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Arkansas weed control

Palmer pigweed has tremendous genetic diversity and could be adapting to rice very quickly.

This may be the year for at least one new rice weed. I could put the bulk of my telephone calls on rice into the following categories: (1) How long will my Command or Facet lay there without a rain? (2) Ricestar. (3) Aim. (4) Palmer pigweed.

Palmer pigweed isn't supposed to be a rice weed, or at least I didn't think so. If I have had one phone call (which I have) on Palmer pigweed control in rice, I have had 50. I consistently get two or three calls on it a day. Of course, the weeds are always 12 to 24 inches tall when I get the call.

I have, for the past several years, had a call or two about Palmer pigweed in rice — usually on the levees. This year they are in the paddies.

As we change herbicides, we often change the weed spectrum. Command is weak on Palmer pigweed. That is one reason it never caught on as a soybean herbicide in Arkansas. That may or may not be the reason for the big increase. I get the impression there is a lot of rice being grown for the first time (at least in a while) in fields that have a history of the weed.

Isn't Palmer pigweed a weed the flood should control? Former rice specialist Bobby Huey would say “that is not a rice weed, it is a junk weed you are paying to pump water for.” If the pigweeds were small and you could cover them with water, no doubt that would be effective. However, pigweeds can grow a foot a week and when they are missed early, the can be 2 to 3 feet tall at flooding. When this happens, the flood can be painfully slow or may not do it at all.

To complicate matters, pigweeds often occur on soils that are difficult to keep a good flood on or that have to be drained for straighthead.

Now that we have established the problem, how do we kill them? While you can bet we will be doing more research on them in rice, we have no database because they just aren't supposed to be rice weeds. We do know that they are a lot like smartweed, small (tiny) ones are easy to kill. Propanil in combination with a lot of things (but especially Storm and Aim) will smoke the little ones.

However, most folks paid them little attention early. They don't have to get very big (6 inches or less) where the non-translocated herbicides only burn them and they come right back.

The primary translocated broadleaf herbicides are 2,4-D and Grandstand. The best one is 2,4-D, but in most of the calls I received, 2,4-D was not an option. Grandstand is only moderately effective on them.

It is over for this year. How much a problem we will continue to have with Palmer pigweed in rice remains to be seen. This weed has tremendous genetic diversity and could well be adapting to rice very quickly. We will do more research. However this year has taught us they can't be ignored. They must be identified quickly and taken out when the are 1 to 2 inches tall.

I hate to see new weed problems for farmers. However, they are job security for weed scientists.

Ford Baldwin is an Arkansas Extension weed scientist.

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