Update from the Field: Weedemic
Posted by Dave Dyson, Agronomist on June 26, 2020
Weed resistance is becoming a nationwide epidemic, a weedemic. Traditional weed control is no longer effective in controlling many weeds. Establishing a long term resistance management program requires thinking out of the box. There are various weeds prone to herbicide resistance, and these weeds require innovative methods of control.
The pigweed family can be prolific and stubborn to kill; a combination that can cause disaster if not controlled early. Redroot pigweed and waterhemp are two common varieties in the pigweed family, Figure 1. Another commonly known member of the pigweed family is Palmer amaranth. Waterhemp and Palmer amaranth are frequently resistant to glyphosate (category 9) and ALS (category 2) herbicides. Category 2 and 9 herbicides are both amino acid synthesis inhibitors. While the three weeds look similar, there are a few distinguishing features to aid in identification. Redroot pigweed has hair on the stem, while waterhemp and Palmer amaranth are both hairless. To distinguish waterhemp and Palmer amaranth, compare the petiole length. Palmer amaranth has a petiole that is longer than the leaf, Figure 2.
Figure 1: Both weeds look similar, but the weed on the right is redroot pigweed because it has hairs on the stem. The weed on the left has been identified as waterhemp because of the hairless stem.
Figure 2: Distinguish between waterhemp and Palmer amaranth by looking at the petiole length compared to the leaf.
Another troublesome weed to consider is giant ragweed. Giant ragweed is becoming glyphosate-resistant (category 9) and glufosinate-resistant (category 10). Figure 3 shows a soybean field sprayed with 32oz/ac of Liberty®. The giant ragweed was below label height when sprayed. After spraying, the giant ragweed was burned to the ground. Three weeks later, the giant ragweed began growing back. Giant ragweed can also host European corn borer, Figure 4. European corn borer can bore into and “hollow” out the weed’s stem, preventing herbicides from translocating to the roots, inhibiting herbicide efficiency.
Figure 3: Giant ragweed burned back but not killed in a LibertyLink® soybean field west of Walton, IN.
Figure 4: European corn borer hollowing out a giant ragweed plant. This picture was taken outside of Young America, Indiana.
Marestail is a newer weed that has made no-till farming more of a challenge in recent years. Glyphosate- and ALS-resistant varieties are abundant in soybean fields. Recently, certain marestail varieties have become glufosinate-resistant as well. Figure 5 shows a marestail weed burned back, but not killed, by an application of Liberty herbicide.
In conclusion, herbicide-resistant weed populations are emerging in soybean fields. PPO inhibitor herbicides (category 14) are being used more frequently to take out resistant weeds. These herbicides can burn the leaf tissue of the soybean plant. To lessen the stress, mix in Phosfix® and Sweet ‘N Eezy® to the herbicide application. The growth regulators in Phosfix and the quick energy boost provided by Sweet ‘N Eezy will allow the soybean plant to recover from an application of a cell membrane disruptor more rapidly.
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Dave Dyson is a regional agronomist for The Andersons’ Farm Centers which are located throughout Ohio, Michigan, and Indiana. He is an Indiana native and grew up on a dairy farm in Miami County. A graduate of Purdue University with a degree in Crop & Soil Science, Dave has a deep knowledge of various agronomic topics and is committed to helping growers improve their crops. If you have any questions, Dave can be reached at david_dyson@andersonsinc.com.
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