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Palmer amaranth can cause considerable yield reduction

Posted Tuesday, December 20, 2011, at 10:33 AM

Palmer amaranth is a very competitive weed. In some fields with "scattered" pigweed plants, the control was considered good.

In a study conducted in soybeans several years ago, one Palmer in 80 feet of soybean row reduced yield 17 percent and one palmer in 60 foot of soybean row reduced yield 27 percent.

Even at low populations, palmer amaranth is a direct problem but another problem is the amount of seed that they produce (up to a million seeds per plant) resulting in challenges in succeeding years.

Palmer amaranth is reported resistant to DNA herbicides, ALS inhibitors, photosystem II inhibitors, and glyphosate as of October 2010. Since that time it has also been found to be resistant to 2,4-D and HPPD-inhibiting herbicides (Callisto, Laudis, and Impact) in Nebraska. Fortunately the plants resistant to 2,4-D and HPPD inhibitors have been found in very limited locations where the herbicides were used repeatedly over several years.

The bad news is that this can easily happen in other fields as these herbicides are commonly used.

There are several good options for good pre-emerge and post-emerge control of waterhemp in corn. In soybeans, options are somewhat fewer, but there are some good control programs.

Glufosinate (ignite) use in glufosinate resistant crops is a good option. Timing of application is critical to get the control you need. The optimum size for gluphosinate application is 3 -- 4 inches and with a plant that can grow an inch a day, even under adverse conditions; the window for ideal application is very small.

Another concern for 2012 is the combination of lower crop price prospects and the higher costs of crop inputs. Managing your fertility inputs can reduce expenses and not reduce yields in the short term. John Lory, University of Missouri fertility specialist conducted research on the effect of not applying maintenance levels of phosphorous (P) and potassium (K).

Results indicated that one year off from maintenance P applications resulted in less than 5% of the field moving into the low category when soils were at the target soil test levels initially. For K, it was projected to take two years of no application to lower any part of the field into the low category.

Their conclusions were that taking a year off applying maintenance P or K applications will lead to a decrease in soil test level in your field but this drop is unlikely to reduce yield in you field if soil test P and K is at or above current MU targets.

This could be a possible strategy to reduce some of the risk in producing a crop in the short term.

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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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