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Timing key concern with glyphosate

Higher-than-normal temperatures in early April put the planting season on a fast track. We are growing more Group IV soybeans and most are Roundup Ready.

Even with Roundup, Touchdown or a generic glyphosate, timing is by far the most critical factor. The key to control in a Roundup Ready system is getting some glyphosate applied by no later than 14 days after soybean emergence (DAE). I believe the ideal timing to shoot for is 10 DAE. That isn't any too early and it gives you a little cushion to make sure it is all sprayed by 14 DAE.

I still get a lot of questions on rates and I still say I have not found rates to be all that important. If I hit the timing at 10 to 14 DAE on the first application, rates from 1 pt/A to 1 qt/A (of the 3 lb/gal acid equivalent formulations like Roundup Ultra) all look about the same.

A second application of glyphosate is needed within seven to 10 days after the first. My second application is normally at 21 DAE. Again, rates haven't been that important. I always include the Monsanto rate of 1 quart, followed by 1.5 pint (of the 3 lb/gal a.e. formation) as my standard to compare the 1 pint, followed by 1 pint rates.

Every now and then you can see small differences in favor of the higher rates. However, this seldom occurs and any differences are usually small.

It doesn't matter to me what rates you use. I will just emphasize again the key to any of the rates is early timing. If you miss the timing of the first application, there is a good chance of failure on weeds such as coffeebean and morningglory, even at the higher rates.

Most of the time in our plots, two applications of Roundup or equivalent are all that is needed. Every now and then there is a benefit to a third application. That is an in-field judgment call to be made about 28 DAE. If the field is clean and the soybeans are about to close the canopy, that is good. If you have weeds down in there and the canopy is closing slowly, hit them again.

That is another reason I like the 1 pt/A (0.8 pint of Roundup Ultra Max) rate. If you have to make a third application, the cost doesn't look so bad if the first applications have been at a pint.

In regards to additives to glyphosate, I add a surfactant if it doesn't have one. Otherwise I just use water.

Classic or Firstrate can help sometimes on coffeebean and larger morningglories. However these differences are often small and again, timing is more important than additives. I don't use any of the other “Roundup additives” everyone else seems to be promoting.

Here is my challenge again: Call me with a failure after you have used a University of Arkansas program of 1 pt/A (or equivalent) of Roundup (or equivalent) at 10 to 14 DAE and then 1 pt/A once a week until there is nothing out there but soybeans. Normally you can count on two shots but if it takes three, do it.

If you do this program, just worry about timing. If you time it right, the rate and additive issues will take care of themselves. It is like Granddad use to tell me every morning getting on the tractor: “Son, check the water and the oil. The fuel will take care of itself.”

Ford Baldwin is an Arkansas Extension weed scientist.

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