Source: Dr Mark Loux, OSU Extension
According to our network of sources, the effectiveness of new soybean trait systems has some growers once again thinking about omitting preemergence residual herbicides from their weed management programs. Some people apparently need to learn the same lessons over and over again. Having gone through this once in the early 2000’s when Roundup Ready soybeans had taken over and we all sprayed only glyphosate all day every day, we think we’re pretty sure where it leads. We’re sensitive to concerns about the cost of production, but the cost-benefit analysis for residual herbicides is way in the positive column. We’re not the ones who ultimately have to convince growers to keep using residual herbicides, and we respect those of you who do have to fight this battle. Back in the first round of this when we were advocating for use of residuals, while the developers of RR soybeans were undermining us and telling everyone that residuals would reduce yield etc, we used to have people tell us “My agronomist/salesman is recommending that I use residuals, but I think he/she is just trying to get more money out of me”. Our response at that time of course was “no pretty sure he/she is just trying save your **** and make sure you control your weeds so that your whole farm isn’t one big infestation of glyphosate-resistant marestail.” And that answer probably works today too – maybe substituting waterhemp for marestail.
We need to state here that a good number of growers kept residual herbicides in their programs through all of this, and we assume they aren’t tempted to omit them now either. For everyone else – maybe interventions are called for. Where the recalcitrant person is repeatedly thumped with a stick while being reminded of what happened last time, until they change their minds.
Weed scientist: so you’re going to use residual herbicides right?
Soybean grower: no
Thump
WS: remember what happened last time – lambsquarters became a problem when every residual herbicide would have controlled it. Change your mind yet?
SB: no
Thump
WS: remember when the weather didn’t cooperate and you ended up spraying 2 foot tall weeds because of no initial control? Do you want this again?
SG: no
WS: so you’re going to use residuals?
SG: not sure
Thump
WS: and you expect your local dealer to clean up whatever mess occurs when you don’t use residuals?
SG: yes
Thump
WS: remember when you burnt out the FirstRate on marestail and then the glyphosate wouldn’t work? Do you want this to happen with dicamba, 2,4-D and glufosinate?”
SG: no
WS: well then
SG: maybe
Gentler persuasive tap
WS: You know how bad a weed waterhemp is right?
SG: yes
WS: what if residuals will help prevent waterhemp infestations
SG: Ok then – yes
WS: ok then
Note: we considered a number of sound effects here – thump, zap, whack…. Thump won out for no particular reason. We could not decide whether getting hit by a stick was more or less acceptable than getting shocked in this context.
The bottom line is that residual herbicides provide both short- and long-term risk management in weed management for a relatively low cost. A non-inclusive list of these:
– reduces weed populations overall and slows weed growth, resulting in more flexibility in the POST application window.
– Reduced risk of yield loss if weather interferes with timely POST application. In the absence of residual herbicides, soybean yield loss can occur when weeds reach a height of 6 inches.
– increases the number of different sites of action used within a season, slowing the rate of resistance development
– reduces the number of weeds that are treated by POST herbicides, which also slows the rate of herbicide resistance development
– residuals control lambsquarters which is not well-controlled by POST herbicides
– the most significant weed problems in Ohio soybean production – waterhemp, giant ragweed, and marestail – cannot be consistently controlled with POST herbicides alone. They require a comprehensive herbicide program that includes residual and POST herbicides. It may be possible to make a total POST system work some years or for a while, but in the end this approach will result in problems with control and speed up the development of resistance.
This whole subject of omitting residual herbicides makes us cranky because we don’t have to guess what will happen. We’ve made our best case here. It’s up to you of course, but we suggest that we not have to come back and have this discussion again. Because next time we’re bringing a few friends, a bigger stick, and a gorilla.
Disclaimer: Parts of this article are meant in pure jest. We would certainly never advocate in earnest the use of physical harm or other methods of persuasion to change the behavior of herbicide users. This goes against everything that the discipline of weed science stands for, and also OSU. Plus – we don’t even know where to rent a gorilla.