After a decent start to the planting season, cold and wet weather has put the brakes on field work for a while. As we look ahead into next week, or whenever it starts to dry out enough to get back in the field, growers are figuring on being able to spray a day or two before it is dry enough to get back into the planter. This has generated a lot of questions over the last few days as we plan out temperature sensitive 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 our burndown products.
What about the temperature effect on glyphosate herbicides?
It temperatures get below the low 40 degree range at night followed by days that don't get above the low-mid 50 degree range, you should consider either waiting a couple days to treat, or some dealers recommend to bump up the 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:
• If it is around the mid-40 degree range or higher at night and the low 50's or above in the daytime, we still worry a little; you can probably spray the next day, but if giant ragweed, marestail or other tough weeds are there, you should still consider bumping the glyphosate application rate up, or tank-mixing the glyphosate with another product that is effective on the tougher to manage species.
• Annual grasses are not too much of a concern with glyphosate, they tend to be easy to control with burndown application rates. But if you have had a heavy frost (some areas reportedly did), you still may want to wait two or three days to spray glyphosate.
What about applying paraquat as a burndown?
Be sure to use the right additives (surfactants, etc.) with Gramoxone (paraquat). As with most burndown herbicides, utilizing the high end of recommended rates of crop oils, methylated seed oils and/or nonionic surfactants is critical.
While adding UAN to the herbicide tankmix 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 the 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 was that 15 gallons per acre was 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 products as long as the plants themselves are not too damaged by the cold temperature (or frost if you had some). In general, winter annuals 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 annuals, then treatment should be “OK” with 2,4-D after a couple days of lows that don't go below freezing. Otherwise, the weeds will need some time to 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 treatment, a concern is any cold, wet soil stress and any cracked open seed trenches we 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 we have risk factors like poor furrow closure or shallow planting. Note that there is a planting delay required for both corn and beans and it varies among products, crops and application rates. Having seen significant crop injury a few times from not following the planting delays, double checking the label for your particular product is important.
What about an approved dicamba product ahead of Xtend beans?
I couldn’t find anything in the labels addressing cool temperature issues, so after some consultation with our own Dr. Bob Hartzler, expecting dicamba to respond to cool temps in a way similar to 2,4-D is probably where we are at. We’ve also talked about this before- a lot- 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 the new dicamba tolerant soybean systems it isn’t just a trite saying; “The label is the law”. Watch the wind speeds and directions, understand the label and follow it.