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Daytime believers

Agriculture.com Staff 07/06/2010 @ 5:21pm

How do cooler morning temperatures and dew affect herbicide performance?" asked Gordon Briggs, a western Michigan grower. Mornings are usually the calmest part of the day for spraying, but Briggs has been concerned about the effectiveness of his early-day sprays.

It turns out, early morning isn't the only time he should be questioning, according to weed scientists.

Reid Smeda has also been intrigued by the topic. In 1996, the University of Missouri (MU) weed scientist noted that the label for glufosinate (Liberty) did not recommend applications two hours before sundown. Smeda determined this was due to reduced weed control that results in some situations when the post-applied herbicide is sprayed late in the evening.

Over the next few years, this finding coincided with experiences by some farmers who applied Liberty or glyphosate (such as Roundup) in early morning or late evening. As the use of these herbicides on genetically modified crops grew, these reports of reduced control increased.

"Looking through the literature, there was no research to document this," says Smeda. "So, we thought maybe it was time to take a look."

Smeda and other researchers, including Brent Sellers, a fellow MU weed scientist, initiated time-of-day application studies over a three-year period. They sprayed weed plots of velvetleaf, common cocklebur, and morningglory species with Liberty and a Liberty/atrazine mix at zero, two, and six hours before sundown at 24-ounces-per-acre and 34-ounces-per-acre rates at two sites.

The verdict? Weed control was lower following late-evening and sunset applications in three out of six site years. In one case, reduced weed control caused a 60% corn yield loss when researchers applied Liberty at sundown vs. six hours before sundown.

That's a point University of Minnesota researchers discovered in a 1998-1999 trial. Reduced weed control resulted when they applied Roundup Ultra and Liberty at early- morning and late-evening times. Meanwhile, a 1999 Kansas State University (KSU) study found similar results with early-morning and late- evening glyphosate applications.

"I was surprised," says Dallas Peterson, KSU Extension agronomist. "I was always skeptical of time-of-day effect. But it does occur."

Unfortunately, these findings throw a wrench in a time-tested production practice. "Farmers like to spray when it's not windy, and the early morning and late evening are the best times to do this," says Peterson.

Spraying in early morning and late evening also enables farmers to dodge herbicide applications during midday heat, which can damage crops. Yet, these findings show there are times when daytime applications work best.

How do cooler morning temperatures and dew affect herbicide performance?" asked Gordon Briggs, a western Michigan grower. Mornings are usually the calmest part of the day for spraying, but Briggs has been concerned about the effectiveness of his early-day sprays.

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