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In-Season Palmer Amaranth Control in Grain Sorghum

By Vipan Kumar and Brent Bean
 
Frequent rainfall over the past couple of weeks in various parts of Kansas has aggravated the problem of Palmer amaranth in grain sorghum. Several sorghum fields have been observed with a heavy infestation of Palmer amaranth (Figure 1). With adequate moisture and high temperatures, these Palmer amaranth populations are rapidly growing and causing concern for some sorghum producers. Palmer amaranth can quickly exceed labeled sizes, especially in fields where no pre-emergence (PRE) herbicide was used. It is important to note that post-emergence (POST) applied herbicide options are very limited in grain sorghum. In addition, the majority of the grain sorghum and Palmer amaranth populations have already passed the growth stage when these POST herbicides can be effective.  
 
Figure 1. Palmer amaranth infestation in grain sorghum fields near Russell (A) and Hays (B) on July 20, 2020. Photos by Vipan Kumar, K-State Research and Extension.
 
POST-applied herbicides for Palmer amaranth control in sorghum
 
A study sponsored by the United Sorghum Checkoff Program was conducted in 2019 at K-State Ag Research Center near Hays, KS to evaluate the effectiveness of POST-applied herbicides for Palmer amaranth control in grain sorghum. The study site had a natural infestation of Palmer amaranth and grain sorghum was planted on June 13, 2019. The test plots were sprayed with AAtrex (atrazine) applied PRE at 32 oz/a rate. The tested POST programs included AAtrex, Clarity, Huskie, AAtrex + Clarity, Huskie + Clarity, Huskie + AAtrex (see Table 1). All POST programs were applied on July 10, 2019 (grain sorghum was 8-to-10 inches tall and Palmer amaranth was 6-to-8 inches tall).
 
Results indicated that AAtrex + Huskie applied POST had up to 95% control of Palmer amaranth at 49 DAT, whereas control ranged from 82 to 89% with POST applied AAtrex, Clarity, Huskie alone or tank-mixture of AAtrex + Clarity and Huskie + Clarity (Figure 2). However, it is important to note that 5 to 17% crop injury was also observed with all Clarity based programs (data not shown).
 
Important note: Do not make any POST application of Clarity if the sorghum is more than 10 inches tall or sorghum will be severely injured or produce damaged seed heads.
 
Table 1. List of POST herbicides evaluated for Palmer amaranth control in grain sorghum.
 
Figure 2. Palmer amaranth control with POST herbicides at 12 and 49 days after treatment in grain sorghum. Graph by Vipan Kumar, K-State Research and Extension.
 
General strategies for Palmer amaranth control in grain sorghum
  • Start clean: Due to limited herbicide options for in-season weed control in grain sorghum, producers should allow enough time for Palmer amaranth to emerge and be controlled with before sorghum planting.
  • Use effective soil-residual herbicides: Producers should utilize a PRE herbicide program with multiple effective sites of action as a foundation for Palmer amaranth control in grain sorghum.
  • In-season control with POST herbicides: Scout fields regularly to check for any new flushes of Palmer amaranth.
  • Be mindful of label restrictions for sorghum size and/or growth stage at the time of POST application.
  • Split atrazine applications (2/3rd of total atrazine in PRE treatment and 1/3rd in POST treatment) to maximize control without exceeding the allowed total application rate (2.5 lb/a per calendar year).
Source : ksu.edu