Materials and Methods

Field experiments were conducted during 2009 and 2010 at the Southern Illinois University Belleville Research Center near Belleville, IL, on a Bethalto silt loam soil (Indorante and Leeper, 2000) with an organic matter content of 1.8% and a pH of 6.2. Wheat (Triticum aestivum) was drill-planted in late October each year before experiment initiation (2008 and 2009) at 1.35 million seeds/ha and harvested in early June the following spring. Within a week after wheat harvest, 0.87 kg acid equivalent/ha glyphosate (Roundup UltraMax; Monsanto, St. Louis, MO) was applied to control any existing weed vegetation.

The experiment was a randomized complete block design with six herbicide treatments and four replications. The PRE herbicide treatments evaluated were 1) s-metolachlor (1356 g⋅ha^–1 a.i.), 2) clomazone (350 g⋅ha^–1 a.i.) and ethalfluralin (1120 g⋅ha^–1 a.i.) premix, 3) s-metolachlor + clomazone and ethalfluralin premix, 4) s-metolachlor + halosulfuron (35 g⋅ha^–1 a.i.), 5) clomazone and ethalfluralin premix+ halosulfuron, and 6) none. Herbicide applications were made with a boom sprayer to cover the entire 1.8-m-wide and 12-m-long experimental unit area. The PRE herbicide applications were applied to the soil on 19 and 21 June for 2009 and 2010, respectively, with a CO₂-pressurized backpack sprayer using 8003 flat-fan spray tips (Teejet Spraying Systems, Wheaton, IL) at 275 kPa in a 187-L⋅ha^–1 water carrier. Within 3 d, ‘Magic Wand’ pumpkin transplants were planted in the soil. The seed of this pumpkin cultivar was obtained from Seedway (Elizabethtown, PA), germinated in standard 72-cell trays in a greenhouse, and hardened off in a cold frame for 3 to 4 d before transplanting. Pumpkin transplants were placed at 1.2-m in-row spacings in rows placed on 1.8-m centers with 10 plants per plot. Within 10 d after planting, 16 and 20 mm of rainfall occurred in 2009 and 2010, respectively, which provided PRE herbicide activation. More than adequate rainfall occurred for the growing season (from late June to mid October) each year, with 369 and 483 mm for 2009 and 2010, respectively. No irrigation was provided because most pumpkin growers in the Midwest do not use a supplemental water source.

Standard cultural practices for Midwest pumpkin production were followed (Egel et al., 2009, 2010). In mid April of each year, 25 kg⋅ha^–1 P and 140 kg⋅ha^–1 K were broadcast-applied to maintain sufficient levels of these nutrients in the soil based on soil tests; and, about 1 week after transplanting, 40, 18, and 34 kg⋅ha^–1 N, P, and K, respectively, were also broadcast. The source of N was NH₄NO₃, P was P₂O₅, and K was K₂O. Experimental plots were later side-dressed by hand with 56 kg⋅ha^–1 N at 5 weeks after transplanting, with the source of N from Ca(NO₃)₂. Pumpkin disease and insect management was achieved by spraying recommended rates of permethrin (Pounce 3.2 EC; FMC Corp., Philadelphia, PA) tank-mixed with chlorothalonil (Bravo; Zeneca, Inc., Wilmington, DE) or azoxystrobin (Quadris; Syngenta Crop Protection, Greensboro, NC) every 10 to 14 d starting at the three- to five-leaf stage until first harvest. When fruit set had begun, copper hydroxide (Kocide; E.I. du Pont de Nemours and Co., Wilmington, DE) was added into the tank mixture to provide control of bacterial diseases.

Both qualitative and quantitative data were collected. Pumpkin plant stunting injury was determined at 28 and 56 d after herbicide treatment (DAT) based on a rating from 0% (no stunting) to 100% (complete growth reduction), compared with no herbicide. Broadleaf and grass weed control was rated from 0% (no weed control) to 100% (complete weed control) at 28 and 56 DAT. Also at 56 DAT, broadleaf and grass weed densities were determined per square meter from two samples in each experimental unit. Weed species that were at the greatest population levels each year included tall waterhemp, giant foxtail, large crabgrass, and redroot pigweed, which are commonly observed weeds in many Midwest pumpkin fields. The total weight and average diameter of each marketable (>5 kg) orange pumpkin for each experimental unit were collected from the combination of two harvests at ≈10 Sept. and 10 Oct. each year.

