Introduction

Peanut has several unique features that contribute to challenging weed management. First, most peanut cultivars grown in the U.S. require a fairly long growing season of 140 to 160 d depending on cultivar and geographical region (Henning et al., 1982; Wilcut et al., 1995). Because of this long growing season, soil-applied herbicides may not provide season-long control, resulting in mid to late season weed problems. Secondly, peanut has a prostrate growth habit, a relatively shallow canopy, and is slow to shade row middles allowing weeds to be more competitive (Walker et al., 1989; Wilcut et al., 1995). Additionally, peanut fruit develops underground on pegs which originate from stems that grow along the soil surface. The prostrate growth habit and pattern of fruit development limits cultivation to an early season control option (Brecke and Colvin, 1991; Wilcut et al., 1995).

Pigweed (Amaranthus spp.) is listed as one of the 10 most common weeds in most peanut-growing states in the United States, with Palmer amaranth (Amaranthus palmeri S. Wats.) ranked as the fourth most common weed in South Carolina (Dowler, 1998). Palmer amaranth is not generally ranked as a troublesome weed in most crops in the U.S., however, it is a common weed in many crops produced around the world. Palmer amaranth is currently found in the southern half of the United States (Anonymous, 1990). In Texas, Palmer amaranth can be found in all areas of the state (Correll and Johnston, 1979), and is a severe problem in many peanut fields, when not properly controlled (P. Dotray personal observation).

Texas panicum (Panicum texanum Buckl.), a large seeded, vigorous, fast growing annual grass is commonly found in peanut fields in parts of Florida, South Carolina, Oklahoma, and Texas (Dowler, 1998). It is listed as one of the most troublesome weeds in all peanut growing states except Alabama and Georgia (Dowler, 1998). During the digging operation, the peanut plant is lifted out of the ground and inverted. A heavy stand of Texas panicum can reduce the effectiveness of the process. The tight fibrous root system becomes intertwined with the peanut plant, causing peanut pods to be stripped from the vine during digging. Peanuts that become detached from the plant remain unharvested in or on the soil (Buchanan et al., 1982).

Weed problems may reduce producer income in several different ways. Herbicide costs range from $37 to $124/ha with a net cost to U.S. peanut producers in excess of $70 million annually (Wilcut et al., 1995). Weeds also increase the need for additional tillage operations with a net loss to producers of $7 to $20/ha (Wilcut et al., 1995). Weeds that escape control then cost producers another $49 to $124/ha due to yield reductions and $7 to $62/ha due to quality reductions (Bryson, 1989; Bridges, 1992). Reductions in harvesting efficiency associated with pod loss is estimated to range from $7/ha in Alabama to $17/ha in Oklahoma and South Carolina (Bridges, 1992). Estimated total income losses from poor weed control, yield and quality reductions, increased cultural inputs, and reduced harvesting efficiency range from $132/ha in Texas to $391/ha in Florida (Bridges, 1992).

This study was conducted to evaluate weed control options for Texas panicum and Palmer amaranth using various soil-applied and POST herbicide combinations. In addition, the profitability of the herbicide combinations was compared to determine the most cost effective herbicide treatment.

Materials and Methods

Field studies were conducted in 1997 and 1998 in a producer's field near Pearsall, TX on a Duval fine sandy loam (fine-loamy, mixed, hyperthermic Aridic Haplustalfs) with less than 1% organic matter and pH 7.2. The producer rotated peanut crops between two fields that were approximately 0.8 km apart. Each year the test area was infested with a natural population of Texas panicum and Palmer amaranth. Texas panicum densities were 10 to 15 plants/m² and densities for Palmer amaranth were 20 to 30 plants/m². The experimental design was a randomized complete block with four replications and a four (soil applied herbicides) by eight (POST herbicides) factorial arrangement of treatments. Each plot was two rows 7.6 m long spaced 97 cm apart.

