Pendimethalin Wash‐Off from Cover Crop Residues and Degradation in a Loessial Soil

Abstract

Tillage and cover crops affect soil biological, chemical and physical properties that control the fate of herbicides in soil. Effects of conventional tillage (CT) and no tillage (NT) and either native winter annual vegetation, hairy vetch (Vicia villosa) or wheat (Triticum aestivum) on degradation of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] were investigated. Effect of pendimethalin sorption on residues of these cover crops on subsequent washoff by simulated rain was also examined. Surface (0 to 3 cm) Gigger soil (fine-silty, mixed thermic Typic Fragiudalfs) from triplicate plots of each tillage by cover crop combination under long-term cotton (Gossypium hirsutum) were fortified with pendimethalin (10 nmole g À1 ), incubated (25 ± 1°C) for 7, 21, 42, and 63 d, extracted and analyzed for pendimethalin by HPLC. Degradation was generally faster in NT than CT soil. Within tillage treatments, degradation was faster under native vegetation and wheat cover than under vetch. First-order degradation, half-lives ranged from 47 d (CT vetch) to 17 d (NT native), however, the data were better described by a nonlinear degradation model. Sorption K D s ( = mmole kg À1 /mmole L À1 ) for pendimethalin retention on residues were determined in a batch study (1:500, residue:0.80 mM pendimethalin, shaken for 24 h, and analyzed by HPLC). Measured K D s were large, ranging from~3000 L kg À1 for native vegetation and wheat to $ 6000 L kg À1 for vetch. When pendimethalin was spray-applied at 1.12 and 2.24 kg ha À1 to triplicate 4-g samples of residues and desorbed by three 2-cm simulated rains, < 1% was recovered. Low recovery was consistent with high sorption and predicted using K D s but indicates that benefits of pendimethalin for weed control in NT systems with dense cover crop residues warrants further investigation.

Cotton (Gossypium hirsutum L.) Response to Pendimethalin Formulation, Timing, and Method of Application

“…While pendimethalin has a reported soil half-life of 74 to 114 days [30], surface applied half-lives of 4 to 6 days can occur due to volatilization, photo-chemical, and other degradation processes [21]. Additionally, increased degradation can occur with no-tillage application [9].…”

“…Soil was Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiadults) with 83% sand, 12% silt, 5% clay, organic matter content of 1 to 1.8%, and pH of 5.6 to 6.1. Conventional tillage was used during all three years of the study to obtain optimal herbicide/soil contact, since pendimethalin has been observed to adsorb to cover crop residue [9]. Delta and Pineland 555 BG/RR was planted in 2005 and Delta and Pineland Flex 445 BG/RR in 2006 and 2007 using a Monosem precision vacuum planter set to deliver 14 seeds per linear meter of row with 0.9 m between row centers.…”

Integrating Herbicides in a High-Residue Cover Crop Setting

“…Primarily, the use of reduced-tillage and cover crops eliminates the ability to utilize preplant incorporated herbicides which offer residual soil activity [11]. Furthermore, cover crop residue can impede preemergent (PRE) herbicide applications from reaching the soil surface, reducing herbicide efficacy [12]. While postemergent chemical weed control can be effective alternatives in these settings, many weed species can prove to be difficult to control if not killed early in the season.…”

Peanut Performance and Weed Management in a High-residue Cover Crop System

“…However, there is uncertainty as to whether an acceptable level of weed control can be achieved in peanut systems that include a high level of cover crop biomass due to a physical barrier of residue impeding the movement of the herbicide to the soil surface. Efficacy of pendimethalin, which is tightly sorbed to plant residue, can subsequently be reduced if substantial amounts of the herbicide are intercepted by the cover crop biomass [27,28].…”