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Pigweed is proving to be stubborn weed

Posted Wednesday, March 9, 2011, at 12:47 PM

Pigweed is a weed that is causing great concern among producers.

Pigweed has developed resistance to several classes of herbicides and can significantly reduce yields. It is probably the main weed that will have to be dealt with in the future.

Just one pigweed per 3 meters of row can reduce soybean yield by 17 percent.

Pigweed is an unbelievably competitive plant.

Pigweed's tap root can go 5 feet deep, it can tolerate very high temperatures, and it's amazingly drought and stress tolerant.

Work at the University of California-Davis in the 1980s showed it had the highest photosynthetic rate of any plant they measured. A single 5-foot plant can produce 500,000 or more seeds, which are easily spread by wind, animals, and/or equipment.

Pigweed can grow from 0.21 to 0.18 centimeters per growing degree day. When daytime temperatures hit the mid-90s and the nighttime low is about 70, if you have 2-inch Palmer pigweeds early morning, they can grow 2.5 inches that day.

Twenty-four hours later, they can be 4.5 inches tall.

In Tennessee in 2010, a county agent sprayed some 8 to 10-inch Palmer pigweeds with glyphosate and nothing happened.

He then sprayed them with Ignite, which will control pigweeds if they're less than 6 inches tall. Ignite burned them, but they came back.

He sent in a chopping crew, but if you leave any root stump at all, it will send out auxiliary buds that will re-grow.

If you pull them up and stack them at the end of the row or the edge of the field, more often than not they will re-root.

It is an unbelievably resilient plant.

Larry Steckel, row crop weed specialist, University of Tennessee Department of Plant Sciences, states that a key component in controlling pigweed is residual herbicides. The amount of control is important.

Steckel notes that if you get less than 90 percent control, pigweed is going to get away from you. Pigweeds that can grow 2.5 inches per day can get too big for adequate control quickly.

A two inch tall plant can be 4 to 5 inches tall in 24 hours which is too big for Flexstar to control. In 36 hours that 2 inch plant can be too big for Ignite to control.

Two days can make a big difference in control.

If you have or suspect Palmer pigweed is in your fields, it is critical that you keep an eye on your fields so you'll know if your preemerge is working.

If you have pigweeds in soybeans, rotate to corn, there are a lot of herbicides that will do a good job of controlling pigweeds in corn.

Steckel believes that the new technologies such as the dicamba trait and the 2,4-D trait will be useful tools to help bail us out of the glypghosate-resistance quandary.

He also emphasizes that we must keep Ignite and Flexstar and the PPOs in play for Palmer pigweed for the next three to five years until these new technologies are available.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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