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Got Waterhemp?

University of Minnesota Extension, Stearns County News
April 12, 2017         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Got Waterhemp?
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (04/12/17) — Palmer Amaranth has gotten a lot of media attention since last fall when it was found in Yellow Medicine County. The key is to know what it looks like; and to make and official identification with MN Dept. of Agriculture. We hope we don’t find more of it scattered around Minnesota. But…

Tall Waterhemp IS scattered around a pretty good chunk of Minnesota and showing up on more farms every year. The key here is also to know what it looks like, to understand why it’s giving us fits, and then to deal with it accordingly – and strategically.

Tall waterhemp is in the amaranth (pigweed) family. It often grows to be taller plant than smooth or redroot pigweed. It tends to be a more “open” plant; and leaves tend to have a little longer and narrower shape. Unlike smooth and redroot pigweed, there are not hairs on the stems or leaves.

The following discussion is based on information provided recently by State and Regional Extension Agronomy staff. The full article with field trial data can be found with an Internet search for “Minnesota Crop News” and looking for the article “Got Waterhemp?...” In Stearns, Benton and Morrison Counties, you’re welcome to give me a call through the County Extension Office or at the Benton County office, 968-5077 or 1-800-964-4929.

One of the key issues for waterhemp is that peak emergence can occur as late as mid-July and continue into September. The second key issue is that waterhemp populations are developing resistance to several key herbicide Site-of-Action groups. All weeds should be managed with the goal of reducing the risk of developing resistance to herbicides.

Most waterhemp populations have been resistance to Group 2 herbicides like Pursuit, Raptor, Permit, First Rate and others for a while. Glyphosate resistant waterhemp was first reported in 2007. Group 14 PPO resistance was confirmed in southern Minnesota for the past 2 years. Group 14 includes products like Cobra, Flexstar, Cadet, and others. Some populations are resistant to all three groups.

As resistance issues build, switching to Liberty or dicamba tolerant soybeans are additional post emerge control options, but late applications or overuse can lead to more resistance.

The “Got Waterhemp?”article suggest a “layered” herbicide application with Group 15 herbicides like Dual, Outlook and Warrant; or a Group 14 herbicide such as Valor pre-emerge followed by an additional Group 15 herbicide application about 30 days after planting. This extends seedling control through the peak waterhemp emergence period. One or both of these applications could include a post-emerge product if fields have weeds that are not controlled by these herbicides.

The layered herbicide concept can be used for corn or soybeans, although specific products used could be different. Farmers would also want to know whether herbicide residuals would affect crops grown the next year. Farm store agronomy reps should be familiar with these concepts and work with farmers toward cost effective weed control strategies that fit budgets as well as they can.

Giant Ragweed is another weed that is moving farther north in Minnesota, and farmers in this area should know what it looks like. It will commonly have leaves that are deeply divided into 3 lobes, sometimes 5 or more. It often shows up first around ditches and wetter field borders.

Corn Planting Note: Dave Schwartz, former Extension Colleague, now a seed sales rep sent an email recently with some corn planting tips. For early planting considerations with variable weather and soil temperature conditions, Dave offers the reminder about seed imbibition – the process seed taking up water. If the soil is too cold, critical embryonic tissue can rupture and lead to corn leafing out underground and not finding its way up. For about the first 48 hours after planting, when corn seeds absorb water, that is critical for soil to be at least 50 degrees where the seed is.

I add it’s important to know that some soil temperature reports are based on a 6 inch depth. In this part of the world, a consistent temperature of 50 degrees at 6 inches may not occur until well into May, often later than best for corn yields. Recommended planting depth is commonly given as 1.75 to 2 inches deep. With warm weather, it could warm to 50 degrees at a 2 inch depth for a pretty good part of the day. When we have cold rain and snow mix in the forecast, or persistently colder weather spells, the imbibition problem is more likely to occur.

It is more common to think about planting corn by the last week of April or first week of May if fields dry out and warm up suitably for making a good seedbed.

Patience often counts as much as persistence; and again make SAFETY a priority as well.

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