Contributed by Albert T. Adjesiwor, Assistant Professor & Extension Weed Management Specialist, University of Idaho
If you are reading this article, chances are that you are already aware that we have Palmer amaranth in the Pacific Northwest (PNW), at least in Idaho and Oregon. Last summer, we found out that the problem is more widespread than we initially imagined. We learned that no crop or situation was “immune” to this pigweed. We found Palmer amaranth in several crops (corn, dry bean, hay, potato, small grains, sugar beet), as well as right-of-way and private property (Figure 1).
One thing was immediately clear, some crops were more susceptible to Palmer amaranth interference than others. Notably, Palmer amaranth was less problematic in small grains for various reasons.
Figure 1. Palmer amaranth found in multiple crops, right-of-way, and private property in Idaho.
What makes small grains the “ideal” rotational crop for Palmer amaranth management?
Palmer amaranth starts to emerge late in the spring (April/May), at a time when small grains are already established. This makes small grains one of the most competitive and best rotational crops for managing Palmer amaranth. Secondly, although Palmer amaranth has developed resistance to several herbicide sites of action, there are still a lot of effective post-emergence herbicides for managing Palmer amaranth in small grains. Thus, in small grains, growers may not even notice they have Palmer amaranth until the grains begin to dry down and the canopy begins to open up.
What would make small grains the “weakest link” in the rotation for Palmer amaranth management?
Palmer amaranth is a desert weed that has adapted to dryland and irrigated agriculture, as well as temperature extremes. As such, this pigweed has a very wide germination window over the growing season (Figure 2). In fact, Palmer amaranth has multiple germination flushes over the season and can emerge when small grains are drying down for harvest or even after harvest.
Figure 2. Palmer amaranth germination at various temperatures compared to other pigweeds (waterhemp and redroot pigweed). Data adapted from Steckel et al. 2004. Weed Science 52: 217-221
I saw this first-hand in Bruneau, ID last summer (Figure 3). Palmer amaranth began to emerge in small grains, possibly sometime in July, and the Palmer amaranth plants were short enough that the combine missed them at harvest. I went back to the same field about three weeks later and all the Palmer amaranth had set seed and they were spread as far as the eye could see (Figure 4). It took just three weeks for them to start producing viable seeds!
Figure 3. Late-emerging Palmer amaranth that were missed by the combine at grain harvest in Bruneau, ID. Photo taken on August 4, 2023.
Figure 4. Late-emerging Palmer amaranth that were left after grain harvest began to produce viable seeds in Bruneau, ID. Photo taken on August 29, 2023.
That’s not surprising at all for Palmer amaranth. This pigweed is very sensitive to day length and temperature swings. As the day length begins to shorten, they get the cue to start producing seeds before they are killed by frost. Thus, waiting too long after harvest to control Palmer amaranth may not be a good idea.
I came to these conclusions:
Small grains would offer the best Palmer amaranth suppression and possibly some of the most effective herbicide options for managing this troublesome pigweed in-crop. However, Palmer amaranth control after grain harvest is critical in preventing viable seed set and the spread of this pigweed.