Nighttime herbicide applications may be less-than-effective, according to recent research at the University of Missouri.

Keith Mohr, graduate student in agronomy, examined how Roundup herbicide performed when applied to weeds at various times of day. He presented results at this year's MU Crop Management Conference, Nov. 28 and 29.

"Over the years farmers have occasionally seen reduced weed control with several herbicides applied at night," Mohr said. "Recently, Roundup Ready soybeans have become a widely used system. When producers started to see variability in weed control with that system following nighttime spraying, it got a lot of attention."

Applicators have moved to increased nighttime spraying for several reasons: the general increase in demand for postemergence applications; consolidation of farms and commercial applicators; and new parallel swathing technology, using Global Positioning System satellites, which allow sprayer operators to track their position accurately in the field. Wind speed is also typically lower at night, which means less chance of drift, Mohr said.

His experiments focused on velvetleaf, which has shown less response to herbicides when treated at night. Such weeds fold their leaves at night, creating much less surface area for herbicide interception. He applied Roundup at reduced rates at 2 p.m., 5 p.m., 6:30 p.m., 7:15 p.m. and 8 p.m. Those times gave from six to zero hours of sunlight after application.

Weeds had 25% to 75% higher fresh weight-meaning they suffered less damage-after an 8 p.m. application versus a 2 p.m. spraying. Mohr also conducted additional experiments at various humidity, temperature and leaf angles in order to trace down the main reasons why velvetleaf was harder to control at night.

He found the folding of velvetleaf leaves was responsible for 82% of the drop in weed control. "Yet we still saw 17% lower weed control at night even when we mechanically kept leaf angle at the same point as in daylight," Mohr said. "So it's not just leaf angle." That observation may also validate farmer reports of poorer control of giant and yellow foxtail, waterhemp, morningglory and other weeds that have little or no leaf movement.

"Given the results, we would recommend that applicators apply full rates of Roundup on fields with velvetleaf and other hard-to-control weeds," Mohr said. "The same is true for fields with a lot of tall, mature, weeds.

"This also means that if you're using reduced rates of Roundup, use those only during the day. The low rate that gave you 100% control in the daylight may not be as good when applied in the evening."

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