An economic analysis was also conducted based on the EROI, defined as the gross return less the PRE herbicide treatment cost (Kammler et al., 2008; Krausz and Kapusta, 1998; Nolte and Young, 2002). Gross return of the herbicide treatment was based on two market prices of $0.44 and $0.88 per kg marketable pumpkin fruit (>5 kg) multiplied by the yield benefit of a particular treatment. The cost of PRE herbicide treatment was based on a 2019 nondiscounted price of herbicides from Southern FS (Cobden, IL) (Table 1). The costs of PRE herbicides alone or in combination based on the application rates used were clomazone and ethalfluralin premix = $222/ha, halosulfuron alone = $71.05/ha, metolachlor alone = $48.77/ha, metolachlor + halosulfuron = $119.82/ha, clomazone and ethalfluralin premix + halosulfuron = $293.05/ham and clomazone and ethalfluralin premix + metolachlor = $270.77/ha. The following equation was used to determine EROI for each replication: EROI = [(Treated yield) – (Nontreated yield)] × ($ Obtained/kg pumpkin) – Cost of PRE herbicide treatment.

Table 1.

Preemergence herbicides and rates used, as well as their cost of application for no-tillage pumpkin herbicide study.

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Data were subjected to analysis of variance (ANOVA) procedures using the GLM procedure of SAS (version 9.4; SAS Institute, Cary, NC) to determine effects of the PRE herbicide treatments on broadleaf and grass weed control, pumpkin stunting injury and yield parameters, and economic return on herbicide investment based on pumpkin yields. Pumpkin injury (percent stunting) and weed control data were arcsine-transformed before ANOVA. Nontransformed means are presented because arsine transformation did not alter the interpretation of the data. Fisher’s protected least significant difference test was used to separate PRE herbicide treatment differences at P < 0.05.

Results

Because interactions were observed between PRE herbicide treatments and growing seasons (P > 0.05) for the dependent variables evaluated (except for broadleaf weed control at 28 DAT), most results are presented by each growing season.

Weed control.

Broadleaf and grass weed control at 28 DAT differed (P < 0.05) among PRE herbicide treatments (Table 2). Although some PRE treatments differed between years (P < 0.05) for grass control (Digitaria sanguinalis and Setaria faberi), s-metolachlor + clomazone and ethalfluralin provided consistent control of these grass weeds over both years. In addition, several PRE treatments evaluated provided control of the broadleaf weeds (Amaranthus retroflexus and Amaranthus tuberculatus) by 28 DAT, including s-metolachlor, s-metolachlor + clomazone and ethalfluralin, and s-metolachlor + halosulfuron. Overall, these results indicate that the PRE herbicide combinations of s-metolachlor + clomazone and ethalfluralin provided the greatest control of broadleaf and grass weeds that emerged in this study for the first month after planting. However, s-metolachlor alone and s-metolachlor + halosulfuron provided similar broadleaf weed control as s-metolachlor + clomazone and ethalfluralin. This indicates that the addition of s-metolachlor to these tank mix combinations provides some additional grass weed control. Without herbicides, straw mulch that covered about 70% to 80% of the soil surface only provided 49% broadleaf control at 28 DAT, and grass control of 8% and 17% for the 2009 and 2010 growing seasons, respectively.

Table 2.

Influence of preemergence herbicide treatment on broadleaf and grass weed control, and pumpkin plant injury at 28 d after treatment (DAT) in 2009 and 2010.

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Weed control also differed (P < 0.05) among the PRE herbicide treatments at 56 DAT. The best option for full-season broadleaf and grass weed control was again s-metolachlor + clomazone and ethalfluralin (Tables 3 and 4). Although this PRE herbicide combination provided better grass control compared with most other treatments, grass control was inadequate by 56 DAT (70% to 73% over the two growing seasons). The clomazone and ethalfluralin combination provided consistently less broadleaf control than those PRE herbicide combinations with s-metolachlor. By 56 DAT, the no-herbicide control in the NT production system (with straw mulch on the surface) had provided only 8% to 22% broadleaf weed control and 0% to 7% grass weed control over the two growing seasons.

Table 3.

Influence of preemergence herbicide treatment on broadleaf and grass weed control, resulting densities, and pumpkin plant injury at 56 d after treatment (DAT) in 2009.

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Table 4.

Influence of preemergence herbicide treatment on broadleaf and grass weed control, resulting densities, and pumpkin plant injury at 56 d after treatment (DAT) in 2010.

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Weed density.

Broadleaf and grass weed densities per square meter indicated the importance of PRE herbicides in NT pumpkin production (Tables 3 and 4). Similar to weed control results, those PRE herbicide treatments containing s-metolachlor had the lowest broadleaf weed densities by 56 DAT over the two growing seasons. In addition, the clomazone and ethalfluralin + halosulfuron treatment also reduced broadleaf weed densities at 56 DAT, because this PRE herbicide combination provided about two and three times less weed densities in 2009 and 2010, respectively, compared with no herbicide.

