Palmer amaranth is one of the most troublesome weeds in the southeast. Effective control is essential in order to avoid reductions of crop yield. Due to widespread resistance to acetolactate synthase (ALS)-inhibiting herbicides, postemergence contact herbicides are often the only in-season option to control Palmer amaranth in peanut. Lactofen is a postemergence protoporphyrinogen oxidase inhibiting herbicide that is commonly used to control Palmer amaranth in peanut. Adequate spray coverage is essential for lactofen efficacy and nozzle selection may affect coverage. Extended range (XR) and air induction (AI) nozzles were used to evaluate spray coverage on water sensitive cards. XR nozzles provided more coverage than AI nozzles. A factorial treatment structure of carrier volume (94, 187, 281 L/ha), nozzle selection (XR and AI) and application timing (5 to 10 cm or 15 to 20 cm tall weeds) was conducted in 2008 in Williston, FL and in 2012 in Tifton, GA to determine the best strategy for controlling Palmer amaranth with lactofen. Palmer amaranth control was recorded 7, 14, and 21 days after treatment (DAT). Nozzle selection was not significant in field trials as a main effect or as an interaction at any location; therefore data were pooled across nozzle type. However, the carrier volume by application timing interaction was significant at each location. In 2008 at Williston, FL and in 2012 at Tifton, GA application at 5 to 10 cm tall Palmer amaranth with 94, 187, or 281 L/ha provided >90% control. Applications made to 15 to 20 cm tall weeds provided less control. Applications made to smaller weeds provided sufficient control at any carrier volume tested, while applications made to larger weeds were least effective at 94 L/ha. Despite reduced coverage by AI nozzles, nozzle type did not translate to differences in herbicide efficacy in the field. Carrier volume did not affect control of small weeds, but on larger Palmer amaranth, control was reduced at smaller spray volumes. Growers should apply lactofen to smaller Palmer amaranth plants for the most effective control.
Palmer amaranth is a C4 summer annual native to the southwest United States and Mexico (
In-season weed management in peanut often relies on postemergence herbicides. However, many of these do not provide season-long control of Palmer amaranth. For example,
Spray technology has evolved toward faster moving spray equipment and lower carrier volumes in an effort to reduce fuel costs from transporting large quantities of water and the need to cover more hectarage per tank-load (
Numerous studies have documented that herbicide application to small weeds increases control. Common waterhemp (
Current information is limited on the effects of application practices on Palmer amaranth control with contact herbicides. Although importance of weed size at application has been well documented, nozzle type and carrier volume have not been thoroughly investigated. The objective of this research was: (1) evaluate the spray coverage area for XR and AI nozzles to determine the range of carrier volumes and (2) conduct a field study that investigated the effect of application timing, carrier volume, and nozzle selection, and possible interactions, on Palmer amaranth control in peanut.
XR Teejet flat fan nozzles (Spraying Systems Company, Wheaton, IL, USA) and AI Teejet flat fan nozzles (Spraying Systems Company, Wheaton, IL, USA) were calibrated to deliver application volumes of 94, 187, and 281 L/ha. Water sensitive cards were placed 50 cm below the spray nozzles. Water was sprayed over each card using each nozzle and carrier volume combination. The cards were then allowed to completely dry before being stored in zip-type plastic bags. A high-resolution flatbed scanner and color identification software program, WinCAM (Regent Instruments Inc., Canada), were used to evaluate the cards for percent spray coverage. Each nozzle and carrier volume combination was replicated across four cards. Percent spray coverage, as indicated by WinCam analysis, was subjected to analysis of variance using SAS (ver. 9.2; SAS Institute Inc.; Cary, NC) to determine significant treatment effects and interactions. Means were separated using Fisher's Protected LSD at P ≤ 0.05.
Field studies were conducted in 2008 and 2012. In 2008, studies were conducted at Sandlin Farms near Williston, Florida on Candler fine sand with less than 1% organic matter. In 2012, studies were conducted in Tifton, Georgia on Tifton loamy sand with 1.3% organic matter. Palmer amaranth was present in natural populations or seed was dispersed prior to the study to achieve a density of 10 to 40 plants/m. Peanut was planted between April 26 and May 10 at both sites with 18 seed per m of row. The study was multifactorial with nozzle type, carrier volume, and Palmer amaranth size as factors. All treatments were replicated four times and were arranged as a randomized complete block. Nozzle types used were the Teejet XR and the air induction Teejet AI to represent two levels of spray coverage. Carrier volumes evaluated were 94, 187, and 281 L/ha. Palmer amaranth size was measured at each location until average heights of 5 to 10 cm or 15 to 20 cm in height was reached. Lactofen (0.21 kg/ha plus crop oil concentrate at 1% v/v) was applied at the appropriate carrier volume, nozzle type, and plant height combinations.
Palmer amaranth control was estimated visually 7, 14 and 21 d after treatment (DAT) using a scale of 0 to 100% with 0 = no control and 100 = plant death (
The interaction between nozzle type and carrier volume was significant, therefore data are presented for both nozzle type and carrier volume. At a carrier volume of 94 L/ha, 21% coverage was achieved with the use of XR nozzles while AI nozzles provided only 11% coverage (
Influence of carrier volume and nozzle type on percent sprays coverage as determined using water sensitive cards analyzed with WinCAM. Error bars indicate standard error (n = 4) and asterisks indicate differences between nozzle types within each carrier volume.
Nozzle type and location were not significant as a main effect or in any interaction. Therefore, data were combined across both nozzle type and location. The lack of interaction for nozzle type was unexpected since the spray coverage study indicated consistent differences in percent coverage. In some cases, as with application at 94 L/ha, coverage with AI nozzles was about half that observed with XR nozzles (
The main effects of carrier volume and Palmer amaranth height at application, as well as the interaction, were significant (P < 0.0001 for each factor). All applications made at 5 to 10 cm in height were 94 to 99% control when rated 7 and 14 DAT (
Palmer amaranth control with lactofen at 7, 14, and 21 DATa.
The influence of carrier volume was not significant within either application stage when rated at 7 or 14 DAT. By 21 DAT, applications made to 5to 10 cm tall plants were also quite effective resulting in greater than 90% control. However, applications made at 15to 20 cm tall with a carrier volume of 94 L/ha provided 80% control, which was statistically less than all other treatments tested. This reduction in control at 21 DAT was due to regrowth of Palmer amaranth plants.
Adequate coverage is essential for maximizing efficacy with lactofen. The results of this research demonstrate that AI nozzles do provide less coverage than XR nozzles, but this difference in coverage does not translate to differences in lactofen efficacy on Palmer amaranth. Applications made at 94 L/ha resulted in reduced control when treating 15–20 cm tall Palmer amaranth plants. But of greater importance is the fact that greater than 90% control was achieved, regardless of nozzle type or carrier volume, when 5–10 cm tall plants were targeted. Therefore, it is essential that peanut producers regularly scout fields to monitor Palmer amaranth height if contact herbicides such as lactofen are to be used. Applications should be made to small weeds to ensure greater control; if larger Palmer amaranth must be treated, increased carrier volume should be used.