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Peanut weed management trials were conducted at the Oklahoma State University Caddo Research Station near Fort Cobb in 2021. Spanish peanut 'Ole' was planted on May 6, 2021,in  36-inch rows. The average temperature for May was 66 F and June was 77 F. The average minimum temperature was 57 F for May (10 d <60 F, 5 d < 50 F) and 66 F for June (7 d <60). This combined with 4.43 inches of rain in May and 7.37 inches in June led to slow development of peanut growth during the late spring and early summer. Additionally, in some situations, poor nodule development was observed and thus plants exhibiting nitrogen deficiency. Under these circumstances, weed control was a challenge. Preemergence (PRE) treatments were applied immediately after planting. For the Gramoxone Tolerance study, the entire trial received an overlay of Prowl (1 qt/A) preplant incorporated (PPI), Valor (2 oz/A) PRE, Cobra (12.5 fl oz/A) + Basagran (1 qt/A) + Butyrac 200 (1 pt/A) + Warrant (1 qt/A) + Shadow (1 pt/A) + Dyne-Amic (6 oz/A) POST, and a mid-season application of nitrogen (120 lb/A 46-0-0). This trial was maintained weed free throughout the growing season. All weed management trials were irrigated and managed for disease throughout the growing season. Trials were visually evaluated for peanut response and weed control. Peanuts were dug (Oct. 8), field dried and harvested (Oct. 19). Peanuts received 3.85 inches of rain between digging and thrashing.

Trial PFCS21-01 was established to evaluate preemergence combinations of Valor (2 oz/A), Outlook (12.8 fl oz/A) and Pursuit for weed control in peanuts. Pursuit was either applied PRE at 4 fl oz/A or as a split treatment PRE (2 fl oz/A) + At-Crack (2 fl oz/A). All treatments included the following applications: PRE-Prowl H2O (32 fl oz/A), At-Crack-Gramoxone (16 fl oz/A) + Zidua (1.75 fl oz/A) + Agri-Dex (1% v/v), POST1-Cobra (12.5 fl oz/A) + 2,4-DB (21 fl oz/A) + Agri-Dex (1% v/v), POST2-Select Max (16 fl oz/A) + Agri-Dex (1% v/v). All treatments that included Pursuit in combination with Valor and/or Outlook controlled Palmer amaranth (pigweed) at least 95% except Outlook + Pursuit PRE season long (Table 1). Treatments that included Pursuit either PRE or PRE + AC (At-Crack) controlled Texas panicum (Coloradograss) over 90% except Pursuit PRE in combination with Valor or Valor + Outlook (Table 2). The only treatments that controlled ivyleaf morningglory at least 90% season long were Pursuit PRE alone or with Valor and Valor + Outlook + Pursuit PRE + AC (Table 3).

Yellow nutsedge populations were highly variable in this trial (Table 4). The only treatments that controlled yellow nutsedge at least 80% late season was Pursuit PRE alone and Pursuit PRE + AC alone and in combination with Outlook. Peanut injury was less than 10% with any treatment combination season long (Table 5). All treatments yielded higher than the untreated check (1990 lb/A), while there were no other differences between treatments (Table 5). Pursuit PRE alone and Pursuit PRE + AC alone or in combination with Outlook or Valor + Outlook all yielded greater than 4,000 lb/A. This research indicates the needs of a residual program that includes multiple modes of action, especially across a diverse weed spectrum.

Trial PFCS21-03 was established to evaluate the response of peanut to POST applications of Gramoxone alone and in combination with Dual Magnum. Gramoxone 3.0 SL was applied at 10.8 (1X) and 21.6 (2X) fl oz/A either alone or in combination with Dual Magnum at 21.3 (1X) and 42.6 (2X) fl oz/A. Applications were made at 14 days after crack (DAC), 28 DAC, or 14 + 28 DAC. All treatments were applied with Induce (non-ionic surfactant) at 0.25% v/v except the treatment that included Agri-Dex.

Peanut stands were not affected by any treatment throughout the growing season (Table 6). All treatments resulted in approximately 10% burning after the initial application (Table 7).  Herbicide rate or the addition of Dual Mangum did not routinely contribute to an increase peanut leaf burn. Peanut stunting was less than 10% during the first 47 DAP with all single applications applied at 14 DAC (Table 8). Two applications of Gramoxone alone or in combination with Dual Magnum at the 2X rate resulted in at least 10% stunting of plants season long. Peanut injury was at least 10% with all 14 DAC on initial evaluation (Table 9).

Peanut injury evaluated after the 28 DAC applications was at least 10% with all treatments except the 14 and 28 DAC applied at the 1X rate. Peanut injury was 10-16% when Gramoxone was applied twice (14 + 28 DAC) alone or in combination with Dual Magnum at the 2X rate. These were also the only treatments that reduced yield when compared to the weed-free check (3490 lb/A). The addition of Dual Magnum or Agri-Dex did not result in an increase in injury when compared to similar treatments that did not include these products. This research indicates that while substantial injury may be observed with Gramoxone, this does not necessarily translate into a yield loss. Additionally, the addition of Dual Magnum to assist in providing residual weed control does not increase the potential for injury.



The authors would like to express our gratitude to the Oklahoma Peanut Commission for support of this research. Additionally, we would like to extend our appreciation to the research staff at the Oklahoma State University Caddo Research Station: Bobby Weidenmaier, Brennan Leighton and Harley Houston. Without the support of these groups and individuals, along with the producers of Oklahoma, the development and results of this research would not be possible.

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