With the 2016 cropping season right around the corner, it's time to plan ahead to start clean in 2016 – that is, not giving weeds, especially resistant weeds, a chance to get started. That's especially the case after 2015, when the weather made it difficult to get in the field for control, specifically for control of early emerging weeds such as marestail and giant ragweed. Some fields were overrun with glyphosate-resistant marestail and waterhemp – the two most problematic weeds in eastern Nebraska.

PROBLEM WEED: In Nebraska, there are about 2.5 million acres affected by glyphosate-resistant marestail, and because of its emergence pattern, it's best to keep it under control up front.

In Nebraska, there are about 2.5 million acres affected by glyphosate-resistant marestail, and because of its emergence pattern, it's best to keep it under control up front, notes Amit Jhala, University of Nebraska-Lincoln assistant professor and Extension weed management specialist.

"Marestail can overwinter, and by early spring, if the field isn't cleared, it can become a big problem for soybean growers," Jhala says. "That's what we saw last year. We had a lot of reports of marestail in soybeans, but by the time growers could get in the field, it was too late. The Roundup Ready soybeans were at V3 to V4 and the glyphosate-resistant marestail was 8 to 10 inches tall. At that point, you don't have any effective herbicide option."

That's why it's best to control early on – applying a pre-plant burndown, usually about a month before planting, if growers can't apply a fall burndown. "Marestail can emerge in early spring and later in the summer, and it's difficult to control," Jhala says. "By the time we plant, marestail might be 3 to 4 inches tall. That's why it's better to control marestail very early spring, sometime in March or early April when weather is cooperative."

Like marestail, giant ragweed, another early-emerging weed, is  resistant to glyphosate in eastern Nebraska. The best approach for control is a pre-plant burndown from late March into May. "By controlling it from the end of March into May, about two weeks before planting corn or soybeans growers can get 80% to 90% control," Jhala says. "Our studies show burndowns with 2,4-D or Sharpen or their tank-mixture have very good giant ragweed control. Giant ragweed is still sensitive to phenoxy herbicides like 2,4-D."

Resistant waterhemp is another problem for Nebraska producers. "Last year, around May and early June, it was very wet and it was difficult for growers to get in the field to plant corn and soybeans," Jhala says. "Then there were fields without any pre-emerge herbicide applied and waterhemp dominated. For waterhemp, applying pre-emergent herbicides within two to three days  of planting is extremely important."

To keep the field free of waterhemp, it's important to follow-up with a post emergence application. But when it comes to post applications to control glyphosate-resistant marestail or waterhemp, options are fairly limited – especially in soybeans, in which case, options are mostly limited to PPO inhibitors.

However, growers have some new tools at their disposal, like LibertyLink soybeans and corn to help combat resistant waterhemp – and marestail if growers weren't able to get a handle on it earlier; however, the Liberty Link varieties must be planted to apply Liberty post-emergence.

Many of these post emerge chemistries, including PPO inhibitors and contact herbicides like Liberty (glufosinate), can't translocate throughout the plant, so timing of application and spray coverage are critical – this means applying when weeds are young and following minimum spray volume requirements, which would be 15 gallons per acre.

Of course, whether starting the year clean or ensuring the chemistry's effectiveness for following years, it's always best to follow the full recommended rate. "If growers use a reduced rate, the weeds might only be partially controlled," Jhala says. "When plants are only partially controlled, there's a greater chance they can become resistant down the road. That's why we always recommend using the labeled herbicide rates."