Introduction
The challenge of weed management in grain sorghum has continued to increase in recent years with the occurrence of herbicide-resistant weed populations (Thompson et al. Reference Thompson, Dille, Peterson, Ciampitti and Prasad2017). Pigweed species have been confirmed resistant to six herbicide sites of action in Kansas (Heap 2019). Yield reductions as high as 57% with 1.6 Palmer amaranth (Amaranthus palmeri) plants m−2 were observed when weeds were transplanted into grain sorghum at developmental stage 2 (Moore et al. Reference Moore, Murray and Westerman2004).
Best management practices indicate that grain sorghum should be planted by early June in Kansas (Ciampitti et al. Reference Ciampitti, Ruiz Diaz, Jardine, Peterson, Hay, Whitworth and Rogers2019). Although planting at this timing can maximize grain yield, it also synchronizes emerging sorghum with the emergence of Palmer amaranth and waterhemp [A. tuberculatus (Moq.) J. D. Sauer], because soil surface (2.5-cm depth) temperatures can often approach and exceed 25 C during the optimal grain sorghum planting time (Guo and Al-Khatib Reference Guo and Al-Khatib2003; Hartzler et al. Reference Hartzler, Buhler and Stoltenberg1999; Jha and Norsworthy Reference Jha and Norsworthy2009). Synchronous emergence of grain sorghum with pigweed may be more influential than pigweed density in determining grain yield loss (Knezevic et al. Reference Knezevic, Horak and Vanderlip1997) and may place grain sorghum at a competitive disadvantage in contrast to other crops.
Grain sorghum producers have few effective herbicide options for controlling pigweed (Hennigh et al. Reference Hennigh, Al-Khatib and Tunistra2010). A systems approach of integrated weed management must be adopted when addressing tough-to-control weeds (e.g., Palmer amaranth and waterhemp) (Owen Reference Owen2016; Thompson et al. Reference Thompson, Dille, Peterson, Ciampitti and Prasad2017).
Because of herbicide resistance and a limited number of registered active ingredients, cultural weed management practices such as narrow row widths (NRWs) must be considered. NRWs generally result in faster canopy closure and increased evapotranspiration efficiency (Steiner Reference Steiner1986). Staggenborg et al. (Reference Staggenborg, Fjell, Devlin, Gordon and Marsh1999) reported grain sorghum yields were increased 10% with NRWs compared with 76-cm row widths in favorable growing conditions. Best management recommendations for Kansas indicate that narrow row spacing should be selected over wide row spacing to increase yield (Ciampitti et al. Reference Ciampitti, Ruiz Diaz, Jardine, Peterson, Hay, Whitworth and Rogers2019), which aligns with integrated weed management strategies to increase crop competitiveness with weeds (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). The use of 38-cm and 19-cm row widths in grain sorghum increases control of Palmer amaranth, tumble pigweed (A. albus L.), redroot pigweed (A. retroflexus L.), large crabgrass [Digitaria sanguinalis (L.) Scop.], and sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby] when compared with wide row widths (≥76 cm) (Besancon et al. Reference Besancon, Heiniger, Weisz and Everman2017; Grichar et al. Reference Grichar, Besler and Brewer2004; Wiese et al. Reference Wiese, Collier, Clark and Havelka1964).
The use of row-crop cultivation has been a long-standing, effective tool for weed management in grain sorghum (Wiese et al. Reference Wiese, Collier, Clark and Havelka1964); however, yield losses due to reduced soil moisture conservation and root pruning have been reported (Dickey et al. Reference Dickey, Jasa and Grisso2013). The use of row-crop cultivation can also increase the potential for soil erosion and must be weighed against integrated weed management benefits (Bates et al. Reference Bates, Gallagher, Curran and Harper2012).
Cover crops (CCs) have provided economic benefit when used as part of a rotational system with grain sorghum (Mahama et al. Reference Mahama, Prasad, Roozeboom, Nippert and Rice2016; Reinbott et al. Reference Reinbott, Conley and Blevins2004). Although CCs have suppressed Palmer amaranth and waterhemp (Cornelius and Bradley Reference Cornelius and Bradley2017; DeVore et al. Reference DeVore, Norsworthy and Brye2013; Loux et al. Reference Loux, Dobbels, Bradley, Johnson, Young, Spaunhorst, Norsworthy, Palhano and Steckel2017;), little to no research has investigated the role of CCs as an integrated pigweed management tool in grain sorghum. Maintaining adequate residue cover in a no-till dryland system can aid weed suppression in grain sorghum (Anderson Reference Anderson2000; Dhuyvetter et al. Reference Dhuyvetter, Thompson, Norwood and Halvorson1996; Thompson et al. Reference Thompson, Brown, O’Brien, Sartwelle and Schelegel1998). An extension of this is to maintain ground cover with winter wheat residue in a double-crop grain sorghum production system (Crabtree et al. Reference Crabtree, Prater and Mbolda1990).
