Introduction

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).

Horse purslane occurs in tropical and subtropical areas throughout the world (Balyan and Bahn, 1986). It has cylindrical green leaves and the seeds germinate at 20 to 45 C (Chandra and Sahai, 1979). Seeds have essentially no dormancy and can germinate soon after they mature, thus allowing multiple generations in a single growing season (Balyan and Bhan, 1986). Although common purslane (Portulaca oleracea L.), a similar species, was rated as one of the ten most common weeds found in Texas peanut fields as early as 1989 (Elmore, 1989), horse purslane only recently has become a problem in certain peanut growing areas of south Texas (author's personal observation). This weed can be a stronger competitor with peanut early in the growing season than common purslane due to a more upright growth habit than that of common purslane (Grichar, 1993). In competition studies, horse purslane reduced mung bean [Vigna radiate (L.) R. Wilcdz.] yields by 50 to 60% when left untreated (Balyan and Malik, 1989). In earlier work on peanuts, Grichar (1993) reported a yield increase with POST herbicides over the untreated check in only one or two years when horse purslane was present.

Hand hoeing is a common practice for horse purslane control in mung bean in India (Balyan and Bhan, 1986). However, very little research has been conducted on control with herbicides. Fomesafen controlled horse purslane 57 to 87% in mung bean (Balyan and Malik, 1989). Grichar (1993) reported that acifluorfen, lactofen, and 2,4-DB controlled horse purslane at least 78% in peanut. It was also shown that tank-mixing these herbicides with other broadleaf herbicides did not improve horse purslane control over aciflurofen, lactofen, or 2,4-DB applied alone.

Smellmelon is becoming more of a problem in south Texas peanut production fields and has become a problem in several crops along the Texas Gulf coast (author's personal observation). The range of smellmelon stretches from Georgia to the southern part of California and as for north as Arkansas (SWSS, 1999). Smellmelon can be a problem at peanut harvest as the melon can become broken apart when run through the combine and increase drying time because of the high moisture content of the melon itself (author's personal observation). In IMI-tolerant corn (Zea mays L.), Thompson et al. (2005) reported that imazapic at 0.07 and 0.14 kg ai/ha applied either preemergence (PRE), early POST, or late POST controlled smellmelon greater than 90%. Tingle and Chandler (2004) reported that smellmelon control was at least 93% with low-, medium-, and high-input herbicide systems. In cotton (Gossypium hirsutum L.), glyphosate systems have provided effective smellmelon control (Livingston et al., 2004; Livingston, 2006). Tingle et al. (2000) reported when smellmelon was allowed to compete with cotton for at least 6 weeks, yield was reduced 7% compared to the weed free but when smellmelon was allowed to compete for 10 to 12 weeks cotton yield was reduced 22 and 27%, respectively.

Palmer amaranth is not ranked as the number one weed or even as a principal weed in major crops, but it is a common weed in many major crops around the world (Grichar, 1997). The pigweeds (Amaranthus spp.) are listed as one of the 10 most common weeds in most all major peanut-growing states in the United States, with Palmer amaranth among the 10 most common weeds in Georgia, Missippi, and South Carolina (Webster, 2005). Palmer amaranth is listed as the most troublesome weed in peanut in Georgia and South Carolina (Webster, 2005). The current distribution of Palmer amaranth is 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). It is a severe problem in many fields in the southern part of the state when not properly controlled (Grichar, 1997).

Monoculture production systems and the repeated use of the same or similar herbicides have led to herbicide resistance in weeds (Culpepper et al., 2006; Peterson, 1999; VanGessel, 2001). Amaranthus species are very sensitive to ALS-inhibiting herbicides and possess characteristics that predispose them to have herbicide resistant biotypes such as high genetic variability, prolific seed production, and efficient pollen and seed distribution (Lovell et al., 1996). The use of soil-applied and POST herbicides with alternative sites of action is necessary to reduce the rate of development of herbicide-resistant weed populations (Shaner et al., 1997).

Although horse purslane and smellmelon are not a severe problem in all peanut growing areas of the state, more efficacy data is needed on herbicide systems which provide effective control. The objectives of this research were to evaluate several soil-applied herbicides alone and in combination with POST herbicides for horse purslane, Palmer amaranth, and smellmelon control and peanut yield response to control of these weed species.

Materials and Methods

Field studies were conducted at the Texas Agricultural Experiment Station research site near Yoakum, TX in 2003 through 2005 on a Tremona loamy fine sand (thermic aquic Arenic Paleustalfs) with less than 1% organic matter and pH of 7.0 to 7.2. The experimental design was a randomized complete block with three replications. Each plot consisted of two rows spaced 97 cm apart and 7.6 m long. All field plots were naturally infested with high populations of horse purslane (6 to 8 plants/m²), smellmelon (6 to 8 plants/m²), and Palmer amaranth (4 to 6 plants/m²). Sprinkler irrigation was applied on a 2- to 3-wk schedule throughout the growing season as needed. Tamrun 96 peanut were planted at a rate of 100 kg/ha and managed for optimum yield according to Texas Cooperative Extension guidelines (Lemon et al., 2001).

