Introduction

Weed management in peanut generally requires a soil-applied dinitroanaline herbicide PPI for annual grass control followed by (fb) multiple applications of POST herbicide mixtures for broad-spectrum weed control (Bridges et al., 1994; Wilcut et al., 1994, 1996). Soil-applied herbicides in peanut typically used include dimethenamid, ethalfluralin, pendimethalin, and S-metolachlor. These herbicides control annual grasses and small-seeded broadleaf weeds such as Florida pusley (Richardia scabra L.), common lambsquarters (Chenopodium album L.), and Amaranthus species (Cardina and Swann, 1988; Wehtje et al., 1988; Wilcut et al., 1991a, 1991b). Unfortunately, these herbicides do not control larger seeded broadleaf weeds including Ipomoea species and prickly sida that are problematic in the Virginia-North Carolina area (Richburg et al., 1996; Webster, 2001; Wilcut, 1991; Wilcut et al., 1989, 1991a, 1991b, 1994). Therefore, control of these species requires POST herbicide applications. To further complicate matters for producers, there are 43 weeds of economic importance in the nine peanut growing states in the United States; seven of these are of economic importance to the Virginia-North Carolina peanut growing area (Bridges et al., 1994). To control these peanut pests, growers use 22 herbicides and mechanical tactics available (Hook, 2000).

Diclosulam is a new triazolopyrimidine sulfonanilide soil-applied herbicide that is registered for broadleaf and perennial sedge weed management in peanut and soybean [Glycine max (L. Merr.] (Bailey et al., 1999a, 1999b; Barnes et al., 1998). A number of peanut varieties have exhibited excellent tolerance to diclosulam treatments (Bailey et al., 2000; Main et al., 2002) and diclosulam provides control of a number of troublesome annual broadleaf weeds (Bailey et al., 1999a, 1999b, 2000; Main et al., 2000; Price et al., 2002).

Diclosulam plus ethalfluralin PPI provides broad-spectrum control of problematic weeds found in the Virginia-Carolina area (Bailey et al., 1999a, 1999b). Many Virginia-North Carolina peanut growers use metolachlor PPI for annual grass control as Texas panicum (Panicum texanum Buckl.) is not widespread and metolachlor provides some control of yellow nutsedge (Cyperus esculentus L.) (Bridges et al., 1994; Grichar et al., 1992). Ethalfluralin and pendimethalin do not control yellow nutsedge (Wilcut et al., 1994). However, metolachlor PPI is not as effective as ethalfluralin PPI for control of common lambsquarters, one of the most common broadleaf weeds in Virginia-North Carolina peanut production.

The objectives of this research were to evaluate weed control, crop response, and peanut yield with diclosulam and or S-metolachlor applied PPI and in a systems approach with registered POST herbicides.

Materials and Methods

Field experiments were conducted at the Peanut Belt Research Station located near Lewiston-Woodville, NC in 1998, the Upper Coastal Plain Research Station located near Rocky Mount, NC in 1998 and 1999, and the Tidewater Agricultural Research and Extension Center near Suffolk, VA in 1998. Soils were a Raines sandy loam (fine-loamy, siliceous, thermic, Typic Kandiudults) with 1.1% organic matter and pH 5.9 at Lewiston-Woodville, NC in 1998, a Norfolk sandy loam, (fine-loamy, siliceous, thermic, Typic Paleudults) with 1.1% organic matter and pH 5.8 at Rocky Mount, NC in 1998 and 1999, and a sandy loam with 1.5% organic matter and pH 6.1 at Suffolk. These experimental sites are representative of the major peanut producing areas of North Carolina and Virginia.

Peanut cultivars included ‘NC 10 C’ at Lewiston-Woodville, ‘NC 7’ at Rocky Mount, and ‘NC-V 11’ at Suffolk. Peanuts were planted 5 cm deep in smooth seedbeds at 120 to 130 kg/ha. Seeding rates were typical for these regions according to state extension recommendations. Pest management practices other than herbicide programs were based on Cooperative Extension recommendations.

