Field studies were conducted in 2005 and 2006 in south Texas and the southern High Plains of Texas to determine peanut response to POST applications of chlorimuron at 9 g ha−1. Treatments included chlorimuron alone, imazethapyr applied 21 days after planting (DAP) followed by chlorimuron applied POST, and chlorimuron plus either 2,4-DB or chlorothalonil in combination applied POST. Postemergence herbicide applications were made 60, 74, and 88 DAP at the southern High Plains location or 67, 81, or 95 DAP at the south Texas location. No difference in peanut stunting was observed with any chlorimuron treatments at the south Texas location. At the High Plains location, chlorimuron alone, imazethapyr followed by chlorimuron, and chlorimuron in combination with 2,4-DB stunting was greater than chlorimuron in combination with chlorothalonil in one of two years. Imazethapyr followed by chlorimuron reduced peanut yield in one year in south Texas. No peanut grade (sound mature kernels plus sound splits) differences between chlorimuron treatments were noted at the south Texas location, but for the southern High Plains location, peanut grade was greater when peanut was treated with imazethapyr followed by chlorimuron compared to the other chlorimuron treatments.
Peanut has several unique features that contribute to challenging weed management. Peanut cultivars grown in the United States require a fairly long growing season (140 to 160 d), depending on cultivar and geographical region (
Weeds compete with peanut for sunlight, moisture, and nutrients and can reduce harvesting efficiency. Weeds are particularly troublesome during digging and inverting procedures (
The Classic® label (E.I. du Pont, Inc. Crop Protection Division, Wilmington, DE) states that chlorimuron will control several weeds that are a problem in Texas peanut production including several
The current commercial POST Florida beggarweed [
Chlorimuron cannot be applied to peanut until 60 d after peanut emergence due to crop tolerance (
Chlorimuron was implicated in yield suppression in 4 out of 15 trials conducted across the peanut-production region (
Imazethapyr is used for control of Palmer amaranth (
Chlorothalanil is commonly used to control foliar diseases while 2,4-DB controls broadleaf weed escapes. The combination of 2,4-DB with many POST herbicides improves control of many broadleaf species, particularly if the broadleaf weeds are larger than recommended (
Because of the low growing nature of peanut, weeds that germinate early and are not controlled, “escape” relatively late in the growing season (
Since chlorimuron may have potential to control some weeds that are a problem in Texas peanut, the objective of this study was to evaluate peanut response to POST applications of chlorimuron alone, imazethapyr followed by POST applications of chlorimuron, or chlorimuron in combination with 2,4-DB, or chlorothalanil, in two different peanut growing regions of Texas.
Field studies were conducted at two locations in the peanut growing regions of Texas in 2005 and 2006 to determine peanut response to chlorimuron alone, following imazethapyr or in combination with 2,4-DB or chlorothalonil. The south Texas study was located at the Texas AgriLife Research Station site near Yoakum, TX on a Tremona loamy fine sand (thermic Aquic arenic Paleustalfs) with less than 1% organic matter and pH 7.0 to 7.2. The southern High Plains location was at the Agricultural Complex for Research and Extension Center (AG-CARES) located near Lamesa, TX on an Amarillo fine sandy loam (fine-loamy, mixed, superactive, thermic Aridic Paleustalf) with 0.4% organic matter and pH 7.8.
Peanut cultivars in south Texas included Tamrun 96 (
The experiment was conducted as a randomized complete block design with a factorial arrangement of four herbicide treatments and three application timings with three replications. An untreated check was included for each experiment. One factor was herbicide treatment which included chlorimuron (Classic®, E. I. du Pont, Inc., Crop Protection Division, Wilmington, DE) alone at 9 g ha−1, imazethapyr (Pursuit®, BASF Corporation, P.O. Box 13528, Research Triangle Park, NC 27709) at 70 g ha−1 applied early postemergence (EPOST) approximately three weeks after peanuts were planted followed by (fb) chlorimuron at 9 g ha−1, chlorimuron plus 2,4-DB (Butoxone 200®, S. R. F. A. LLC, One Hallow Lane, Lake Success, NY) at 220 g ha−1, and chlorimuron plus chlorothalonil (Bravo Weather Stik®, Syngenta Crop Protection, P. O. Box 18300, Greensboro, NC) at 1.26 kg ha−1. The other factor was chlorimuron application timings of 67, 81, or 95 DAP in south Texas or 60, 74, or 88 DAP in the southern High Plains. All POST applications included a non-ionic surfactant (Induce®, Helena Chemical Co., 7576 N. Ingram Ave. 101, Fresno, CA 93711) at 0.25% v/v.