Herbicide treatments included pendimethalin at 1.12 kg ai/ha applied PPI, flumioxazin at 0.07 and 0.1 kg ai/ha applied PRE, pendimethalin applied PPI followed by flumioxazin at 0.1 kg/ha applied PRE, imazethapyr at 0.07 kg ai/ha applied PRE, pendimethalin applied PPI followed by imazethapyr applied PRE, pendimethalin plus diclosulam at 0.026 kg ai/ha applied PPI, S-metolachlor at 1.4 kg ai/ha applied PRE, pendimethalin applied PPI followed by S-metolachlor applied PRE, dimethenamid at 1.12 kg ai/ha applied PRE, pendimethalin applied PPI followed by dimethenamid applied PRE, pendimethalin applied PPI followed by POST applications of acifluorfen at 0.42 kg ai/ha, bentazon at 1.12 kg ai/ha, diclosulam at 0.026 kg ai/ha, imazapic at 0.07 kg ai/ha, imazethapyr at 0.07 kg ai/ha, 2,4-DB at 0.28 kg ai/ha, lactofen at 0.22 kg ai/ha, and a split application of imazapic with one-half (0.035 kg ai/ha) applied early POST plus a second application (0.035 kg/ha) applied three wks later (late POST).

Pendimethalin was incorporated immediately after application prior to peanut planting with a tractor-driven rotary tiller that had an incorporation depth of 6 cm. PRE herbicides were applied immediately after peanut planting. Rainfall or irrigation followed within 7–10 d to activate PRE herbicides. POST herbicides were applied three to four wks after planting when all weeds were 10 to 20 cm in height. Early POST applications of imazapic were made approximately three wks after planting when weeds were approximately 10 cm tall with the late POST application of imazapic approximately three wks later when weeds were 10 to 15 cm tall. An untreated check was included each year.

Acifluorfen, bentazon, diclosulam, lactofen, and 2,4-DB treatments included a crop oil concentrate¹ at 2.3 L/ha while imazapic and imazethapyr included a non-ionic surfactant² at 0.25% v/v. Herbicides were applied with a compressed-air backpack sprayer equipped with Teejet 11002 DG flat fan spray tips³ that delivered a spray volume of 190 L/ha at 180 kPa. Clethodim at 0.18 kg ai/ha 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 panicum (Panicum texanum Buckl.). Weed control was estimated visually on a scale of 0 to 100 (0 indicating no control and 100 indicating complete control), relative to the untreated check. Weed control ratings taken four wks after LPOST herbicide application are presented. Peanut yields were obtained by digging each plot separately, air-drying in the field for 4 to 7 d, and harvesting peanut pods from each plot with a combine. Weights were recorded after soil and foreign material were removed from plot samples.

Data were subjected to analysis of variance and means were separated using Fisher's protected LSD test at P = 0.05. Weed efficacy data were arcsine square-root transformed before analysis, but only the nontransformed data are reported because transformation did not affect data interpretation. There were no significant year by treatment interactions, so data are combined over years.

Results and Discussion

Palmer Amaranth Control

Pendimethalin provided less than 42% control of Palmer amaranth while flumioxazin at 0.07 and 0.1 kg/ha provided 72 and 85% control, respectively (Table 1). Imazethapyr applied PRE provided 100% control of Palmer amaranth while dimethenamid and S-metolachlor alone controlled 74 and 90%, respectively. The addition of pendimethalin applied PPI to dimethenamid, flumioxazin, imazethapyr, or S-metolachlor applied PRE did not improve control over the PRE herbicides used alone. However, the addition of a POST herbicide following pendimethalin applied PPI improved Palmer amaranth control over pendimethalin alone. In earlier work using only POST herbicides, imazapic at 0.04 to 0.07 kg/ha controlled Palmer amaranth at least 95% when applied EPOST while imazethapyr provided at least 90% control in 2 of the 3 yr (Grichar, 1997). The poor control in one year was attributed to taller Palmer amaranth at time of EPOST treatments. Acifluorfen at 0.42 and 0.56 kg ai/ha controlled Palmer amaranth at least 94% in 2 of 3 test years while lactofen controlled Palmer amaranth 99% in two years and 80% the other year (Grichar, 1997). Bentazon usually does not control pigweed species (Buchanan et al., 1982; Grichar, 1994; Wilcut et al., 1994). The partial control of Palmer amaranth with the POST application of bentazon may actually be the interference from smellmelon and horse purslane and the poor control of those weeds with bentazon (author's personal opinion).

Table 1

Horse purslane, Palmer amaranth, and smellmelon control and peanut response with soil-applied herbicides alone or in combination with POST herbicides.^a,b

Amaranthus spp. can be controlled with the dinitroaniline herbicides such as pendimethalin (Wilcut et al., 1994); however, metolachlor PPI or PRE controls pigweed less consistently than dinitroaniline herbicides (Wilcut, 1991; Wilcut et al., 1994). Dimethenamid is used in corn, soybean (Glycine max L.), grain sorghum [Sorghum bicolor (L.) Moench], and peanut (Anonymous, 1998) and controls several broadleaf weeds 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 Sendter) better than metolachlor or pendimethalin (Tonks et al., 1999).

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