Being “Number 1” is often thought of as a good thing. Just ask the Royals, Villanova or the Denver Broncos. Being number 1 is something earned, however, while it’s because you’re the best at something, it’s not always a positive. Take being “Public Enemy Number One” for example. This year several publications and weed specialists have declared a “Weed Enemy Number One” for 2016 – Palmer Amaranth. Not that other weeds aren’t headaches, but why single out Palmer Amaranth?
• It’s a member of the Amaranth genus commonly called pigweeds and includes weeds such as redroot pigweed and water hemp. These are fast growing aggressive broadleaves with a large capacity to produce seed. They are tough weeds and can be difficult to control as they mature.
• Water hemp is native to much of the southern half of the North America. It was used by Native Americans as a vegetable source and the seed as flour. It has a tendency to accumulate nitrates so it can be potentially harmful or even toxic to livestock. And it is spreading north to areas of the U.S. where it typically wasn’t common.
• What has made it a particularly troublesome weed in corn and soybean production is the species has developed resistance to glyphosate herbicides. Glyphosate, the most familiar brand being Roundup®, use has skyrocketed over the last twenty years with the release of GMO crops that are tolerant to glyphosate including corn, soybeans, canola, alfalfa, cotton, and Kentucky blue grass. The development of crops modified to allow the application of Roundup during the cropping season was in response to herbicide resistance problems. Water hemp resistance to Roundup was first identified in 2005.
• There are many other problem weeds resistant to Roundup including other pigweed species so why single out water hemp? It is typically the most aggressive of the pigweed species. It robs crops of moisture and nutrients as well as outcompeting crops for space. Under good conditions it can grow up to three inches per day and is more tolerant of stress than many common crops.
• Plants can produce over 100,000 seeds in a crop field and around 500,000 without competition. These seeds thrive in minimum and no- till situations and the acreage under conservation tillage has grown significantly over the last thirty years. As small seeds their preferred germination zone is within one inch of the soils surface and conservation tillage provides an ideal environment. And conservation tillage, especially no-till, typically has relied on Roundup since it doesn’t need to be incorporated into the soil.
• If not checked during the growing season, palmer amaranth can reduce both corn and soybean yields by over 70%. And this doesn’t include reducing the physical quality of the product and harvest difficulties.
• Palmer amaranth has developed resistance in this country to the following herbicide modes of action: ALS inhibitors, triazines, HPPD inhibitors, dinitroanilines, and glyphosate. It isn’t important to necessarily how each mode of action works but the resistance developed to multiple modes.
Hopefully, it is obvious why Palmer amaranth has its reputation. Next week how to deal with this and other problem summer weeds.