Posts tagged ‘roundup ready’

Glyphosate Management in Soybeans

The results of weed-crop interference studies show that weeds in soybeans should be treated with herbicide before they exceed a size of 6 to 8 inches in order to ensure that weed interference is not a limiting factor in soybean yield. Weeds may not have reached this size in later-planted soybeans, or where pre-emergence herbicide activity reduced weed populations and growth. In soybeans planted late May through June, POST herbicides can be applied sooner after planting when weeds are small, which maximizes POST herbicide activity and reduces the need for higher glyphosate rates.

Glyphosate rates should generally be based on weed size and age, environmental conditions, and the previous history of glyphosate effectiveness in the target weed population. You can use the lowest labeled rate of glyphosate, 32 oz/A Roundup Original and similar products, and achieve 100% control when the following are met: 1) weeds are less than 6 inches tall; 2) weeds have not survived tillage or a previous herbicide application; 3) glyphosate is applied following use of a pre-emergence herbicide; and 4) environmental conditions are extremely favorable for herbicide. Increasing the glyphosate rate to a range of 48 oz/A to 64 oz/A of Roundup Original and similar products can greatly increase effectiveness when weeds are more than 6 inches tall and/or other conditions are not optimum for herbicide activity.

Glyphosate-resistant corn increases the potential for volunteer corn problems in soybean when glyphosate is used for weed control. Fortunately, good volunteer corn control options exist in glyphosate-resistant soybean. Most postemergence grass herbicides are very effective in controlling volunteer corn. The exception is that Poast Plus can be less effective. Postemergence grass herbicides can be tank mixed with glyphosate, but the adjuvant requirements may be greater than the typical ammonium sulfate added with glyphosate and may depend if a glyphosate formulation requires no additional surfactant or if surfactant is required. The question of whether volunteer corn needs to be controlled depends both on the competition from the corn and the potential for dockage because of corn in the harvested soybeans. Based on previous studies from University of Wisconsin, soybean yield loss from volunteer corn is in the neighborhood of 1% yield loss for every 75 to 100 “clumps” of volunteer corn per acre.

June 17, 2009 at 7:30 am

End of Season Weed Survey

On Tuesday, September 23, 2008 I spent the better part of the day conducting a comprehensive weed survey in Van Wert County. Why, you ask, would I drive 180 miles within a one county area to look at weeds? The answer is simple: to better serve my local clientele’s needs. By knowing what weeds are problematic to area farmers, I can better plan winter meetings, etc. At the end of the day, I scouted 80 randomly selected soybean fields. This project is actually part of a larger project in Ohio, and I am just one participant. I sent my data to Mark Loux for his assessment, but I thought I’d share with you my results.

I surveyed summer annual weeds only, and did not look for winter annual weeds. Also, I limited my survey to soybean fields only.  Basically, this saved time for me; by looking only for summer annuals in soybean fields I did not have to walk into a field and I could make my observations from the edge of the field. Please note that my observations are qualitative and subject to my interpretation. I was asked to score the weed infestation on the following scale: 1=Occasional (A plant of the species as an occasional individual plant), 2=Large patches (A patch(s) of 8 or more plants of individual species scattered in field), or 3=Widespread (Numerous patches or individual plants of the species across the field).

Rankings (from most often to least often weed found in soybeans)

#1 Marestail- I found this weed in nearly 20% of the soybean fields I randomly selected. This weed has become prevelant in Van Wert County and is very difficult to control. This weed seems to be more abundant in southern Van Wert County. The average infestation was 1.8 (on a scale of 1-3, see above for explanation of scale).

#2 Volunteer corn – My guess is that I was seeing volunteer Roundup Ready corn, but I have no way of knowing that for sure. Average infestation of 1.1.

#3 Common ragweed – No surprise here. Common ragweed is found in nearly every soybean field, but is usually easily controlled by glyphosate. These escapes were probably due to a too early application of glyphosate. Average infestation of 1.7.

#4 Giant ragweed – Fortunately, this weed does not flourish throuhout Van Wert County and seems to be isolated to certain fields. Like Marestail, this weed ranks at the top of the list with regards to difficulty of control. Average infestation 1.0.

#5 Foxtail – I was surprised to see foxtail make it on the list. Foxtail is easily controlled by glyphosate and/or selective grass herbicides. Also, the infestation was high with an average score of 2.5.

#6 Lambsquarters – Lambsquarter becomes very difficult to control when the weather turns dry and hot, and that is exactly what happened during August. Average infestation of 1.0.

#7 Pigweed – I was also surprised to see pigweed on the list. It is not normally a weed that I’ll find in a field this late in the season. Usually this weed does well along field edges but not in the field. Average infestation 1.0.

There were other weeds I noted, but not in enough frequency or infestation to make the list. Velvetleaf, cocklebur and smartweed were noted in one or two soybean field edges. Bear in my that this survey is done for soybeans only (since soybean acres outnumber corn acres nearly 2:1 in Van Wert County). Had I stepped into some corn fields I probably would have found more velvetleaf and cocklebur. In my experience these weeds can become prevalent in corn fields in Van Wert County.

Let me know if you have a different observation or wish to augment my observations.

September 25, 2008 at 7:00 am


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