d seed aerially. Research has shown that marestail seed can reach altitudes of 450 ft. and travel 100 miles in a single flight with moderate winds.
Resistant weeds also spread mechanically on equipment from field to field and region to region. For example, in fall 2010, Christy Sprague, Michigan State University Extension weed specialist, was called to a farm infested with glyphosate- and ALS-inhibitor-resistant Palmer amaranth, a weed not native to Michigan. Sprague went into detective mode to figure out how a weed generally considered a problem in the South and Mid-South had made it to Michigan.
“In 2011 and 2012, we received more reports of glyphosate-resistant Palmer amaranth on farms four to five counties away from the first one,” Sprague says. “A common factor was that manure from dairy farms had been applied to many of the infested fields. We speculated that the seed may have been brought in with hay or feed from another state. A lot of Michigan dairies feed cotton seed.”
In one exceptional case, Palmer amaranth appeared only on the edge of a field. “We speculated that it may have blown off a truck from a corn seed delivery,” Sprague says. “It turned out that cotton seed had been hauled in the truck before it was loaded with corn seed.”
By 2012, glyphosate-resistant Palmer amaranth had been found in nine Michigan counties, with a lot of it from manure applications. “The Palmer amaranth in Michigan looks like it can be as aggressive here as it is in the South,” Sprague says.
Last summer, Mike Owen, Iowa State University Extension weed specialist, found that 60% of waterhemp populations sampled in Iowa were resistant to three or more herbicide modes of action. The samples were gathered as part of a study sponsored by the Iowa Soybean Association.
“In Iowa, it’s likely that you may have two or three different types of resistance in the same waterhemp population,” Owen says. “Farmers have to understand that we already have widespread herbicide resistance in Iowa. We clearly need to do something by going back to integrated pest management practices and diversifying as much as possible.”