Category Archives: Weeds

Late Burndown in Soybean

Picture 1. 3 foot tall goosegrass, fall panicum and Palmer amaranth that need to be burndown before soybean planting

The question of the week is on how to burndown fields that are grown up messes before planting soybeans (Picture 1).  The problem is when fields are heavily infested with 3’ tall goosegrass, Palmer amaranth, horseweed and volunteer corn that is tasseling there are no good answers. Continue reading

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Managing Palmer Amaranth and Grass Burndown Escapes

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Picture 1. Grass and Palmer amaranth escaping paraquat burndown in emerged cotton

There have been numerous fields where goosegrass, junglerice and Palmer amaranth were not controlled at burndown and the crop has emerged.  In some cases, the wet weather delayed burndown to where Palmer amaranth and those grasses were too large to be effectively controlled with one pass of Gramoxone (Picture 1).  In other cases, tillage used to help speed up field drying or to build beds partially buried pigweed and grasses which make effective burndown from Gramoxone very problematic (Picture 2). In a few cases, dicamba and glyphosate were used for burndown and they did not control those weeds due to resistance (Picture 3). Continue reading

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Pre-Emerge Herbicide Injury in Soybean

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The call of the last week was Pre-emerge herbicide injury in soybeans.  Several folks reported that their soybeans were “going backwards”. This is not a surprise as in wetter springs the probability of seeing injury from Pre-applied herbicides is greatly increased. That coupled with an increase in soybean acres this spring would increase the probability of herbicide injury in that crop. I would expect more of the same as we move into June with all the rain in the past few days.

Picture 1. Metribuzin injury on soybean

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Fomesafen Carryover into Corn and Grain Sorghum

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Carryover injury from fomesafen (Flexstar, Prefix, Reflex, Intimidator, Marvel, Dawn, Rhythm, etc.) is starting to show in a few corn fields.  This is because some  applications in late June and July in 2023 coupled with a very dry fall allowed the herbicide to persist into winter.  Recent rains then allow any herbicide carryover to more completely get into the soil solution.

Fomesafen carryover injury in corn and grain sorghum looks similar.  Expect to see stunting and interveinal chlorosis as pictures below sent from crop consultants clearly shows.

Striping of corn leaf veins characteristic of fomesafen carryover

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Herbicide Corn Maturity Cutoffs

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Due to little time and very few good spray days, some corn fields have yet to have their layby application. Corn is putting on a new leaf every 3 days with all the heat and water. Therefore, time is short before it will be too mature to apply many herbicides safely over the top.

How large can corn be before glyphosate could potentially cause injury? The glyphosate label states it can be applied up through the V8 corn growth stage. My experience has been if glyphosate is applied to corn larger than that it can at times cause ears to be barren.  This phenomenon is difficult to predict ahead of time as it can vary due to maturity of the corn at application, hybrid and weather.

Crop stage and/or crop height is used to determine the cutoffs for a given herbicide.  Often the label states the limits at whichever (crop stage or crop height) comes first.  Crop stage can easily be done by counting the number of leaf collars that are fully exposed (Ex: 4 collars = 4 leaf) (Picture below).  If tankmixing, cutoff is always determined by the most limiting factor.

4 – Leaf Corn

Johnsongrass Escaping Glyphosate Burndown

Postemergence Corn Herbicides Crop Height Cutoff Crop Stage Cutoff
Accent 20 inches 6 leaf
Acuron GT 30 inches 8 leaf
Atrazine (Bicep, Degree Xtra, Acuron, etc.) 12 inches
Callisto* 30 inches (without atrazine)
Capreno* 20 inches 7 leaf (without atrazine)
Corvus 2 leaf
Dicamba (Clarity, etc.) 8 inches (1 pt/A) or
36 inches (1/2 pt/A)
5 leaf

Distinct 24 inches
Glyphosate – RR corn only 30 inches 8 leaf
Halex GT 30 inches 8 leaf
Armezon/Impact 8 leaf
Laudis 8 leaf
Liberty – LL corn only 24 inches 7 leaf
Diflexx 36 inches 10 leaf
Realm Q 20 inches 7 leaf
Resicore 11 inches
Resolve Q 20 inches 7 leaf
Roundup PM – RR corn only 8 leaf
Shield Ex 20 inches 6 leaf
Status 36 inches 10 leaf
Steadfast Q 20 inches 7 leaf
Storen 8 leaf
2,4-D 8 inches

*Note that if atrazine is added to herbicides such as Callisto or Capreno for example, the cutoff would be reduced to 12 inches.


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Cotton Scout School (May 21, 2024)

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The UT Cotton Scout School is scheduled for Tuesday, May 21st, at the West Tennessee Research and Education Center (605 Airways Blvd, Jackson). There is no fee, and preregistration is not required. Registration begins at 8:00 AM with the program starting at 8:30. Pesticide points in categories 1, 4, 10 and 12 will be offered and a BBQ lunch will be provided. Content will include classroom and hands-on training with an optional go-to-the-field session after lunch. Topics covered will include cotton development and identification and symptoms of insect pests, plant diseases, and weeds.

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