ISSUE 7 June 25, 2009
CONTROLLING UNCONTROLLED RAGWEED
How do I control ragweed that survived the first application? The answer to this depends upon how they survived treatment with glyphosate. If most were controlled and the survivors were still substantially affected, then it may be possible to apply glyphosate again and obtain adequate control. The addition of another product with activity on ragweed could certainly improve control, especially where plants are large. The question here is whether the survival is indicative of a low level of resistance, and an alternative to glyphosate should be used if there are doubts that a second glyphosate application will work.
Where many plants survived the first glyphosate application and showed little response to glyphosate, which is indicative of resistance, it will be necessary to apply a postemergence alternative to glyphosate that is effective on ragweed. Possibilities here include Status, Hornet, Impact, Callisto, Laudis, and NorthStar, among others. In Liberty Link corn, Ignite would be an effective option. Impact, Callisto, and Laudis are most effective on ragweed when mixed with atrazine. Labeled rates and recommended adjuvants should be used to maximize effectiveness of alternatives to glyphosate. It is possible to add some glyphosate to improve control of other weeds. However, be sure to add crop oil concentrate or MSO if recommended on the label of the alternative herbicide, instead of relying on the surfactant that is in most glyphosate products.
CAN I EAT "DRIFTED-ON" VEGETABLE GARDEN PRODUCE?
The answer to this "no", except where the herbicide is labeled for the garden plant of concern (such as Callisto or Laudis on sweet corn). Since most corn and soybean herbicides are not labeled for use on vegetables, there is no established tolerance for the allowable herbicide residue in the produce, and therefore no guidelines for "safe" use have been established.
HOW MUCH N WILL WEEDS ROB FROM CROP?
A Michigan State University timing study on post emergent weed control showed that 95 percent of weed control or better can be achieved when weeds are 9 inches tall, but yields were reduced 25 bushels per acre (Everman et al., 2008). Post-emergent herbicides provide a greater window of opportunity to control weeds, but delaying application allows weeds to use available N and other nutrients from the intended crop. In a two-year study in Wisconsin, there was no yield loss when weeds were controlled at the 4-inch stage, but delaying application on 12-inch weeds resulted in an average 9 percent yield loss. Looking at the data from another view point, 2006 data showed the MRTN rate was 96 lbs per acre when weeds were controlled at 4 inches, compared with an MRTN rate of 200 lbs per acre when weeds were controlled at 12 inches. Timely weed control will ensure valuable nutrients are used for crop production rather than weed production. (Boerboom et al., 2008.)
EPA APPROVES LABEL UPDATE FOR LAUDIS
The EPA recently approved several label updates for Laudis herbicide. Label updates include:
Laudis controls grass and broadleaf weeds such as ragweeds, lambsquarters, velvetleaf, waterhemp and woolly cupgrass in corn fields. The Laudis formulation includes an effective safener that ensures crop safety.
Extension Weed Specialist
WEED CONTROL IN ROUNDUP READY SUGARBEET
For those growers unable to apply glyphosate to Roundup Ready sugarbeet for the first time due to wet soil conditions, apply the maximum rate of glyphosate allowed. The maximum glyphosate rate for Roundup Ready sugarbeet is 1.125 pounds acid equivalent per acre (lbs ae/A). This equates to 32 fluid ounces per acre (fl oz/A) of Roundup-branded products, 48 fl oz/A of 3.0 pounds acid equivalent per gallon (lbs ae/gal) products, and 39 fl oz/A of 3.7 lbs ae/gal products. This glyphosate rate can only be applied up to the eight-leaf stage of sugarbeet. This rate should be applied to any field with weeds greater than two to three inches in height or with difficult to control species such as wild buckwheat, lambsquarters, and common and giant ragweed.
The timing of sequential glyphosate applications should be based upon weed density and height and whether individual plants survived the initial application. Generally, sequential applications should be applied 18 to 30 days after the preceding application. The greater the weed density the sooner glyphosate should be applied sequentially. Once weeds reach three inches in height, regardless of weed density, glyphosate should be applied a second time. If plants survived the initial application of glyphosate, apply glyphosate at the highest legal amount 14 to 21 days later, regardless of weed density and height. Once sugarbeet has developed beyond the eight-leaf stage, glyphosate can only be applied at 0.75 lb ae/A. This is equivalent to 22 fl oz/A of Roundup-branded products, 32 fl oz/A of 3.0 lbs ae/gal products, and 26 fl oz/A of 3.7 lbs ae/gal products.
If common or giant ragweed have survived the initial glyphosate application, clopyralid can be mixed with glyphosate at a rate of 4.0 fl oz/A. This rate of clopyralid will not control all ragweed plants in the population, but should keep the ragweed from competing with the crop, unless high densities are present. If other weed species, not susceptible to clopyralid, survive a glyphosate application in Roundup Ready sugarbeet, one option is to continue to apply the highest legal rate of glyphosate in subsequent applications. The only other option is to cultivate and remove any remaining surviving plants by hand.
Agronomist - Weed Science