By Clarke McGrath

After a decent start to the planting season, cold and wet weather has put the brakes on fieldwork for a while. Looking ahead to whenever it starts to dry out enough to get back in the field, farmers are figuring on being able to spray a day or two before it’s dry enough to get back into the field with the planter.

This situation has generated a lot of questions over the last few days as we plan for temperature-sensitive herbicide applications like no-till burndowns. If the rain moves out as predicted and temps stay close to the forecasted range, it could influence the effectiveness of some of the burndown products. 

What about the temperature effect on glyphosate herbicides?
If temperatures get below the low 40 degree Fahrenheit range at night followed by days that don't get above the low-to-mid 50 degree range, you should consider either waiting a couple days to treat, or some chemical dealers and crop consultants recommend bumping up the application rate slightly—to mitigate the risk of not controlling some of the larger winter annuals and spring annuals, especially broadleaf weeds. Here are some other key points and recommendations:

What about applying paraquat as a burndown?
Be sure to use the right additives (surfactants, etc.) with Gramoxone (paraquat). As with most burndown herbicides, using the high end of recommended rates of crop oils, methylated seed oils and/or nonionic surfactants is critical.

While adding liquid nitrogen (UAN) to the herbicide tank-mix can impede glyphosate activity, it seems to enhance Gramoxone activity, making UAN a great choice for corn burndown treatments when you are applying UAN as the nitrogen source anyway. Also, keep in mind that tank-mixing triazines (atrazine for corn, or metribuzin such as Sencor in soybeans) increases the speed and efficacy of Gramoxone in burning down weeds. 

Use flat fan nozzles with Gramoxone and try to get your carrier up to 15 gallons per acre or so. The label goes down to 10 gallons per acre, but my experience has been that 15 gallons per acre is more consistent on bigger weeds.

Is 2,4-D as sensitive to cold temperatures?
In my experience, the 2,4-D products aren't as cold-sensitive as many other burndown herbicide products as long as the plants themselves are not too damaged by the cold temperature (or frost if you had some). In general, winter annual weeds won't sustain much damage from cold temperatures, so you should key off of the spring annuals that are up.

If there is around 50% undamaged leaf tissue on the spring annual weeds, then treatment should be “OK” with 2,4-D after a couple days of low temperatures that don't go below freezing. Otherwise, the weeds will need some time to reinitiate new growth before you spray with 2,4-D as a burndown.

In corn where you are using a burndown herbicide treatment, a real concern is any cold, wet soil stress and any cracked-open seed trenches you may see in a tough spring like this one so far. Try to avoid applying 2,4-D under these conditions if there is potential of a rain driving acetanilides and 2,4-D into the seed zone soon before and/or after planting, as this can cause seedling damage. There are some acceptable alternatives to 2,4-D for any fields where you have risk factors like poor furrow closure or shallow planting.

Note there is a planting delay required for both corn and beans and it varies among herbicide products, crops and application rates. Having seen significant crop injury a few times from not following the planting delay requirements, I can tell you double-checking the label for the burndown herbicide product you are using is important. 

What about an approved dicamba product ahead of Xtend beans?
I couldn’t find anything in the labels of these products addressing cool temperature issues. So, after consulting with Iowa State University Extension weed management specialist Bob Hartzler, here’s our answer: Expecting dicamba to respond to cool temperatures in a way similar to 2,4-D is probably where we are at. 

We’ve also talked about this before with farmers and chemical dealers—we discussed it a lot at meetings this past winter. But having had calls on the topic already this spring, it is worth mentioning again. It is true for all pesticides, and with the spotlight on using the new dicamba tolerant soybean systems, it isn’t just a trite saying: “The label is the law.” Watch the wind speeds and wind direction when spraying. And be sure to understand and follow label recommendations for using the product. 

Editor’s Note: Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Contact him at [email protected].