Palmer amaranth is a dioecious species having male and female plants. A single female is capable of producing more than 1.5 million seed when allowed to grow with little competition and commonly produces 150,000 to 200,000 when competing with cotton or soybeans.
This highly prolific characteristic allows pigweed to completely dominate a field under less than optimum management. An example would be to assume only one female plant escaped that produced 150,000 seed in year one. Pigweed seed are not long lived in the soil, and predation by soil insects and microorganisms is high. But if only 10% of the seed survive and an effective herbicide program is imposed that controls 99% of these, 150 plants are present the following year. If only 50 of these are females and this same scenario of 10% survival and 99% control is carried out one more year, there are now 7,500 surviving plants. For this reason, there is no economic threshold for pigweed management. Near zero tolerance is required each year.
Utilizing residual herbicides and maintaining full labeled rates of all herbicides are essential to control these weeds. Mechanical removal of scattered weeds escaping herbicide programs is an effective means to reducing the soil seedbank.
Weed scientists feel that other weeds will also evolve to resistance populations under heavy selection pressure where glyphosate is the major herbicide used. Following good resistance management practices for pigweed may also reduce the occurrence of future weeds that are resistant to glyphosate.
A carefully planned herbicide program may provide effective control of Palmer amaranth and other pigweed species in soybean. Various herbicides may be used interchangeably and selected based on weed spectrum, price and availability. In addition to herbicides, certain practices such as drill seeding, irrigation to activate pre herbicides and crop rotation can have significant impact on the success of managing Palmer Pigweed.