Most PRE herbicide treatments over the two growing seasons had similar grass weed densities, but all were less than those provided by the no-herbicide control (Tables 3 and 4). These results indicate that all PRE herbicide treatments evaluated provided some level of grass control in NT pumpkin production. Thus, PRE herbicides can definitely have an influence on reducing both small-seeded broadleaf and grass densities in NT pumpkin production by the end of the growing season.

Pumpkin plant injury.

Pumpkin plant stunting differed among PRE herbicide treatments at both 28 and 56 DAT (Tables 2–4), and was most related to weed competition, because no growth reduction or phytotoxicity was observed as a result of the PRE herbicides used. At 28 DAT, all PRE herbicide treatments provided 20% or less plant stunting as a result of low weed competition, with only the no-herbicide treatment showing significant amounts of stunting (Table 2). However, by 56 DAT, the s-metolachlor + clomazone and ethalfluralin combination resulted in the least amount of pumpkin plant stunting compared with all other treatments over both growing seasons (20% to 27%), which was directly related to the greater amount of broadleaf (88% to 90%) and grass (70% to 73%) weed control that was provided by this treatment in both growing seasons (Tables 3 and 4). In comparison, the s-metolachlor alone and s-metolachlor + halosulfuron PRE treatments both resulted in 47% or greater pumpkin plant growth reduction by 56 DAT, whereas the clomazone and ethalfluralin, and clomazone and ethalfluralin + halosulfuron combinations had 70% or more growth reduction resulting from a lack of broadleaf and/or grass weed control.

Pumpkin fruit size and diameter.

Pumpkin fruit size and diameter differed among the PRE herbicide treatments (P < 0.05) (Table 5). The combination of s-metolachlor + clomazone and ethalfluralin, as well as s-metolachlor alone, consistently provided the largest pumpkin fruit size and diameter over both years. Although the no herbicide consistently provided the smallest pumpkin fruit size and diameter over both years, the two treatment combinations with halosulfuron were generally similar to the no-herbicide treatment.

Table 5.

Effect of preemergence herbicide applications on ‘Magic Wand’ pumpkin fruit size and diameter, and yield using no-tillage production practices during 2009 and 2010.

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Pumpkin yield.

Pumpkin fruit yields differed (P < 0.05) among PRE herbicide treatments (Table 5). Results indicated that s-metolachlor + clomazone and ethalfluralin provided the greatest yields over both growing seasons, although this treatment was closely followed by s-metolachlor alone, especially during 2009. Compared with the high yields provided by the s-metolachlor + clomazone and ethalfluralin treatment, clomazone and ethalfluralin resulted in a 40% and 39% yield reduction during 2009 and 2010, respectively, whereas both PRE treatments containing halosulfuron had more than 42% and 63% yield reductions for 2009 and 2010, respectively. Moreover, the no-herbicide treatment provided 76% and 95% yield reductions for 2009 and 2010, respectively, compared with the higher yielding PRE herbicide combination of s-metolachlor + clomazone and ethalfluralin.

PRE herbicide EROI.

The PRE herbicide EROI indicated similar trends as the yield data. The PRE herbicide treatment that consistently provided the highest EROIs for the two pumpkin price points over the two growing seasons was s-metolachlor + clomazone and ethalfluralin, with s-metolachlor alone providing slightly lower EROIs (Table 6). Although clomazone and ethalfluralin provided similar EROIs at both price points in 2009 to clomazone and ethalfluralin + halosulfuron, the clomazone and ethalfluralin treatment was higher than clomazone and ethalfluralin + halosulfuron in 2010, and was similar to the two pumpkin price point EROIs provided by s-metolachlor alone during this year. In all years and at both price points, the lowest EROI was obtained in the s-metolachlor + halosulfuron treatment.

Table 6.

Effect of preemergence herbicide applications on economic return on investment (EROI) for pumpkins grown in no-tillage at two commodity price points during 2009 and 2010.

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Discussion

The use of appropriate weed management practices are essential to obtain the highest possible pumpkin yields in an NT production system (O’Rourke and Petersen, 2016; Rapp et al., 2004; Walters, 2011; Walters et al., 2008). Pumpkin productivity in NT production systems correlates highly with full-season weed control (Walters and Young, 2012; Walters et al., 2008). Although pumpkin is a cucurbit crop well suited for NT culture, acceptable weed control is often difficult to achieve, and inadequate weed management will generally result in significant yield and monetary losses. The use of PRE herbicides in NT pumpkin systems is critical because the 30- to 40-d growth period following pumpkin seeding or transplanting is most important to minimize weed growth and development, because resulting vine growth over the soil surface after this time will provide some shading and weed suppression (Walters and Young, 2012). However, weed management systems are necessary to maximize yield and fruit size in NT pumpkin production, with PRE herbicide selection playing a critical role. In addition, the use of effective PRE herbicides in combination with cover crop or other crop residues integrated into NT pumpkin planting systems are also another important consideration for growers to enhance weed control (Rapp et al., 2004; Walters, 2011; Walters and Young, 2010; Walters et al., 2008). Forcella et al. (2015) also indicated that pumpkins grown in cold temperate regions have a moderate potential for herbicide-free production in rolled-crimped cereal rye mulch, although some yield reductions will most likely occur in those not augmented with PRE herbicides. Alternative tillage systems, such as NT, along with the integration of cover crops is an effective method to prevent soil erosion and can be used to make crop production truly sustainable (Jelonkiewicz and Borowy, 2009). Although cover crops can play a role in providing some weed control, the addition of PRE herbicides is still critical to maximize yields in NT pumpkin production.

