Renewable Farming

“I’m not convinced the solution to weed resistance will come in a jug.”

 “It will be in cultural things we do. Multiple things we do for weed control.”  That comment came today from Dr. Jason Norsworthy, chair of Weed Science at the University of Arkansas.  He was lead speaker at the first “Knowledge Exchange” seminar hosted by Spraytec, a Brazilian firm introducing a new line of crop nutrient products into the United States.

Dec. 13, 2016  By Jerry Carlson — My take-home from Norsworthy’s presentation was: Chemical weed control leads inevitably into a more costly, complex dead end. Any weed control practice, when repeated continually, leads to selection of weeds that adapt and survive, he told about 120 farmers at the all-day seminar. 

In his own university greenhouse, his experiments showed it’s possible for Palmer Amaranth to develop dicamba herbicide resistance in as few as three generations. 

Thirty states have glyphosate resistant Palmer Amaranth. This weed can produce 1.7 million seeds per plant.

Norsworthy observed that “Iowa agriculture lacks diversity. There’s one herbicide, one trait on almost every acre. My first talk on weed resistance in Iowa was in 2008. I showed a photo from Arkansas of resistant Palmer Amaranth. This weed now exists in half of Iowa’s counties. At the rate we are going, it could soon be your No. 1 problem weed in Iowa, and possibly in Illinois.”

Here are several “bullet points” from Norsworthy’s presentation today:

We approach weed control by doing the same thing over and over and expecting a different answer. Einstein defines that as insanity.

  • We found our first PPO resistance in 2016. We can’t continue relying on one tool and considering it sustainable.
  • Roundup Ready beans and cotton arrived in 1996. By 1998, 98% of the beans in Arkansas were Roundup Ready. It was so easy. By 2008, glyphosate resistant Palmer Amaranth was on 80% of the farms in Arkansas. We lost 25% of our farmers in Arkansas in 2016. We lost them to RR weeds.”
  • American Soybean Association Director Eric Maupin, who farms in Tennessee, warns: “We are quickly approaching the day when we will be unable to grow soybeans in parts of the U.S. because there are no effective herbicide options.”
  • Mares tail was the first resistant weed in Delaware in 2000. By 2022, every acre will have at least one glyphosate resistant weed.
  • Palmer Amaranth would the primary focus. Perhaps marestail. Palmer grows 2.5 inches a day after it is 4 to 6 inches tall. However, Palmer can’t persist too long in the soil.

You can get a 96% reduction in Palmer with a moldboard plowing and a cover crop. Plant into the cover crop, such as cereal rye, when it is 4 feet tall.  Plowing alone can give you a 75% reduction in Palmer emergence.

To delay onset of resistant weeds, start with a clean field, with a residual herbicide. 99% of our Southern farmers have a residual herbicide. Then every two to three weeks we spray a contact herbicide. We need a crop canopy to stop weeds from emerging.

  • Prefix at planting followed by Liberty at 1-3 inches: You will have to spray again with Liberty with every flush.   You need a residual herbicide. Spray bare soil; get over “see ‘em, spray ‘em.”
  • Apply recommended herbicide rate with a good adjuvant at the correct weed size. 2 to 4 inches is where the label is. Don’t use low rates; this selects for resistant weeds.  You need thorough leaf coverage. I can kill every weed in the field with 75% coverage of the leaf. Absorption depends on quality of the adjuvant.

Some of the label restrictions on Dicamba herbicide:

1. You cannot add anything in the Dicamba tank; possible exception: an adjuvant.

2. Must spray when wind is 3 to 10 mph. Only by ground.

3. Can’t spray if rain in forecast next 24 hours.

4. Only TTi nozzles.

5. 110 foot buffer on downwind side. If a sensitive crop is adjacent, no spray at all in your field.


Temperature fluctuation at the soil surface triggers weed germination. Cover crops stabilize the soil temperature.  A crop canopy, or a cover crop, reduces soil temperature variations within 5 to 7 degrees from night to day. In bare soil, the germination zone varies by 20 to 25 degrees, signaling to weeds that this is a good time to germinate and emerge. I favor drilled beans; they can grow a good canopy in 4 to 6 weeks.