As we near harvest, many of you might be thinking the pigweed (Palmer amaranth) war is over, nothing could be further from the truth. While economic thresholds exist for some weeds, the threshold for resistant pigweed should be zero – do not tolerate this weed in your fields.
Jha & Norsworthy (2009) found two to three consistent emergence periods occurring from early May through mid-July in soybeans and that cohorts during the peak emergence periods from early May to mid-July need greater attention in weed management. However, an older paper by Keeley et al. (1987) examined the influence of planting date on growth of Palmer amaranth. They found pigweeds germinate from March through October. Seeds planted from March to June initiated flowering 5-9 weeks after sowing, and 3-4 weeks for those sown from July to October. This is important, because after you harvest your crop and think you are done for the year, the pigweeds in your field can begin a seed increase project. This is important because Sauer (1988) states that pigweed seeds (no species named) germinate gradually over decades following soil disturbance. Keeley et al. (1987) showed that pigweed seeds sowed at depths up to 1” germinated very well, and seeds buried deeper than 2” do not germinate. They aren’t dead, they are just waiting to be in the right spot, pigweeds need light to germinate. This is why Jha & Norsworthy (2009) only found two to three emergence periods – after the soybeans canopied, the pigweed didn’t have enough light to germinate. Jha & Norsworthy (2009) did not monitor emergence periods post harvest.
I hope you are inspired to at least monitor your fields post harvest. Winning against pigweed requires that we take things personal and remain vigilant and ready for action.
Jha, P., & Norsworthy, J. K. (2009). Soybean Canopy and Tillage Effects on Emergence of AMAPA (Amaranthus palmeri) from a Natural Seed Bank. Weed Science, 57(6), 644-651.
Keeley, P.E., C.H. Carter, and R.J. Thullen. 1987. Influence of planting date on growth of AMAPA (Amaranthus palmeri). Weed Science 35:199-204.
Sauer, J.D. 1988. Plant migration: The dynamics of geographic patterning in seed plantspecies. University of California Press, Berkeley, California. 282 pp