ISSUE 9   June 29, 2006

ERRATUM

Last week in the Pest Report it was incorrectly stated that Assure II was not labeled on dry bean. Assure II is labeled on dry bean. However, no dry bean acreage was reported as being applied with Assure II in the 2004 Pesticide Use Survey, so the acres applied to sunflower and flax remain at 85,000 acres.

 

CLARIFICATION OF ROUNDUP APPLICATION WINDOW ON SOYBEAN

Crop and Pest Report #7 included an article attempting to clarify how late Roundup can be applied to soybean. It would be best to disregard the article and follow label directions which state that applications can be made through flowering. Indeterminate soybean varieties in ND will flower through R5 (See Crop and Pest Report # 8 on flowering) and possibly into R6 with rain. The Roundup labels state that applications can be made through flowering.

From a practical standpoint, ground that had excess and standing water will be seeded late. These soybeans may move from vegetative to reproductive stages quickly because of the day length. Growers will likely apply Roundup to control weeds while soybean plants are flowering. So Roundup can be sprayed as long as the crop is flowering but the 14-day preharvest interval (PHI) must be observed.

 

SECTION 18 APPROVED FOR POAST ON BUCKWHEAT

The ND Dept of Ag has issued a crisis exemption for Poast to control grass weeds in buckwheat. Available herbicides registered for buckwheat are for pre-plant application only. Without postemergence weed control, buckwheat growers would incur major yield losses, which has been documented and is required for Section 18 exemption.

The exemption allows up to two ground or aerial applications of Poast at a maximum rate of 2.5 pints per acre through July 6, 2006. No applications can be made within 21 days of harvest. Applicators must follow all directions, restrictions and precautions on the EPA-approved labeling and the crisis-use directions.

Buckwheat is considered a minor crop in the U.S. with about 1 million acres in production. North Dakota, New York, and Pennsylvania are the leading buckwheat-producing states.

 

EFFECT OF RAINFALL ON GLYPHOSATE

I often receive the following question. "I've sprayed glyphosate when I was not expecting rain for at least 3 hours. Unfortunately, a light rain began to fall about 20 to 30 minutes after a glyphosate treatment. How can I tell if the herbicide is working or if I'll need to respray?" Although it may not have relevance in many parts of the state, the principle is important.

To start, what is the definition of a light rain? Is it a trace, 0.01 inch, 0.05 inches, or more? After a quick search of the literature, I cannot find any reports on the volume of rain that it takes to wash glyphosate off leaves. Many research papers report on the effect of the time between herbicide application and rainfall on the level of weed control (i.e., rain fastness). In many cases, the researchers use 1 inch of simulated rain, much more than a light rain. I think many people have had the experience of making a postemergence herbicide application only to be followed by a shower that developed out of nowhere. Many times these applications are still successful in controlling weeds. If the leaf was not fully wet by the rain, I think it is likely that enough herbicide will remain to still be effective. If the leaf was fully wet by the rain, then it is much harder to know how much herbicide may have been washed from the leaf. We donít know if the leaf just got to the point of being fully wet without losing herbicide or if it was wet plus being washed. Of course, leaves of some weeds appear to be wetted easier than others, which will increase the risk of a light rain impairing performance.

It is tough to tell if the glyphosate is working because the chemical is slow acting. The first weeds that might show symptoms are likely to be sensitive to the lowest rates. If you have rain and lose half of the glyphosate from sensitive plants like foxtails, you might see symptoms on foxtails (turning off-color) by 5 days after spraying and still get full control of them. However, if only half the glyphosate remains, you may not get control of broadleaf weeds, which are more tolerant. I would focus on checking for glyphosate activity on broadleaf weeds such as volunteer sunflower or lambsquarters after 5 days and see if they are starting to get wilty or turning off color. This would be a good sign. Unfortunately, this still does not tell us if they got a full dose of glyphosate or if a percentage was washed off. Some weeds might recover if some herbicide was washed off . If noticeable symptoms are not seen after 5 days, I would re-spray if the weeds are so large that you cannot wait any longer.

This applicator needs to monitor this field because it rained soon after the glyphosate application. However, all applicators are wise to monitor fields that were rained on, even after longer intervals. Regardless of formulation, glyphosate is not rapidly absorbed by the leaves of weeds, and there is a risk of wash-off. Last year, weed scientists at Univ of WI sprayed moderately sized lambsquarters with 32 oz/a Glystar Plus or 22 oz/a WeatherMax (both provide 0.75 lb a.i./ac glyphosate) and then washed the plants with a simulated rain at 0.5, 1, 2, and 4 hours after the applications.

Lambsquarters control was reduced with both formulations, even with a 4-hour interval between the application and the simulated rain. The WeatherMax was slightly better than Glystar Plus at the intermediate times, but control was still less than either formulation without rain. The reduction in control due to rain should be less with smaller or more sensitive plants. Regardless, it is worth a quick field check to make sure you are getting the weed control you need in cases when it has rained within a few hours after the application.

 

Lambsquarters control (%)

Simulated rain

GlyStar Plus

WeatherMax

0.5 hr

56

51

1 hr

41

61

2 hr

45

79

4 hr

69

81

No rain

93

96

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


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