Herbicide treatments at planting included no soil-applied herbicides, preplant incorporation applications of ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl) benzenamine] at 0.84 kg ai/ha, dimethenamid [2-chloro- N -[(1-methyl-2-methoxy)ethyl]- N -(2,4-dimethyl-thien-3-yl)-acetamide at 1.4 kg ai/ha and S-metolachlor [S-2-chloro- N -(2-ethyl-6-methyphenyl)- N -(2-methoxy-1-methylethyl)acetamide at 1.12 kg ai/ha. POST herbicides included no POST herbicides, acifluorfen {5-[2-chloro-4-(trifluormethyl)phenoxy]-2-nitrobenzoic acid} at 0.56 kg ai/ha, aciflurofen at 0.28 kg ai/ha plus bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one,2,2-dioxide] at 0.56 kg ai/ha, bentazon at 1.12 kg ai/ha, imazapic {(±)-2-[4,5-dihydro-4-methyl-4-4(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid} at 0.07 kg ai/ha, imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid} at 0.07 kg ai/ha, pyridate [O-(6-chloro-3-phenyl-4-pyridazinyl) S-octylcarbonothioate] at 1.0 kg ai/ha, and 2,4-DB [4-(2,4-dichlorophenoxy) butanoic acid] at 0.28 kg/ha. Acifluorfen, bentazon, acifluorfen plus bentazon, and 2,4-DB, were applied with a petroleum oil adjuvant³ at 2.3 L/ha. Imazapic and imazethapyr were applied with a nonionic surfactant⁴ at 0.25% (v/v) of the spray volume. Ethalfluralin, dimethenamid, and S-metolachlor were incorporated immediately after application with a power-driven rotary tiller that had an incorporation depth of 6 cm. A nontreated check was included for comparison. Herbicides were applied with a compressed-air bicycle sprayer using Teejet⁵ 11002 flat-fan nozzles that delivered a spray volume of 190 L/ha at 180 kPa. POST herbicides were applied 3 to 4 wk after planting (WAP) when peanut was 10 to 15 cm tall. “AT-108” and “Virugard” were planted on May 1, 1997 and May 20, 1998, respectively, at the rate of 100 kg/ha immediately after preplant incorporated (PPI) herbicides were applied.

Weed control and crop injury were visually estimated 2, 4, 8 and 14 wk after planting. Visual estimates were based on a scale of 0 (no weed control or peanut injury) to 100% (complete weed control as peanut death) relative to the nontreated control. Stand reduction, stunting, and foliar necrosis and chlorosis were used when making the visual estimates. The full impact of herbicide programs were reflected best in the 14-wk evaluations of weed control. Therefore, the 14-wk evaluations are the only evaluation presented.

Peanut yield was obtained by digging each plot separately, air drying in the field for 6 to 16 d, and harvesting peanut pods from each plot with a PTO-driven peanut combine. In 1997, peanuts were left in the field for 16 d before they were combined due to heavy rainfall which prevented access to the field and prevented adequate drying. In 1998, peanuts were harvested 6 d after digging. Weights were recorded after soil and foreign material were removed from the plot samples. Data were analyzed by a four by eight factorial analysis (soil-applied herbicide by POST herbicide). Significant differences among treatments were determined using analysis of variance and means were separated by Fisher's Protected least significant difference at P ≤ 0.05. Transformation of treatment means for Palmer amaranth and Texas panicum control did not change the statistical analysis. Therefore, nontransformed data are presented

Gross value ($/ha) was determined as a product of pod yield (kg/ha) and market value ($/kg). Net return ($/ha) was determined by subtracting herbicide costs from gross value. Costs other than those for herbicides were held constant over the entire experiment. Prices for each herbicide are the average of quotes provided by three major agricultural suppliers in south Texas.

Results and Discussion

There was a soil-applied by POST herbicide interaction for weed control, peanut yield, and net returns. Lack of year by treatment interactions allowed pooling of data over years for Texas panicum and Palmer amaranth control. A treatment by year interaction was significant for peanut yield and net return. Therefore, data are presented separately by year for these parameters.