Weed species evaluated included common lambsquarters (Chenopodium album L.), entireleaf morningglory (Ipomoea hederacea var. integruiscula Gray), goosegrass [Eleusine indica(L.) Gaertn.], pitted morningglory (Ipomoea lacunosa L.), and smooth pigweed (Amaranthus hybridus L.). These weeds are among the most common and troublesome weeds in North Carolina-Virginia peanut production (Webster, 2001). At the time of POST treatments, broadleaf weeds had one to six leaves with densities ranging from 1 to 35 plants per species per m². Plot size was four 91-cm rows that were 6.1 m in length. POST herbicides were applied 14 to 20 days after peanut emergence. These application timings are typical of commercial POST systems in North Carolina and Virginia peanut (Wilcut et al., 1991a, 1991b, 1994, 1995).

Weed management systems one, two, and three received diclosulam PPI at 17, 26, or 35 g ai/ha, respectively. Systems four through six received S-metolachlor PPI at 1.42 kg ai/ha plus diclosulam PPI at 17, 26, or 35 g/ha, respectively. Systems seven and eight received S-metolachlor PPI at 1.42 kg/ha plus diclosulam PPI at 17 and 26 g/ha, respectively, fb acifluorfen at 280 g ai/ha plus bentazon at 560 g ai/ha POST. Systems nine and 10 received S-metolachlor PPI at 1.42 kg/ha fb acifluorfen at 280 g/ha plus bentazon at 560 g/ha POST or imazapic POST at 71 g ai/ha. Acifluorfen plus bentazon POST is the commercial standard for annual broadleaf weed control in North Carolina and Virginia peanuts while imazapic is the commercial POST standard for yellow and purple nutsedge (Cyperus rotundus L.) control (Bailey et al., 1999a, 1999b; Richburg et al., 1994, 1996; Scott et al., 2002; Wilcut, 1991; Wilcut et al., 1994). System 11 received S-metolachlor PPI at 1.42 kg/ha while system 12 was the nontreated check. A nonionic surfactant³ at 0.25% (v/v) was applied with all POST herbicide treatments.

A randomized complete block design with three replicates of treatments was utilized at all locations. Visual estimates of crop tolerance and weed control were made early (mid-June) and late season (late August to early September). Weed control and crop tolerance were visually estimated on a scale of 0 to 100% where 0  =  no control and 100%  =  complete death of the weeds or crop (Frans et al., 1986). Because weed control at the end of the season influenced peanut yield and harvest efficiency, only late season evaluations of weed control will be presented (Wilcut et al., 1994, 1995). The two center rows of each plot were harvested in mid-October using conventional harvesting equipment. Final yields were adjusted to 7% moisture.

Results and Discussion

Crop Injury

Only data from the 3 WAT evaluations are presented for crop injury. Crop injury from S-metolachlor plus diclosulam treatments or either treatment alone consisted of slight stunting that never exceeded 5% and was transitory (Table 1). This level of injury is typical from these herbicides in North Carolina and Virginia (Bailey et al., 1999a, 1999b, 2000; Price et al., 2001; Wilcut et al., 1991a, 1991b).

Table 1 Peanut injury and late-season weed control with diclosulam alone and diclosulam systems^a.

Pitted Morningglory

Pitted morningglory was not controlled by S-metolachlor (2%) and control was improved to 93% with acifluorfen plus bentazon POST or imazapic POST (Table 2). Excellent control of Ipomoea morningglories including pitted morningglory has been seen with imazapic POST in peanut ( Bailey et al., 1999; Richburg et al., 1996; Wilcut et al., 1996). Acifluorfen is also widely used for pitted morningglory control in peanut (Wilcut et al., 1994, 1995). Diclosulam PPI controlled pitted morningglory 63 to 91% with control increasing with increased rate of application. The registered use rate of diclosulam (26 g/ha) controlled 72%. Control with diclosulam PPI or S-metolachlor plus diclosulam PPI was similar within a given rate of diclosulam. All diclosulam PPI treatments fb acifluorfen plus bentazon POST controlled pitted morningglory at least 98%. Bailey et al., (1999b) reported 90 to 99% pitted morningglory control in North Carolina with diclosulam and ethalfluralin PPI, while Grichar et al., (1999) reported greater than 98% control in Texas.

Table 2 Late-season weed control ratings and peanut yield with diclosulam alone and diclosulam systems.^a

The authors thank the station personnel at the Upper Coastal Plain Research Station, Peanut Belt Research Station, and the Tidewater Agricultural Research and Extension Center for the assistance in this research. Appreciation is also extended to the Peanut Growers Associations of North Carolina and Virginia, and Dow AgroSciences for partial funding of these research endeavors.

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