Herbicide Use During Cold Weather

Sunlight inactivates some herbicides by ultra violet (UV) light. Trifluralin and Eptam degradation is minimal because incorporation is done soon after application. Achieve, Poast, and Select are highly sensitive to UV light and will degrade rapidly if left in nonmetal spray tanks for an extended period of time or if applied during mid-day. To avoid UV breakdown, apply Achieve, Poast, and Select soon after mixing, after 4:00 pm, and with superior oil adjuvants. Glyphosate is not sensitive to UV light but research has shown reduced weed control if glyphosate is applied after 4:00 pm or before 10:00 am.

Ideal temperatures for applying most POST herbicides are between 65 and 85 F. Weeds may be killed slowly below 60 F. Some herbicides may injure crops if applied above 85 F. 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.

Temperatures following herbicide application determine crop safety or injury. Cold temperatures may influence crop safety and weed control from herbicides. Plants degrade herbicides by metabolism, but plant 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. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.

Wild oat is more sensitive to fenoxaprop (Puma) during cool rather than warm/hot conditions. Green and yellow foxtail are warm season grasses and may shutdown under cold conditions resulting in reduced control. Grass and broadleaf weeds are controlled more effectively when plants are actively growing. Cool or cold conditions at and following application of Achieve and products containing fenoxaprop give greater grass weed control but also may cause crop injury. Other ACCase herbicides of Assure II, Poast, Fusilade DX, and Select provide better grass control in warm weather when grasses are actively growing.

Cold temperatures, including freezing conditions following application of ALS herbicides, Sencor, and bromoxynil may increase crop injury of respective crops with little effect on weed control.

Basagran, Cobra, Flexstar, Liberty, paraquat, Stampede, and Ultra Blazer may not cause crop injury when cold temperatures follow application but less weed control may result.

2,4-D, dicamba, MCPA, Stinger, Starane, glyphosate (resistant crops) have adequate crop safety and provide similar weed control, but weed death is slowed when cold temperatures follow application.

Recommendations for applying products containing fenoxaprop, ALS herbicides, and Sencor is to delay application until daytime temperatures exceed 60 degrees F and after active plant growth resumes.

Adjuvants may also affect crop safety and weed control. Oil additives may increase risk of crop injury but may be necessary for greatest weed control. Refer to each herbicide label for specific information on adjuvant use during stress conditions. Use an oil additive if risk of crop injury is acceptable for those herbicides that allow use.

Glyphosate
Glyphosate applied POST will control annual and perennial weeds in preplant, in-crop use on resistant crops, and post-harvest in conventional or reduced-till systems.

Glyphosate at 0.5 pt/A of a 3 lb ae/gal concentration controls foxtails, at 0.75 pt/A controls volunteer small grains and at 1 pt/A controls wild oat and downy brome less than 4 inches tall. Use higher rates on larger weeds, tolerant weeds, or if plants are under moisture stress. Use 3 to 10 gpa by ground or 3 to 5 gpa by air when glyphosate is applied at low rates. Glyphosate at 2 pt/A should be applied when quackgrass is at least 8 inches tall (3 to 4 leaf stage) and actively growing; at 4 to 6 pt/A when Canada thistle is actively growing and just before the bud stage. Tillage can be performed the same day as glyphosate application for annual weeds but tillage should be delayed for 3 days following application. Some glyphosate formulations contain adjuvants that enhance uptake allowing less time between application and tillage or application.

Glyphosate can be used in the spring before or after planting but before emergence of several crops. Potential for crop injury exists when glyphosate plus 2,4-D or dicamba mixtures are applied immediately before or after planting due to the PRE soil activity of 2,4-D and dicamba. Glyphosate at the equivalent of 2.66 pt/A of a 3 lb ae/gal concentrate is required to control fall planted rye or wheat prior to seeding crops in spring.

Dew on plant foliage at application may reduce weed control. Glyphosate should be applied in low spray water volumes to produce spray droplets having a higher concentration of glyphosate. Dew on leaves dilutes spray droplets and negates the effect of low spray volumes at application.

ALWAYS add AMS to any glyphosate products at 8.5 lbs/100 gal water or 1 lb/A if less than 12 GPA. Addition of AMS increased weed control even under good growing conditions and/or lack of salts in water. Allow sufficient time for AMS to dissolve before application.

NIS Approved for use in Water:
Only the Glypro, Rodeo, and some 4 lb ae/gal formulations of glyphosate can be applied on water because they do not include adjuvants are not toxic to fish and aquatic life. For any herbicide used in and around water, add only approved surfactants for effective weed control. Some surfactants labeled for use in and around water are: Activate Plus, Agridex, Class Act NG, Induce, Liberate, LI-700, Preference, R-11, Widespread, and X-77.

Glyphosate product rates based on formulation and acid equivalent (ae) and active ingredient (ai) rate. Pounds ae or ai are found on glyphosate product labels.

                                 0.38 ae 0.57 ae 0.75 ae 1.125ae 1.5 ae
3 lb ae (4 lb ai) =        16 fl oz 24 fl oz 32 fl oz 48 fl oz 64 fl oz
3.7 lb ae (5 lb ai) =     13 fl oz 20 fl oz 26 fl oz 40 fl oz 52 fl oz
4 lb ae (5.4 lb ai) =     12 fl oz 18 fl oz 24 fl oz 36 fl oz 48 fl oz
4.17 ae (5.1 lb ai) =    12 fl oz 18 fl oz 24 fl oz 36 fl oz 48 fl oz
4.5 ae (5.5 lb ai) =      11 fl oz 16 fl oz 22 fl oz 32 fl oz 44 fl oz
5 ae =                        10 fl oz 15 fl oz 20 fl oz 30 fl oz 40 fl oz

Registered Glyphosate Products:

* Full = No additional NIS needed. Partial = Additional NIS needed.
None = Additional NIS at full rate required.

**AMADS = 1-aminomethanamide dihydrogen tetraoxosulfate (Monocarbamide dihydrogen sulfate)

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