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Coverage is key piece to weed puzzle

The amount of herbicide that actually reaches the target, especially with postemergence herbicide applications, can greatly influence the level of weed control.

Eric Prostko

April 16, 2024

4 Min Read
plant growing from jigsaw puzzle
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As I traveled the road this winter with my Ichabod Crane-like partner delivering the weed science gospel, I was often humbled by the many positive comments I received from growers regarding my Tailgate Talk articles. I always appreciate feedback, especially when it is positive!   

In a conversation with a Jenkins County, Ga., grower, who just so happens to like my articles, I mentioned to him that I am getting so old now that I am running out of things to say and do not want to be overly repetitive. He said farmers have so much on their minds, that friendly reminders are often welcome.  Guess I will keep plowing this field then. 

One of the many things I discussed with growers this year at winter meetings was how weed control is like a puzzle. There are many and when one piece is not in the right place, the puzzle is not complete. One of the pieces of the weed control puzzle that I would like to talk about in this article is coverage.   

The amount of herbicide that actually reaches the target, especially with postemergence herbicide applications, can greatly influence the level of control.  Generally, coverage with contact herbicides (paraquat) is more critical than with systemic herbicides (glyphosate), but it is never a bad thing when coverage can be maximized.  As a grower, you have complete control over many factors that will influence coverage including droplet size, gallons per acre (GPA), boom height and tractor speed.  

Related:Do wide soybean rows make sense?

When I began my professional weed science career in 1987, nobody was talking about droplet size on a regular basis.  Way different than today since the introduction of auxin-tolerant crops highlighted the need to focus more attention on droplet size with the goal of minimizing physical drift.  Simply, larger spray droplets help reduce drift, but they also result in reduced coverage.  Recent UGA studies have shown a 6% to 11% reduction in coverage with ultra-course droplets (>650 microns) compared to very course droplets (401-500 microns).   

Perhaps the best advice I can give you about droplet size is to have at least two different types of spray tips on the boom.  One tip/pressure combo that produces coarse droplets (326-400 microns) and another tip/pressure combo that produces the larger droplets required for the Enlist and XtendFlex cotton/soybean systems. 

For the record, I use 11002AIXR spray tips in all my research plots, except for Enlist and XtendFlex trials, which results in a coarse droplet (20” spacing, 15 GPA, ~40 PSI). Coarse droplet sizes can be obtained with many different tips/pressure systems from the various tip suppliers. It has been my experience, applying many different herbicides, that coarse droplets offer a reasonable balance between coverage and drift protection. 

Water is the cheapest liquid on the farm, yet most growers don’t like the hassle of carrying it from field to field. UGA studies on GPA effects on coverage showed a 5% to 10% reduction in coverage with 10 GPA compared to 15 GPA. Not much difference in coverage between 15 GPA and 20 GPA.   

If you drive around Georgia, you can’t help but notice the new and fancy 90-foot to 120-foot red/green/yellow sprayers cruising across fields. Often, boom heights are too high for optimum coverage (Figure 1). UGA research on the effects of boom height on coverage showed 21% to 22% less coverage with a six-foot boom height compared to 2-foot boom height.  If two-foot is not possible due to land terrain or terraces, set the boom as low as practical.  

Lastly, tractor speed can also influence spray coverage.  Generally, faster tractor speeds will result in less coverage (and more physical drift).  I realize that many growers today have more acres to cover, and time is of the essence. I am not convinced that driving a sprayer at 18 MPH is a great idea.  I just want you to consider slightly lowering your sprayer speed if you have not been satisfied with your weed control results.   

When putting together your weed control puzzle, don’t forget to focus on spray coverage with a particular emphasis on droplet size, GPA, boom height, and tractor speed.  These are some of the very few things on the farm that you have 100% control of.  Maximizing coverage can result in better weed control, especially when environmental conditions and weed sizes are not optimum.   

As always, good weed hunting! 

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