Palmer Amaranth Control

Acifluorfen alone or acifluorfen plus bentazon controlled Palmer amaranth at least 70% while imazapic or imazethapyr alone controlled at least 80% (Table 1). Grichar (1997) reported imazapic POST provided greater than 95% Palmer amaranth control in 3 years when used alone while acifluorfen, acifluorfen plus bentazon, or imazethapyr provided greater than 90% control in 2 out of 3 years.

Table 1 Late season Palmer amaranth control using soil-applied and POST herbicides^a.

Bentazon, pyridate, or 2,4-DB POST without a soil-applied herbicide failed to control Palmer amaranth (Table 1). Bentazon does not control pigweed species (Buchanan et al., 1982; Grichar, 1994; Wilcut et al. 1994, 1995). Pyridate is active against yellow nutsedge Cyperus esculentus L., Florida beggarweed [Desmodium tortuosum (Sw.)DC.], and tall morningglory [Ipomoea purpurea (L.) Roth] (Grichar, 1992; Hicks et al., 1990; Jordan et al., 1993; MacDonald et al., 1988). Birschbach et al. (1993) reported that pyridate in combination with atrazine [6-chloro-N-ethyl-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine] controlled an average of 98% triazine-resistant smooth pigweed (Amaranthus hybridus L.). In Europe, pyridate has been used extensively to control triazine-resistant weed biotypes (Birschbach et al., 1993)

When these POST herbicides were applied following PPI applications of ethalfluralin, dimethenamid, or S-metolachlor, Palmer amaranth control was at least 84% (Table 1). Ethalfluralin alone controlled 79% Palmer amaranth while dimethenamid and S-metolachlor alone controlled 59 and 69%, respectively. Pigweed spp. can be controlled with ethalfluralin (Wilcut et al., 1994). Metolachlor applied PPI or PRE controls pigweed less consistently than dinitroaniline herbicides (Wilcut, 1991; Wilcut et al., 1994).

Dimethenamid is used in corn (Zea mays L.), soybean (Glycine max L.), grain sorghum [Sorghum bicolor (L.) Moench], and peanut (Anonymous, 1998). Several broadleaf weeds are controlled or suppressed by dimethenamid including nightshade species (Solanum spp), pigweed species, and common lambsquarters (Chenopodium album L.) (Gaeddert et al., 1997; Owen et al., 1998; Tonks et al., 1999). In field potato (Solanum tuberosum L.) studies, dimethenamid effectively controlled annual grasses but provided less consistent annual broadleaf weed control (Arnold and Gregory, 1994; Arnold et al., 1998; Sarpe et al., 1994). Other potato studies have shown that dimethenamid controlled common lambsquarters, redroot pigweed (Amaranthus retroflexus L.), and hairy nightshade (Solanum sarrachoidas Sendtner) better than metolachlor [2-chloro-N-(2-ethyl-6-methylpheny)-N-(2-methoxy-1-methylethyl)acetamide] or pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] (Tonks et al., 1999).

Texas Panicum Control

Imazapic applied POST controlled Texas panicum 80% when used without a soil-applied herbicide (Table 2). Imazapic will control Texas panicum control when applied to Texas panicum less than 2.5 cm tall, while imazethapyr alone provides inconsistent control when applied to small Texas panicum (authors personal observation). Imazapic will control rhizome and seedling johnsongrass [Sorghum halepense (L.) Pers.], Texas panicum, large crabgrass, southern crabgrass, [Digitaria ciliaris (Retz.) Koel.], and broadleaf signalgrass (Wilcut et al., 1993; 1995).

Table 2 Late season Texas panicum control using soil-applied and POST herbicides^a.

Ethalfluralin alone controlled 94% Texas panicum while dimethenamid and S-metolachlor controlled this weed less than 70% (Table 2). The dinitroaniline herbicides provide excellent control of annual grasses (Buchanan et al., 1982; Chamblee et al., 1982; Wilcut et al., 1994) and are the only soil-applied herbicides registered for use in peanut that will provide full-season control of Texas panicum (Wilcut et al., 1987a,b, Wilcut et al., 1995).