Effective weed management systems are critical in NT pumpkin production to produce the greatest yields with highest revenue potential. In our study, appropriate PRE herbicide investments were shown to provide significant yield improvements and increased EROIs in NT pumpkin production systems (Tables 5 and 6). Pumpkin yields and EROIs were significantly improved with specific PRE herbicides and/or mixtures. Research has generally indicated that tank mixtures of various herbicides are necessary in general to maximize weed control in pumpkin production (Brown and Masiunas, 2002; Kammler et al., 2008). Our results over both growing seasons indicated that s-metolachlor applied in combination with clomazone and ethalfluralin generally provided an increased EROI compared with other PRE treatments, which included s-metolachlor alone and in combination with halosulfuron, as well as clomazone and ethalfluralin alone and in combination with halosulfuron. Furthermore, similar to results obtained by Kammler et al. (2008), our study indicated that the treatments with the highest pumpkin yields provided the greatest EROIs regardless of herbicide costs. Although some small-scale pumpkin growers in the Midwest still show reluctance to use PRE herbicides as a result of concerns of crop injury or lack of experience/knowledge regarding product application and/or use (Nathan Johanning, personal communication, University of Illinois Commercial Agriculture Extension), our results show the advantages of using these herbicides to obtain effective weed management with high EROI.

Although multiple studies have evaluated PRE herbicide effectiveness in NT pumpkin production (Harrelson et al., 2007; Rapp et al., 2004; Walters and Young, 2010; Walters et al., 2008), growers must first consider the specific weed species composition that is the most troublesome in their fields before choosing the correct PRE herbicides and/or mixtures to use when trying to maximize pumpkin yields and revenues. However, the addition of s-metolachlor as a PRE herbicide for pumpkin production has improved weed management in this crop. In previous NT pumpkin PRE herbicide studies, s-metolachlor was not included, and PRE herbicide experiments focused on clomazone and ethalfluralin as the standard PRE herbicide, which was often combined with halosulfuron (Walters and Young, 2010; Walters et al., 2008). These studies generally indicated that pumpkin fruit sizes and yields were the greatest with the PRE application of clomazone and ethalfluralin + halosulfuron. However, in our study, the herbicide mixture of s-metolachlor with clomazone and ethalfluralin, or s-metolachlor alone, was the most consistent herbicide combination over years to provide the greatest weed control, fruit yields, and EROI under the weed populations evaluated. The other herbicide mixtures evaluated generally resulted in less weed control, low fruit yields, and/or low EROI, which included s-metolachlor + halosulfuron, clomazone and ethalfluralin, and clomazone and ethalfluralin + halosulfuron. Our research indicates that the addition of s-metolachlor to clomazone and ethalfluralin is the key to greater PRE broadleaf weed control (A. retroflexus and Amaranthus rudis) in NT pumpkin production systems compared with combinations of clomazone and ethalfluralin with halosulfuron, which ultimately resulted in higher yields.

The cost of PRE herbicide applications in NT pumpkin production is an important investment consideration, because costs vary among the available labeled products (Table 1). However, the correct PRE herbicide(s) used for control of the weed species occurring in NT pumpkin fields can substantially increase revenues. The small investments of PRE herbicides and mixtures were shown to provide significant monetary returns in NT pumpkin production systems. In general, those growers who are reluctant to use PRE herbicides often become more willing to include them in a weed management program after observing the weed control, improved crop yield, and labor and time savings they provide (N. Johanning, personal communication). Thus, the judicious use of PRE herbicides is an important part of any effective weed management program for NT pumpkins (Walters, 2019), and will provide significant monetary savings, resulting in increased revenues.

Weeds in alternative tillage production systems must be managed to provide high yields and profitability, and effective use of PRE herbicides are essential to promote sustainability. The greatest economic return on herbicide investment was provided by the tank mix of s-metolachlor with clomazone and ethalfluralin. Although growers often look for ways to reduce input costs in NT pumpkin production systems, PRE herbicides are clearly an important investment that improves pumpkin yields and revenues.