ISSUE 4   June 4, 2009

TIMING THE FIRST GLYPHOSATE APPLICATION IN ROUNDUP READY SUGARBEET

Glyphosate should be applied to Roundup Ready sugarbeet when weeds reach 1 to 2 inches in height and before sugarbeet reaches the 4- to 6-leaf stage of development. Weed competition and subsequent yield loss is likely to occur if glyphosate is applied beyond this time period except for sparse weed populations. Apply glyphosate at 0.98 pounds acid equivalent per acre (lbs ae/A) in the initial application. A glyphosate rate of 0.98 lb ae/A is equivalent to 28 fluid ounces per acre (fl oz/A) of Roundup branded products, 41 fl oz/A of 3.0 lbs ae per gallon (lbs ae/gal) products, and 34 fl oz/A of 3.7 lbs ae/gal products. If weeds are greater than three inches in height at the time of the initial glyphosate application, glyphosate should be applied at 1.125 lbs ae/A. This rate is the maximum rate allowed in a single application and is equivalent to 32 fl oz/A of Roundup branded products, 48 fl oz/A of 3.0 lbs ae/gal products, and 39 fl oz/A of 3.7 lbs ae/gal products. Once sugarbeet reaches the 8-leaf stage of development, the maximum rate of glyphosate that can be applied in a single application to Roundup Ready sugarbeet is 0.75 lb ae/A.

Jeff Stachler
Agronomist - Weed Science
jeff.stachler@ndsu.edu

MAXIMIZING GLYPHOSATE ACTIVITY IN GLYPHOSATE-RESISTANT CROPS The goal of all growers should be to maximize glyphosate activity whenever it is applied to glyphosate-resistant crops. This goal should maximize profitability and weed control and reduce the risk for herbicide resistance. Below are the most important management strategies for improving glyphosate activity. For additional information, consult pages 88 and 89 in the 2009 North Dakota Weed Control Guide and pages 52 and 53 in the 2009 Sugarbeet Production Guide.

1. Apply glyphosate to small (< 4") annual weeds.
2. Apply glyphosate to perennial species in the bud to early-flowering stage of development.
3. Apply the most effective rate for the most difficult to control species in the field. For many annual species the minimum rate of glyphosate should be 0.75 pounds acid equivalent/acre (lb ae/A). Species such as lambsquarters, velvetleaf, wild buckwheat, common ragweed, giant ragweed, common mallow, smartweeds, biennial wormwood, horseweed/marestail, and waterhemp can be difficult to control with glyphosate. Consider using the maximum single-use rate of glyphosate for control of these species, especially if a reduction in control has been observed over time. Rates of glyphosate greater than 0.75 lb ae/A usually improves control of perennial species, compared to lower rates. Multiple glyphosate applications can also improve control of perennial species.
4. Always add ammonium sulfate (AMS) to glyphosate mixtures. Ammonium sulfate should be added at a minimum of 1.0 pound per acre if using greater than 12 gallons per acre of spray volume or 4 to 6 pounds per 100 gallons of spray mixture (lbs/100 gal) for most of North Dakota. If water hardness is greater than 1600 ppm apply minimum of 8.5 to 17 lbs/100 gal.
5. Allow at least a 6 hour rainfast period for all glyphosate formulations for maximum activity. A shorter rainfast period can be acceptable for the most susceptible species. Lambsquarters control is usually reduced if the rainfast period is less than 6 hours.
6. Apply glyphosate during the warmest and most humid weather conditions to maximize activity. Cold weather a few days before and after application usually reduces control. Increasing the rate of glyphosate during cold weather can reduce some loss of weed control.
7. Most glyphosate formulations include nonionic surfactant (NIS) at a high enough concentration for maximum activity. However, some glyphosate formulations do not include NIS. For these formulations add a quality NIS product at 0.5 to 1.0 % v/v. Some weed species, especially lambsquarters, can be more effectively controlled with the addition of NIS at 0.25 %v/v to all "surfactant-loaded" glyphosate formulations. Some glyphosate formulations prohibit the addition of NIS.
8. Glyphosate activity is influenced by the time of day of the application. Maximum activity occurs between 8:00 AM and 8:00 PM. Velvetleaf, common ragweed and giant ragweed control may be the most negatively affected by the time of day of the glyphosate application.
9. Application of glyphosate in low water volumes improves glyphosate activity. However, when spraying large weeds and/or dense weed canopies, higher spray volumes usually improves glyphosate activity.
10. Glyphosate is strongly and irreversibly absorbed to clay particles and organic matter. Therefore dust of any amount, especially initiated by the wheels of the sprayer, will cause a reduction in glyphosate activity. Two methods for decreasing this problem are to drive slower and apply higher spray volumes near the sprayer wheels. These solutions will not completely solve the problem. Do not over over-apply glyphosate if using higher volume nozzles near the sprayer wheels. The best method may be to off-set subsequent applications from the first application.
11. When mixing other herbicides with glyphosate, add the most effective adjuvant for the herbicide being added. This strategy will maximize the activity of the herbicide(s) being added to the glyphosate mixture. If the herbicide being added to the glyphosate mixture recommends the addition of an oil adjuvant for maximum activity, include a high surfactant oil concentrate (page 132 of 2009 North Dakota Weed Control Guide), increase the rate of AMS if not using maximum rates, and increase the rate of glyphosate. If possible, do not mix antagonistic herbicides with glyphosate.
12. Know the concentration of glyphosate in the formulation being used. There are currently six different glyphosate concentrations available. This will ensure the application of the correct amount of product.
13. Scout fields closely for plants surviving glyphosate applications. React accordingly this growing season and subsequent seasons to eliminate these plants from the population.