Herbicides at the south Texas location were applied with a compressed-air backpack sprayer equipped with Teejet 11002 DG flat fan spray tips (Spraying Systems Company, P.O. Box 7900, North Avenue, Wheaton, IL 60188) which delivered a spray volume of 190 L ha−1 at 180 kPa. At the High Plains location, herbicides were applied with a CO2 pressurized backpack sprayer using Teejet 110015 TT flat fan nozzles calibrated to deliver a spray volume of 140 L ha−1 at 207 kPa. The test areas were maintained weed-free with a preplant incorporated treatment of pendimethalin (Prowl 3.3 EC®, BASF Corporation, P. O. Box 13528, Research Triangle Park, NC 27709) at 1.12 kg ha−1. At the south Texas location, clethodim (Select®, Valent Corp., Walnut Creek, CA 94596) at 180 g ha−1 was applied over the entire test area when annual grasses were at the six- to eight-leaf stage with a tractor-mounted sprayer to control Texas millet [
Peanut injury, expressed as stunting, was visually estimated on a scale of 0 to 100 (0 indicating no stunting and 100 indicating complete stunting or plant kill), relative to the untreated check (
Peanut injury data were transformed to the arcsine square root prior to analysis of variance, but are expressed in their original form for clarity because the transformation did not alter interpretation. Visual estimates of peanut injury, yield, and grade were subjected to analysis of variance to test effects of POST herbicide and application timing. Means were compared with the appropriate Fisher's Protected LSD test at the 5% probability level. The untreated check was not included in peanut stunting analysis but was included in peanut yield and grade analysis. There was an embedded factorial of chlorimuron herbicide by application timing; therefore, data were subjected to ANOVA unstructured.
Analysis of variance indicated that the two-way interactions between the four chlorimuron treatments and three application timings were not significant for all measured variables. Therefore, data for the main effects were combined for presentation. Data is presented by year because a different peanut cultivar was planted each year at both locations. Climatic and weather conditions varied between the two locations; therefore, no attempt was made to combine data over locations.
Peanut stunting observations recorded approximately six weeks after the last chlorimuron POST application indicated that, in 2005 in south Texas, 2% or less stunting was visible. In contrast, in 2006, chlorimuron alone or imazethapyr followed by chlorimuron applications reduced visual peanut growth (5 and 6%, respectively) when compared with the untreated check (
Peanut response to chlorimuron alone and in combination with chlorothalonil, imazethapyr, or 2,4-DB.a,b
In 2005 at the south Texas location, chlorimuron in combination with imazethapyr resulted in a significant (3150 kg ha−1) reduction in yield as compared to the nontreated control (4210 kg ha−1) (
Peanut grade results were inconsistent at both locations (
Peanut growth was not affected by chlorimuron timing application at the south Texas location in 2005 (
Peanut response to chlorimuron alone and in combination with chlorothalonil, imazethapyr, or 2,4-DB applied at different timings.a,b
Peanut yield was reduced for all chlorimuron timing applications at the south Texas location in 2005 but not 2006 as compared to the nontreated control (
The application timing restriction for chlorimuron was based upon field research conducted in the late 1980's (
In summary, the use of chlorimuron for weed control in Texas is a concern due to negative crop response and the weed control spectrum may be limited. Chlorimuron applied 74 to 81 DAP resulted in reduced yield as compared to the nontreated control at south Texas, and for the High Plains experiments in one of two years. Plant stress has been implicated as a factor in peanut yield reduction associated with chlorimuron timing applications (
The Texas Peanut Producers Board provided funds for this research. Kevin Brewer, Dwayne Drozd, Lyndell Gilbert, Bill Klesel and A. J. Jaks provided technical assistance.
1 Texas AgriLife Research, 3507 Hwy 59E, Beeville, TX 78102.
2 Texas Tech University, Texas AgriLife Research, and Texas AgriLife Extension Service, Lubbock, TX 79403.