The last couple of springs, I have gotten calls from farmers and consultants who had just assessed early season Palmer amaranth control. They typically called about fields that had severe glyphosate-resistant Palmer amaranth numbers up with the crop.

These were fields that had no chance of being harvested without drastic action. After we discussed salvage options, which usually entailed either replanting or chopping crews, we often visited about other fields with less pressure. Often the consultant or grower would say they were not worried about a particular field because the pigweed control was pretty good. The phrase often used to describe the pigweed pressure in these fields was “they are scattered.”

I often wonder how much yield loss can be expected from just a “scattered” population of Palmer in a given field. This information is deficient in cotton, so we put together a Palmer amaranth cotton competition study this spring to try to get some answers. We just harvested this trial and the results were surprising to me.

In the plots with no Palmer, lint yield was 1,340 pounds per acre. Just one Palmer amaranth in 60 feet of cotton row competing from emergence to harvest reduced cotton yield to 950 pounds per acre. I really did not expect one Palmer in that large an area to reduce yield by 30 percent. I was expecting about half that yield loss.

Two Palmer in 60 feet of row reduced cotton yield by 38 percent. Four Palmer in 60 feet of cotton row reduced yield by 600 pounds, or 45 percent. Eight or more Palmer per 60 feet of row basically took the crop.

How about in soybeans? Dick Oliver conducted a Palmer completion study in soybeans several years back which is published in the

 

Journal of Weed Science

. In his study, a Palmer in 80 feet of soybean row reduced yield 17 percent, and one Palmer in 60 feet of soybean row reduced yield 27 percent.

Based on the results from these studies in both cotton and soybeans, a Palmer population had better be

 

very

scattered not to be a significant drag on yield.

Soybean and cotton yields have been disappointing in many areas. I know a large part of the reason was the very dry and hot summer. However, based on this data I would suggest that some of the disappointment in yield can be attributed to the Palmer pigweed that is scattered across many fields.

Palmer pigweed is so common that it is becoming part of the background when looking at a field. However, rest assured, it is a yield-limiting pest even at relatively low populations.

Of course, the effect of the scattered Palmer does not end with the yield reduction. The other big problem is that just a few female Palmer pigweeds can produce millions of seed. Fields with that kind of Palmer seed in them will be a serious challenge next year.

I rode with a few growers over the last week as they were picking cotton and combining soybeans. The fields I observed from the cab overall had good weed control. However, in every case there was indeed scattered Palmer across the field. As you are harvesting, note the Palmer in these fields and start planning your 2012 weed control now.

lsteckel@utk.edu