A recent survey in Nebraska reported Palmer amaranth and waterhemp are among the top five most troublesome and difficult to control weeds in corn-soybean production systems.
This is because of a number of reasons, including widespread occurrence, a season-long emergence pattern, prolific seed production, evolution of resistance to herbicides, and ability to outcross that can easily spread herbicide resistance from resistant to susceptible species.
Most weed management strategies depend on weed density, potential yield loss and associated cost. However, weed escapes from postemergence herbicide and late-season weed emergence are usually ignored once normal yield has been achieved.
Considering the late-season emergence pattern and prolific seed production, few female plants of Palmer amaranth can contribute notably to the seedbank. Therefore, labeled late-season herbicide applications should be investigated, particularly in soybean fields where preemergence herbicide was not applied and postemergence herbicide is the only option in a no-till production system.
Furthermore, late-season sequential herbicide applications would not only suppress weed cohorts, but also diminish the seed production of the surviving plants, leading to a reduction in seedbank replenishment.
Field experiments were conducted in a grower’s field infested with glyphosate-resistant Palmer amaranth in Nebraska for two years to evaluate single and sequential application of postemergence herbicides in Roundup Ready 2 Xtend soybean.
Dicamba applied when soybean was at four trifoliate (V4) stage or in sequential applications at V4 followed by R1 (flowering initiation) stage provided 86% to 97% control of glyphosate-resistant Palmer amaranth 21 days after treatment, and 91% to 100% control at soybean harvest, reduced Palmer amaranth seed production in the range of 557 to 2,911 seeds per female plant, and secured the highest soybean yield.
Here is the list of best management practices to reduce weed seed production: Understand the biology of present weed species as it is important to know when they emergence for effective control; use a diverse weed management approach focused on reducing weed seed production and the number of weed seeds in the soil seedbank; plant into weed-free fields using weed-free crop seeds; scout fields routinely, use multiple mode of action preemergence herbicide at planting crops for controlling pigweeds; apply labeled herbicide rate at recommended weed height; use cultural management techniques such as narrow row spacing and integrating cover crops when possible that suppress weeds through crop competitiveness; prevent field-to-field or in-field movement of weed seed, and manage weed seed at harvest to prevent a buildup of the weed seedbank.
Jhala is a Nebraska Extension weed management specialist.