It has been my experience, after serving 31 years in Extension, that herbicides are often the first thing blamed when things go wrong. It is very common for me to hear, “I applied herbicide X last year and it is now causing injury to my crop this year!”

This phenomenon is typically known as herbicide carryover (Figure 1). It is also my experience that other problems such as nematodes/soil fertility/pH issues are just as likely to cause herbicide-like symptomology (Figure 2).

A recent informal poll of my Extension weed science colleagues across the South would suggest that true herbicide carryover is less of a problem region-wide than others such as off-target movement, sprayer contamination, and mixing errors.

It’s important to note that my trusted sidekick and friend ASC (not to be confused with AOC) ranked herbicide carryover as more common than I did since he lives in the world of specialty crops (cucurbits, greens, watermelons, etc.) and has also been dealing with the Cadre (imazapic)/cotton dilemma since we both arrived in Georgia way back in 1999.

Figure 2. Simulated field corn injury from Strongarm (diclosulam) - top left and sting nematode injury - bottom right. Credit: Eric Prostko

For the record, my colleagues from Missouri (KWB), Texas (PAD/SAN), and Virginia (MLF) also ranked herbicide carryover as more common than I.

Herbicide carryover is not an easy issue to understand. Carryover can be influenced by many things such as the herbicide, rate, time of application, rotational crop sensitivity, and a myriad of environmental factors (OM, soil texture, pH, rainfall, temperature). Because of this, it takes a lot of time and money to hammer out reliable crop rotation restrictions. As a reminder, microbial degradation is one of the main ways that herbicides are broken down in the soil and is favored by warm temperatures and ample moisture.

Related:Peanut growers get a new herbicide

The herbicide label is the best source of information regarding crop rotation restrictions. After reading one for an hour or two, you should be able to find the section that contains the recommended plant-back restrictions for various crops. Most of these are based on good science. Some are not. In certain instances, longer rotation restrictions are listed on an herbicide label because of limited or no data especially on smaller acre or less valuable crops.

Here is one example. Sinbar 80WDG (terbacil) was first registered for use in 1966 in various perennial crops such as apples and peaches with a use rate of 32-48 ozs/A.In 2011, Sinbar was labeled for use in watermelons at 2-4 ozs/A.The labeled rotation restriction for any crop after an application of Sinbar at any use rate is two years.

Related:The generic herbicide formulation wasn’t the problem

Recent research in Georgia has shown that peanut and grain sorghum would only require a ~240 day and ~100 day plant-back restriction, respectively, after an application of Sinbar at the watermelon use rates.Prior to this research, a peanut or grain sorghum grower might have automatically assumed that a production problem might have been related to carryover. But I am not anticipating that the Sinbar label will change anytime soon.

I am not trying to say that herbicide carryover is not important. Herbicides can and do cause carryover problems to rotational crops in many scenarios. It’s very complicated. However, in the Southeast, especially south Georgia, environmental and soil conditions favor the rapid breakdown of many herbicides.

The best way to prevent herbicide carryover problems is to read and follow the label. Remember that other issues such as low pH and/or nematodes can mimic herbicide injury symptoms and must be considered when investigating a field problem. When searching for answers to field problems, get as much input as possible from those that you trust. Hopefully that list of folks includes your local county extension agent and your friendly neighborhood Extension weed specialist.

As always, good weed hunting!