Jeff Stachler
Agronomist - Weed Science
jeff.stachler@ndsu.edu

Rich Zollinger
Extension Weed Specialist
r.zollinger@ndsu.edu

Temperatures over the last month have fluctuated greatly. Cold temperatures two weeks ago caused a reduction in glyphosate activity. Individual plants of lambsquarters and annual smartweed species were not completely controlled at a research location while other plants and other species were completely controlled. Cold weather in early June of 2008 also caused a reduction in glyphosate activity. The current forecasted temperature for Wednesday morning of this week is the low to mid-30 F. This will likely cause glyphosate applications in the next several days to be less effective.

The following paragraph about glyphosate activity during cool and cold weather can be found on page 88 of the 2009 North Dakota Weed Control Guide. Weed control from glyphosate applied during cool and cold weather will take longer but the end result (weed control) will usually be the same as from application in warm weather. Ideal temperatures for applying POST herbicides are between 65 and 85 F. Speed of kill will be slower during cold weather also. Use higher rates to overcome reduced control if cold temperatures occur a few days before or if forecasted after application. Cold weather is a stress to plants. Weeds with low level resistance may not be controlled whether in good or adverse conditions. Proper timing of glyphosate application is critical for adequate weed control. Glyphosate applied during cold weather and to large weeds will result in less weed control. AMS enhances weed control and can partially overcome reduced control of stressed plants.

Waiting for warmer temperatures usually is not a viable option because plants become older and larger making them more difficult to control as well. The best recommendation is to carefully scout fields 10 to 14 days after the initial glyphosate application to determine if plants present at the time of application were controlled. If plants have survived the initial application, allow them to resume normal growth before applying the second glyphosate application or apply a second glyphosate application 21 days after the initial application. Apply the maximum labeled-rate of glyphosate remaining for the second application. Another option would be to apply an herbicide with an alternative mode of action to control the surviving plants, however, plant height is likely to be too tall for other herbicides to effectively control the surviving plants.

Jeff Stachler
Agronomist - Weed Science
jeff.stachler@ndsu.edu

Rich Zollinger
Extension Weed Specialist
r.zollinger@ndsu.edu