NDSU Crop and Pest Report
Weeds


ISSUE 2   May 13, 2004

CAN GLYPHOSATE INACTIVATION FROM DUST/DIRT BE REDUCED OR ELIMINATED?

(Side note - There is probably little concern of this with the recent rains but it is good to review the concept.)

Inactivation of glyphosate from dust is different than inactivation from antagonistic salts in water so the proven methods to eliminate hard water antagonism may help but actually not do much to eliminate inactivation from dust. Glyphosate is tightly bond to soil components and organic matter and this binding is non-reversible. That is one reason why glyphosate has no residual activity even though the herbicide can be detected by analysis for several days or weeks after application.

Salt interaction with glyphosate is reversible with ammonium sources of nitrogen but requires ammonium sulfate (AMS) to inactivate and precipitate the hard water cations. Because glyphosate binding with soil and organic matter is non-reversible, AMS would not help to overcome the deactivation but would help to increase glyphosate activity through water conditioning and increased absorption.

What can a grower do? Well, as mentioned in the side note above, rain would be the most obvious solution but in the lack of rain, I am not sure there is much that can help except use AMS and USE HIGHEST RATES OF GLYPHOSATE ALLOWED. I just got off the phone with a weed scientist from NE and he said he saw glyphosate for $9.00/gallon so for those prices just use more glyphosate.

Many universities have tested adjuvants with glyphosate in an attempt to determine adjuvant enhancement. Many mid-west universities have tested some of these adjuvants which lower spray solution pH (namely, Engame which is longer on the market) through ingredients like sulfuric acid. Under drought conditions like the mid-west had in 2001 and 2002, Engame did perform better in chem-fallow conditions than other adjuvants. There may be other products on the market that contain caustic materials like sulfuric acid but the wear on sprayer parts and hoses and applicator exposure may offset the benefits of using these types of products. I have not looked into the research to see if these ph adjusting adjuvants were used with glyphosate on RUR crops or if they cause crop injury.

This bring up another issue which affects glyphosate activity - humidity. Glyphosate is much less effective under dry, less humid conditions as humid environments. Glyphosate is, perhaps one of the most sensitive herbicides to humidity - the more humid the better glyphosate works so this is another reason to stress higher rates under drought conditions.

Since some weeds are not controlled well with glyphosate like, wild buckwheat, nightshades, dandelion, puff-ball size kochia, growers may add other herbicides to glyphosate to improve activity. Harmony GT is effective in controlling wild buckwheat and is labeled for preplant use in wheat and barley, and corn. It is not expected that Harmony GT would decrease the activity of glyphosate but Harmony GT activity would be dependant on the environmental condition as glyphosate so dry conditions and dust may decrease Harmony GT efficacy same as glyphosate. Aim is labeled for preplant application in many crops and is popular because of the rapid activity and control of many SMALL weeds. Aim may decrease glyphosate activity only of the quick burn and desiccation will limit glyphosate absorption.

 

HOW DOES COLD WEATHER AFFECT HERBICIDE ACTIVITY?

Ideal temperatures for applying most POST herbicides are between 65 and 85 F. Weeds may be killed slowly when temperatures remain 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 influence crop safety or injury. Cold temperatures may affect crop safety and weed control from herbicides. Plants metabolize 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. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.

Wild oat is more susceptible to fenoxaprop (Puma) during cool rather than warm/hot conditions. Green and yellow foxtail are warm season grasses and may stop growing 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, Discover, 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. 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.

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.

 

STAGING SMALL GRAIN FOR HERBICIDE APPLICATION IN PLANTS WITH MISSING LEAVES

Below freezing temperatures around the state has caused wheat and barley leaves to die and fall off. In these situations, how are small grain plants staged for correct herbicide application?

No. 1 Rule = all leaves (missing and present) must be counted. If the first two leaves are frozen off and the plant grows 3 new leaves, the plant is physiologically in the 5-leaf stage not the 3-leaf stage.

There are a couple of techniques available to determine total number of leaves. One is to use the NDAWN weather system to determine growing degree days. Refer to pages 11 and 12 in the 2004 ND Weed Control Guide for number of growing degree days to reach each leaf stage. Another method would be to dig up a few plants and count leaf sheaths attached to the main stem. Each leaf arises from a leaf sheath and the leaf sheath should be present on the main stem even though the leaf may be missing.

Some herbicides that require precise small grain staging to avoid crop injury are:

  • 2,4-D or any products containing 2,4-D - do not apply before the 5-leaf stage.
  • Dicamba - do not apply after the 5-leaf stage.
  • Assert - apply before jointing.
  • Everest - apply before jointing (4-leaves + 2 tillers).
  • Puma on barley - apply before 5-leaf stage
  • Dakota and Cheyenne - do not apply after 6-leaf stage.
  • Silverado - apply before jointing.
  • Refer to pages 6-12 of the 2004 ND Weed Control Guide for correct crop stage for herbicide application in small grains.

    Richard Zollinger
    NDSU Extension Weed Specialist
    r.zollinger@ndsu.nodak.edu


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