Gear Up Your Glufosinate Game Plan

JEFFERSON CITY, Mo. (DTN) -- With no over-the-top dicamba herbicide presently labeled for postemergence weed control, farmers have one fewer option this season. The result, in many cases, is greater reliance on glufosinate, but getting the most from this active ingredient isn't as simple as loading the sprayer and rolling.

A new study confirms glufosinate effectiveness can vary depending on weather conditions, but there are steps farmers can take to maximize weed control with this contact herbicide.

"We already place a pretty good amount of pressure on glufosinate, but with no dicamba, we're likely going to be placing even more pressure on it," said Kevin Bradley, University of Missouri Extension weed scientist. "It's going to be important for farmers to plan ahead to get the most from their applications."

Bradley was part of a team -- led by Christopher Landau, a postdoctoral research agronomist with the USDA Agricultural Research Service -- who took a retrospective look at how weather affects the probability of successful weed control with one postemergence application of glufosinate. The researchers used a database of more than 10,000 herbicide evaluation trials from 14 U.S. states and one Canadian province and focused on control of waterhemp, giant foxtail and morning glory species, all of which are common, troublesome weeds in both corn and soybeans. Their analysis identified weather variables before, during and after application that most influence glufosinate efficacy, including solar radiation (i.e. amount of sunshine), relative humidity, air temperature and rainfall.

FORGET THE HAY, TAKE TIME TO SPRAY

It's been considered sage advice to "make hay when the sun shines," but those rays may be better used to get the most out of a glufosinate application. Sunshine is essential for the herbicide's mode of action. When clouds rolled in on the day of application, weed control decreased by varying degrees, depending on the species, the study found.

"I think that's one of the main reasons we fail with glufosinate," Bradley said. "We've got so many acres to cover, and if the wind is favorable, most guys aren't going to slow down for a cloudy day. But I've answered a lot of calls about failures when that's exactly what occurred. So, it's important to bring more attention to that."

HEAT & HUMIDITY

The average air temperature both five days before and after an application played a factor in glufosinate efficacy. The probability of successfully controlling weeds increased when average air temperatures were 24 degrees Celsius (about 75 degrees Fahrenheit) or more before an application. Success decreased after application when the average temperature was 25 degrees C (77 degrees F) or less.

The study also showed that overall, low relative humidity on the day of application was generally detrimental to glufosinate efficacy. There appeared to be a threshold of about 70% relative humidity, below which the probability of successful control decreased. Higher levels of humidity improved weed control, especially for waterhemp.

Bradley recounted a conversation he had with a fellow weed scientist years ago who was having trouble getting glufosinate to work.

"Here in Missouri, we usually have no problem having enough humidity, but he was in Canada, and he didn't have the same situation," Bradley said. "So, in different geographies with different humidity levels, you can see much different activity out of glufosinate."

RAINFALL RESPONSE

The researchers also looked at the amount of precipitation that fell before or after a glufosinate application. Though not as influential as other weather factors, weed control probabilities were reduced when total rainfall was 20 millimeters (about 0.79 inches) or less in the five days prior to spraying.

After an application, the effect of rainfall varied by weed species. For controlling giant foxtail, for example, glufosinate efficacy decreased when rainfall reached 100 mm (about 3.94 inches) or more in the five days after spraying. However, the opposite was true when attempting to control waterhemp.

"For me, the solar radiation and the humidity are the two biggest weather factors," Bradley said. "When we're spraying research plots, we're not doing it on a cloudy day. Humidity also is part of the equation."

OTHER FACTORS

While the realities of spraying fields within narrow application windows won't always allow farmers to consider every weather factor, there are other ways to help ensure they are getting the biggest bang for their glufosinate buck.

"Volume is probably the biggest one," Bradley said. "Apply 15 to 20 gallons per acre. If you're willing to go to 20 (the maximum labeled rate), it's worth it."

Adding ammonium sulfate (AMS) also can help with glufosinate's efficacy, especially in the face of low humidity or high pH water. AMS can be added to the tank at a rate of 1.5 pounds per acre all the way up to 3 pounds per acre.

"Time of day also can play a role," Bradley added. "It usually takes the grass weeds until about 10 a.m. to orient themselves in a way where you'll get good coverage. So, spraying from 9 a.m. to 3-4 p.m., somewhere in that range, gives the best chance of success for your application."

Lastly, spraying when weeds are still small -- preferably less than 3 inches tall -- will go a long way toward success.

"This molecule is incredibly sensitive to weed size," Bradley said. "We often place it in some situations that are just setting it up for failure. If you're going out and trying to control 12-inch weeds, you're expecting too much."

The retrospective analysis was published in the journal, Weed Science, and can be found here: https://doi.org/….

Jason Jenkins can be reached at jason.jenkins@dtn.com

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