Introduction
Palmer amaranth is a C4 summer annual native to the southwest United States and Mexico (Ehleringer 1983). The potential yield loss from Palmer amaranth competition has been well documented in several crops, including corn, cotton, and soybean (Massinga et al. 2001; Rowland et al. 1999; Klingaman and Oliver 1994). In peanut, yield loss of 28% was associated with Palmer amaranth season long competition, and peanut canopy cover was also reduced (Burke et al. 2007). Palmer amaranth is ranked as the most troublesome weed in peanut production in Florida, Georgia, and South Carolina (Webster 2009). The rapid growth rate of Palmer amaranth, when compared with other Amaranthus spp., combined with seed production of up to 1.2 billion seed ha−1 (greater than 250,000 seeds per plant), necessitate the need for effective control of this weed to economically grow peanut (Burke et al. 2007; Sellers et al. 2003).
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, Norsworthy et al. (2008) noted paraquat to be highly effective, but the herbicide is only labeled for use 28 days after peanut emergence and possesses no soil activity (Anonymous 2012a, Senseman 2007). The acetolactate synthase (ALS)-inhibiting herbicide imazapic also controls Palmer amaranth from both foliar and soil activity while having a more flexible application criteria (Grichar 2007). However, due to development of ALS resistance in Florida and neighboring states, the efficacy of this herbicide cannot be guaranteed (Heap 2013). The protoporphyrinogen oxidase (PPO) inhibiting herbicides aciflourofen and lactofen are often the only postemergence options for growers attempting to control Palmer amaranth in peanut. But considering the contact activity of these herbicides, aciflourofen and lactofen require good coverage of young, actively growing weeds for maximum weed control (Anonymous 2012b).
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 (Etheridge 1999). In an effort to reduce the drift potential of these faster moving sprayers, many growers employ the use of drift-reducing nozzles. Air induction (AI) nozzles, for example, produce larger droplets, which are less susceptible to drift, than extended range (XR) nozzles at the same pressure (Etheridge 1999; Miller and Lane 1999; Ellis et al. 2002). Although these larger droplets reduce drift, spray coverage may be reduced. Knoche (1994) reported that smaller droplets from XR nozzles were more effective than larger droplets when applying postemergence herbicides at a constant carrier volume. Recent research reported that control of velvetleaf (Abutilon theophrasti Medik.) and common lambsquarters (Chenopodium album L.) with fomesafen, a PPO herbicide, was improved as carrier volume was increased for both XR and AI nozzles (Sikkema et al. 2008). However, Ramsdale and Messersmith (2001) noted that paraquat provided effective grass control regardless of nozzle type or carrier volume. Other studies have shown that nozzle type, carrier volume, and spray pressure provide varying levels of control that are herbicide- and weed species-specific (Buhler and Burnside 1984; Brown et al. 2007; Sikkema et al. 2008).
Numerous studies have documented that herbicide application to small weeds increases control. Common waterhemp (Amaranthus rudis Sauer) was controlled more effectively at 5 cm than at 10 cm with several herbicides (Hager et al. 2003). Mayo et al. (1995) demonstrated that Palmer amaranth control with lactofen herbicide declined from 99% control 14 days after planting (DAP) to 56% control 28 DAP. This trend continued with other contact herbicides tested. Acifluorfen provided 84% control 21 DAP and declined to 24% control when applied 35 DAP (Mayo et al. 1995). The lactofen registration label (Anonymous 2012b) limits application to Palmer amaranth having a maximum of 6 leaves, which Dotray et al. (1996) reported to be approximately 4 to 8 cm in height. Since Palmer amaranth can grow up to 5 cm/day, making applications with these herbicides before plants exceed the registration labeled height can be challenging (Sellers et al. 2003).
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.