Section 3 - Weed Management

12 Glyphosate Weed Control Tips

1) Dew on plant foliage at application (or rain soon after) may reduce weed control. Glyphosate is very water soluble, which simply means that it can be dissolved in water. Dew on leaves dilutes herbicide concentration in spray droplets, and negates the effect of low spray volume at application. Allow at least a 6 hour rainfast period for all glyphosate formulations regardless of label rainfast recommendation.

2) Glyphosate is affected by humidity. Weed control increases as humidity increases.

3) Glyphosate is not deactivated by sunlight. Time of day application studies show that activity of glyphosate if greatest when applied after 10:00 am and before 4:00 pm.

4) Use the lowest water volume (gpa) allowed on the label. Low spray water volumes produce spray droplets of high glyphosate concentration which results in greater absorption.

5) Use drift management techniques, either through nozzles or adjuvants. Glyphosate is a non-selective, non-residual, translocated foliar herbicide. Glyphosate can cause severe injury or death of plants intercepting even a small amount of downwind spray droplet drift. Use drift reducing nozzles or drift reducing adjuvants to help manage drift.

6) Glyphosate is not volatile. Glyphosate does not produce fumes or vapor after application similar to dicamba or esters of 2,4-D, MCPA. Off-target movement of glyphosate is from droplet or particle drift, not volatility.

7) Always add AMS to glyphosate. AMS enhances absorption and translocation, deactivates antagonistic hard water salts, and the ammonium in AMS makes glyphosate-NH4 in the spray droplet, which is easily absorbed into the plant. The NDSU recommendation is to add AMS at a minimum of 1 lb/A if using greater than 12 gpa spray volume or 4 to 6 lb/100 gallons of water. Glyphosate labels generally recommend AMS at 8.5 to 17 lb/100 gallons water. However, analysis of water across the state has shown that lower rates of AMS are adequate. AMS at lower rates will overcome antagonism from most ND water. Some locations, particularly in western ND, have hard water that exceeds 1600 ppm or even 2500 ppm of hardness and do require AMS at 8.5 to 17 lb/100 gal water. Growers should know their water quality to determine AMS rate. Allow sufficient time for AMS to dissolve before application.

8) Add high quality NIS if the glyphosate label allows use. NIS is used because oil adjuvants can be antagonistic. Research has shown greater weed control even when NIS was added to full load glyphosate formulations. Use reputable adjuvants from major adjuvant manufacturers. Do not believe claims of cutting herbicide rates by 50%. Data show generally less control from some “AMS replacement” adjuvants as compared to NIS plus AMS.

9) Glyphosate applied in cool/cold weather will kill weeds – it just takes longer. Ideal temperatures for applying most POST herbicides are between 65 and 85 F. Weeds may be killed slower when temperatures remain below 50 F. Cold weather is a stress to plants. AMS and NIS can be used overcome the reduced control of stressed plants. Absorbed glyphosate will remain in the plant until warm temperatures cause weeds to resume glyphosate translocation to growing points.

10) Weed control is reduced when glyphosate is applied to desiccated plant tissue affected by frost. Below freezing temperature may burn off top growth and desiccate plant tissue. Plant material injured by freezing temperatures will not translocate herbicides. Application to new plant growth is required for optimum herbicide activity.

11) Dust inactivates glyphosate. If glyphosate absorption is slowed during cold weather by hardening of the cuticle (which may or may not be true) then glyphosate on the plant surface may be susceptible to inactivation from dust, especially if the wind blows. This applies also using slough water for spraying. Mud and soil in slough water will inactivate glyphosate. Addition of NIS or AMS will not overcome inactivation from dirt. Glyphosate is strongly and irreversibly absorbed to clay particles and organic matter.

12) DO NOT USE REDUCED GLYPHOSATE RATES. The price of glyphosate has decreased to affordable levels, and weed control is relatively inexpensive compared to conventional weed control strategies. Reducing glyphosate rates may encourage the development of resistant weed biotypes, and eventually reduce the effectiveness of this key weed control tool.

Controlling Volunteer Roundup Ready Crops

Go online to www.ag.ndsu.edu/weeds/w253/w253-4d.htm for ratings of relative herbicide effectiveness. Under favorable conditions control may be better than indicated, and under unfavorable conditions herbicides may give erratic results. Dry and cool weather increases herbicide persistence while wet and/or warm weather reduces herbicide persistence.

Organic Matter, Spray Water Can Affect Herbicide Performance

Some herbicides are partially adsorbed and inactivated by soil organic matter, both high and low. Thus, test organic matter to help make herbicide product decisions. OM levels change very slowly, so testing once every 5 years should be adequate.

Spray carrier water can also reduce the effectiveness of herbicides. Water high in sodium bicarbonate reduces the effectiveness of 2,4-D and MCPA amines (not esters), Poast, glyphosate, and dicamba. High salt levels in spray water can reduce weed control in nearly all situations. Calcium and, to a lesser degree, magnesium are antagonistic to 2,4-D and MCPA amine, dicamba, and glyphosate.

Analysis of spray water sources will determine possible effects on herbicide efficacy. Water samples can be tested (for about $25) at the following laboratory: NDSU Soil and Water Environmental Laboratory, (701) 231-7864, Waldron 202, NDSU, Fargo, ND 58105-5575.

Reducing Spray Drift

Adjust boom as close to the target as possible while maintaining uniform spray coverage. Choose nozzles with a wide angle over narrow angle nozzles.

Some herbicides volatilize under warm or hot temperature and cause plant injury from vapors or fume drift. Temperature on the soil surface often is several degrees warmer than air temperature. Herbicide vapor can drift further and over a longer time than spray droplets. Wind blowing away from susceptible plants during application will prevent damage from droplet drift but a later wind shift toward the susceptible plants could move damaging vapors to the plants.

Low volatile esters of 2,4-D or MCPA may produce damaging vapors between 70 to 90 F. Amine formulations are essentially non-volatile even at high temperatures. To minimize the risk of drift injury, dicamba and ester formulations of 2,4-D and MCPA should not be used near susceptible plants. Several sprayer nozzles designed to reduce spray drift are available; these drift -reducing nozzles are flat-fan types and are adapted for conventional spray equipment. Small plastic cones that fit around individual nozzles reduce drift by approximately 25 to 50%, and spray shields which enclose the entire boom reduce drift by approximately 50 to 85%. Spray shields provide greater drift reduction when winds are low and droplets are relatively large. Therefore, spray shields should not be used as a substitutefor other drift control techniques but as a supplement to all other applicable methods of drift reduction.

Spray drift can be reduced by increasing droplet size and droplet size can be increased by reducing spray pressure, increasing nozzle orifice size, special drift reduction nozzles, additives that increase spray viscosity, and rearward nozzle orientation on aircraft.

For more detailed informationon managing spray and vapor drift, efer to NDSU Extension Circular A-657, “Herbicide Spray Drift” and Circular WC-751 “Documentation for Suspected Herbicide Drift Damage,” both of which can be found on the Internet under NDSU online weed control publications: www.ag.ndsu.edu/pubs/weeds.html.

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