Update from the Field: Smoke ‘em, Don’t Burn ‘em
Posted by David Dyson, Agronomist on June 11, 2018
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Weed resistance is becoming a nationwide epidemic. Long-term resistance management requires more than pest control aimed only at minimizing crop loss in any one season. In this week’s post, I would like to show some of the weeds prone to resistance and provide some suggestions for controlling or managing weed resistance.
The pigweed family can be very prolific and stubborn to kill, a combination that can cause disaster if not controlled early. The picture in Figure 1 shows two of the more popular varieties of pigweeds. Redroot pigweed can be distinguished from its nastier two cousins Waterhemp and Palmer amaranth because it has “hair” on the stem. Waterhemp and Palmer amaranth are both “hairless” and most likely Roundup/ALS resistant. You can identify Waterhemp from Palmer amaranth by comparing the petiole length, as Palmer amaranth’s petiole is longer than its leaf.
Figure 1: Both weeds look similar, but the weed on the right is identified as redroot pigweed because it has hairs on the stem. Since it is hairless, the weed on the left is identified as waterhemp.
Giant ragweed has always been a thorn in the sides of farmers, but there is a new variety of giant ragweed that is Roundup and maybe Liberty resistant. Figure 2 shows a Liberty Link soybean field sprayed with 32oz/ac of Liberty and the giant ragweed, which was within label height, was burned down to the ground and is now growing back. Giant ragweed can also host European corn borer (Figure 3), creatures that can bore into and “hollow” out the weed, preventing chemicals from translocating to the roots and killing the whole plant.
Figure 2: Giant ragweed burned back but not killed in a Liberty Link soybean field west of Walton, IN.
Figure 3: European corn borer hollowing out a giant ragweed. This picture was taken outside of Young America, IN, 2017.
Marestail is a newer pest that has made no-till farming more of a challenge in recent years. Roundup/ALS resistant varieties are abundant out in soybean fields, and I have found some varieties that may be Liberty resistant, too. Figure 4 shows a Marestail weed burned back, but not killed by an application of Liberty herbicide.
Figure 4: Marestail weed coming back to life 2 weeks after an application of Liberty herbicide. This picture was taken near Fostoria, OH, 2017.
In conclusion, we are starting to see new weed species becoming resistant to herbicides that have traditionally been able to kill everything in our soybean fields. This can and needs to scare producers into changing the way we look at applying herbicides. First, is rotation. We need to stop growing continuous soybeans. Rotate into corn and let some of the corn herbicide reduce the weed populations. Second, stop relying on one herbicide to carry the weed load. We need to be utilizing at least four modes of action on every soybean acre. Third, we need to adjust the spray schedule. Troublesome weed spraying needs to start as soon as the combines roll and continue through canopy. We have a greater chance of success if we apply herbicides to weeds under six inches. My fourth and final recommendation is to change up your soybean genetics. Alternate between Roundup-ready, Liberty Link, Dicamba-tolerant, and HPPD-resistant soybean varieties.
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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.