Ethalfluralin in combination with POST herbicides controlled 88 to 97% Texas panicum which was not better than ethalfluralin alone (Table 2). Only dimethenamid in combination with imazapic provided better Texas panicum control than dimethenamid alone (Table 2). S-metolachlor in combination with imazapic or imazethapyr controlled at least 84% Texas panicum which was better than S-metolachlor alone. Dimethenamid controls many annual grasses, such as foxtails (Setaria spp.), barnyardgrass [Echinochloacrus galli (L.) Beauv.], and large crabgrass [Digitaria sanguinalis (L.) Scop.] but control of woolly cupgrass [Eriochloa villosa (Thunb.)Kunth], wild-proso millet (Panicum miliaceum L.), broadleaf signalgrass [Brachiaria paltyphylla (Griseb.) Nash], and Texas panicum is inconsistent (Grichar et al., 1996; Mueller and Hayes, 1997; Rabaey and Harvey, 1997). Metolachlor provides little or no Texas panicum control (Wilcut et al., 1995).

Peanut Yields

Peanut yields were lower in 1997 because peanuts were not harvested for 3 wk after digging due to extremely wet conditions. Peanut yields from plots without any herbicide were less than 700 kg/ha in both years (Table 3). In 1997, when comparing POST herbicides only, imazethapyr increased yield over the untreated check. When soil-applied herbicides alone were compared, ethalfluralin increased peanut yield. Dimethenamid or S-metolachlor alone did not result in a yield increase over the untreated check in 1997 but did increase yield over the untreated check in 1998 (Table 3). All ethalfluralin and dimethenamid herbicide combinations resulted in a yield increase over the untreated check (Table 3). Only S-metolachlor followed by acifluorfen POST did not result in a yield increase over the untreated check.

Table 3 Peanut yields from herbicide treatments^a.

In 1998, when used alone, acifluorfen and imazapic applied POST increased peanut yield over the untreated check (Table 3). Ethalfluralin, dimethenamid, and S-metolachlor and all POST herbicide combinations increased yield over the untreated check. Competition from Texas panicum and Palmer amaranth can severely reduce peanut yield (Grichar, 1997; Wilcut, et al. 1987a). Not only does the competition from these weeds reduce peanut yield but their extensive root system interferes with harvesting efficiency (Buchanan et al., 1982).

Net Returns

Imazapic and imazethapyr were the most expensive herbicides used, while 2,4-DB was the least expensive (Table 4). Returns closely followed trends in yield. Although imazapic and imazethapyr were the most expensive herbicides to use they provided the greatest return (Table 5). Only in 1997, when imazapic was used without a soil-applied herbicide, was the net return for imazapic lower then with several other herbicides. In soybean, economic returns were found to be less with an extensive weed control system compared to a less extensive system (Bridges and Walker, 1987).

Table 4 Herbicide costs for 1997 and 1998 averaged over three distributors^a.

Table 5 Net returns per hectare for each herbicide treatment^a,b.

In 1997, when comparing POST herbicides applied alone, only imazethapyr increased return over the untreated check (Table 5). Ethalfluralin alone increased return, while dimethenamid and S-metolachlor alone did not. Herbicide combinations which included soil-applied herbicides, with the exception of dimethenamid fb 2,4-DB POST and S-metolachlor fb acifluorfen POST, increased returns over the untreated check.

In 1998, acifluorfen, imazapic, and imazethapyr POST alone increased returns over the untreated check (Table 5). Ethalfluralin, dimethenamid, and S-metolachlor alone or in combination with a POST herbicide increased returns over the untreated check.

Results from this research demonstrate that although imazethapyr and imazapic are the most expensive herbicides, they provide the greatest net returns. Lesser inputs resulted in reduced weed control and lower net returns. Based on this data, the added cost of a soil- applied herbicide resulted in improved weed control and increased net returns over POST herbicides alone. Although many growers in the southwest feel that a total POST program using imazapic or imazethapyr is sufficient, this research shows that a soil-applied herbicide is important in order to maintain season-long weed control and increase net returns.

This research was supported by grower funding administered through the Texas Peanut Producers Board. Mr. Martin Graham, farm foreman, LDS Church Farm provided assistance and Ms. Karen Jamison helped in manuscript preparation.

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