60-year shift in weeds tells ag’s story

Quick! Take this quiz before you look at the chart. First, were perennial weeds more common today or in the 1940s? Second, which weeds have always given Midwesterners more fits, grasses or broadleaves? Finally, how many perennial weeds were among the recent top five common weeds?

“It’s probably not a surprise that the five most common weeds in 1948 were all perennials,” notes Bill Johnson, a Purdue University weed control specialist. No. 1 was Canada thistle. Back then, there were precious few ways to control it except by mowing repeatedly.

The entire top five most common weeds were broadleaves in 1948. Quackgrass, a perennial, made it in the 1950s. Johnsongrass climbed into the No. 3 spot in the 1970s, before there were effective ways to control it with glyphosate.

Canada thistle made its last appearance at No. 1 in 1987. With Roundup Ready crops, farmers have reduced thistle problems.

Two relatively new weeds to appear are common waterhemp and Palmer amaranth.

Key Points

• Perennial weeds dominated farming 60 years ago.

• Broadleaves outnumber grasses as the most common weeds.

• Glyphosate-resistant weeds rank high today.

Today’s top five

No perennials and one grass made the top five in 2007. Ironically, the grass is volunteer corn. Technically, four of the five are glyphosate-resistant, assuming volunteer corn carried over from glyphosate-resistant corn.

“Handling volunteer corn is more of an issue,” Johnson says. That’s especially true in corn after corn.

The list of glyphosate-resistant weeds worldwide is 16, Johnson notes. They include, along with date of first discovery, rigid ryegrass, ’96; goosegrass, ’97; horseweed (marestail), 2000; Italian ryegrass, ’01; hairy fleabane, ’03; buckhorn plantain, ’03; common ragweed, ’04; giant ragweed, ’04; ragweed parthenium, ’04; johnsongrass, ’05; Palmer amaranth, ’05; common waterhemp, ’05; sourgrass, ’06; wild poinsettia, ’06; junglerice, ’07; and liverseedgrass, ’08.

Sixteen species have turned up resistant in 14 years, Johnson notes. That’s 1.1 species per year.

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Chart provided by Bill Johnson, Purdue University

This article published in the February, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.