Last fall there was a notable amount of Palmer amaranth in many corn fields in areas of the state where we had confirmed dicamba resistance. As a result, we have recently conducted some field screens on these pigweed populations to herbicides used in corn and soybean.
Figure 1
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
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.
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.
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.
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.
Zidua (pyroxasulfone) applied via impregnated fertilizer in cotton has become popular in Tennessee. My understanding is that about 25% of the cotton acres last year had Zidua applied in this fashion. With the increasingly poor Palmer amaranth and grass control we are experiencing with dicamba + glyphosate, overlaying residual herbicides must be the foundation for weed control. Continue reading
Palmer amaranth emergence is very rapid now. This is about a month early for Palmer emergence to be at this pace in Tennessee. With the spread of dicamba resistance in our Palmer amaranth population Liberty is our last best hope to control dicamba-resistant pigweed escapes in cotton and soybean. This is just a quick reminder that the time of day that Liberty is applied has a large determination on the success or failure of that application. Continue reading
It is apparent from numerous calls that many growers this spring are planning to cut rates on PRE-applied herbicides in soybean and cotton. I can understand the mindset with the depressed commodity prices, however I fear it may be the catalyst for Palmer amaranth history to repeat itself. Continue reading