As the field corn harvest season begins and we get closer and closer to peanut and cotton harvest, it is a great time to evaluate the success (or not) of your weed control programs.
The success of any weed control program is based upon these 3 simple rules:
- Starting clean.
- Using a preemergence/residual herbicide that gets activated by irrigation or rainfall (more than 1 depending upon the crop).
- Applying timely postemergence herbicides (before weeds are bigger than 4”).
If you follow these simple rules and get some help from Mother Nature, then you will have a better chance of achieving the success observed in Figure 1. For comparison, note the nasty non-treated control plots. If your fields do not look like this, here are 10 tough questions that you must ask yourself (and be honest):
- Did I really start clean? A 2 foot tall pigweed plant in a 6 inch tall peanut crop was not dropped from the sky by extraterrestrials!
- Did the preemergence herbicide I used at planting get activated by moisture within 7 to 10 days? Depending upon the herbicide, you might get some residual control after that time, but you will still likely be dealing with an escaped flush of weeds before that residual control begins.
- When I applied my postemergence herbicides, were the weeds really 3-4” tall (Figure 2.) I am 100% certain that it is impossible to accurately measure weed heights with a RULER while driving a pick-up truck, tractor, or ATV!
- Did I use the right herbicide? I recently counseled a soybean grower (via the county agent) who applied Liberty (glufosinate) to wild poinsettia. That did not turn out very well. More often than not, you only get 1 chance with a POST herbicide. You absolutely must know what weeds are in the field and what herbicides will work the best.
- In 2019, did I apply an herbicide somewhere between May 14 and June 4? If so, it probably did not work well due to the fact that most Georgians were suffering from a mini-drought and high temps that were 6-7° above normal.
- Did I apply a POST herbicide that got washed off before it had a chance to work? At this time of year, random pop-up showers are common. On my small hobby farm, I have seen it rain on one side of the farm but not at my house. I suspect that this type of thing happens all over Georgia during the summer and you might not be in the field to know when it does.
- Did I mix multiple agri-chemicals in a single tank to try and solve every problem (real or fictional)? Not uncommon at all for herbicide antagonism (less control) to occur when multiple products are tank-mixed.
- What were my expectations for these herbicides? If a herbicide is rated by your friendly neighborhood weed specialist as excellent on a certain weed species, that means that it should provide better than 90% control of that species (assuming environmental conditions are favorable and plants are in susceptible stage of growth). With that said, an herbicide that provides a solid 95% control of Palmer amaranth, would still leave 50,000 plants uncontrolled if the starting population was 1,000,000.
- Did I use enough water (GPA) and the best nozzle tip possible? If you are an awesome applicator and Mother Nature has been kind, you probably will not notice many negative GPA or nozzle/pressure/droplet size effects. But if things are not going so good, i.e. dry weather and big weeds, then a higher GPA (15 vs. 10) and a more appropriate nozzle tip is likely warranted.
- Was I driving my spray rig at warp speeds? I know that time is your most precious resource. But, there is data out there that would suggest that spray coverage is reduced as tractor speed increases.
Successful weed control programs do not happen by accident. It takes much forethought and diligence. Most of the weed control successes or failures that occur on your farm will depend upon the management decisions that YOU make or don’t.
You have got to be honest with yourself! As always, good weed hunting!
Figure 1. A successful soybean weed control program from 2019 weed control research plots on UGA Ponder Farm near Ty Ty, GA (non-treated controls on left, right and behind treated plot.