Two particularly troublesome pigweeds are Tall Waterhemp and Palmer Amaranth, which have and are continuing to develop resistance to herbicides. In Illinois, Tall Waterhemp is resistant to 6 different classes of herbicides and Palmer Amaranth is resistant to 3. If that is not bad enough, members of the pigweed or amaranth family can cross pollinate between species which aids in the rapid spread of resistance as well.
Weed scientists across the Corn Belt, including Dr. Hager, are recommending a "zero" tolerance for palmer amaranth escapes. This is due to the fact that one female plant will produce 460,000 Seeds and if you control 95% of those seeds, there is still the potential for 23,000 plants to survive.
A weed free field is the key starting point. Tillage or a herbicide burndown plus a residual herbicides are essential when Palmer is present. This must be followed by vigilant scouting and herbicide applications combining post-emergence and residual products when new seedlings are identified.
If Palmer is identified after it is too big for herbicide control, hand removal is the best option. Any field in which Palmer reaches maturity should be harvested last and left untilled. Leaving any seed on the surface will allow for natural forces to reduce the viability of seed are enhance germination the following spring when tillage or burndown herbicides can reduce the population potential. Deeply burying the seed may lead to a decade of Palmer problems as future tillage brings old seed up to the surface. Research shows that viable seed left near the surface will "burn out" in about 4 years if no future seeds are added to the seed bank.
Zero tolerance for pigweed escapes is the key to successfully keeping these weeds in check.