Winter wheat as a CC offers similar suppression of Palmer amaranth and waterhemp in cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] when compared with cereal rye (Secale cereale L.) (Wiggins et al. Reference Wiggins, Hayes and Steckel2016). When planting a summer annual grass crop such as grain sorghum, a leguminous CC could be selected to avoid challenges of crop establishment and nitrogen immobilization (Mahama et al. Reference Mahama, Prasad, Roozeboom, Nippert and Rice2016). However, leguminous CCs seldom produce the biomass and ground cover necessary to adequately suppress Palmer amaranth and waterhemp (Cornelius and Bradley Reference Cornelius and Bradley2017; Wiggins et al. Reference Wiggins, McClure, Hayes and Steckel2015). Research has indicated that with appropriate agronomic practices (e.g., adequate nitrogen fertilization), establishment of summer annual grass crops such as corn (Zea mays L.) after cereal rye does not consistently reduce grain yields (Appelgate et al. Reference Appelgate, Lenssen, Wiedenhoeft and Kaspar2017; Duiker and Curran Reference Duiker and Curran2005).
The repeated use of any one of these weed management tactics (herbicide, NRW, row-crop cultivation, or CCs) will eventually select for tactic-resistant biotypes (Shaner Reference Shaner2014). Thus, a combination of these practices in pursuit of an integrated weed management plan should be implemented as a system. In developing strategies, all farmers can be placed on a continuum ranging from the mindset of weed control (i.e., simplistic, short-term focus) to weed management (i.e., consideration of environmental, economic, and cultural aspects) to cropping systems–based decisions (i.e., complex, integrated decisions across many years), which is truly an integrated, sustainable approach (Cardina et al. Reference Cardina, Webster, Herms, Regnier and Buhler1999). By incorporating weed management decisions at the cropping systems level, the selection for resistance to a given practice will be delayed and create a more sustainable system overall (Gallandt et al. Reference Gallandt, Liebman, Huggins and Buhler1999). Unfortunately, limited research has investigated how to incorporate many of these best management practices into a dryland grain sorghum cropping system. The objective for this research was to assess the influence of a winter wheat CC, row-crop cultivation, three row widths, and herbicide program, each alone and in combination, on pigweed height, density, and biomass in dryland (limited rainfall, nonirrigated) grain sorghum in Kansas.
Materials and Methods
Field experiments were conducted in Riley (39.12567°N, 96.613488°W), Reno (37.931114°N, 98.029392°W), and Franklin (38.539265°N, 95.244301°W) counties on Kansas State University Department of Agronomy Experiment Fields during 2017 and 2018 for a total of 6 site-years. Riley and Reno counties contained an indigenous population of Palmer amaranth, whereas Franklin County contained an indigenous population of waterhemp. Sixteen treatments were established using a one-way treatment structure consisting of combinations of four components: the absence or presence of a herbicide program, three row widths (76, 38, and 19 cm), CCs, and row-crop cultivation of only the 76-cm row width. Cultivation of 19- and 38-cm row widths is not practical. The 76-cm row width with no cover crop (NCC) treatment is referred to as standard management (SM) in this article.
Winter Wheat CC Component Establishment and Termination
‘Gallagher’ winter wheat was no-till planted at 134 kg ha−1 in 19-cm row widths at all locations (Table 1). At spring green-up (i.e., Feekes 4), the CC was topdressed with 56 kg ha−1 urea fertilizer (46% nitrogen). The CC was terminated with 1,065 g ha−1 glyphosate (Roundup PowerMAX®; Monsanto Co., St. Louis, MO) at Feekes 10.5.1 “anthesis” (Table 1). Aboveground biomass of CC was harvested from one representative 0.25-m2 area in each replication per site-year at grain sorghum planting, dried, and weighed (Table 2).
Table 1. Winter wheat cover crop planting and termination dates, grain sorghum planting dates, herbicide application and row-crop application dates, and site characteristics for each site-year.
a Abbreviations: mEq, milliequivalents.
b All soil characteristics assessed from a 0- to 7.6-cm soil sampling depth.
c Fine-silty, mixed superactive, mesic Pachic Argiudolls.
d Fine-loamy, mixed, superactive, mesic Udic Argiustolls.
e Fine, smectic, thermic Abruptic Argiaquolls.
f Fine, smectitic, mesic Aquertic Argiudolls.
g Loss on ignition (Ball Reference Ball1964).
h Adjusted to pH 7 (Rich Reference Rich1969).
Table 2. Winter wheat cover crop aboveground dry biomass at grain sorghum planting and soil nitrogen concentration at grain sorghum planting and at grain sorghum maturity.a
a Soil sampled from 61-cm soil cores from CC and NCC plots.
b Abbreviations: CC, winter wheat cover crop; NCC, no winter wheat cover crop.
c PPM represents total concentration of nitrate plus ammoniacal nitrogen.
Grain Sorghum Establishment
Experiments were established in a randomized complete block design. Plots at all sites were 3-m wide and 9-m long, with four replications per site. Immediately before planting the grain sorghum, the entire experimental area received an application of 841 g ha−1 paraquat (Gramoxone® SL 2.0; Syngenta Crop Protection, LLC., Greensboro, NC) to control all emerged pigweed and a broadcast application of 112 kg ha−1 urea. Grain sorghum hybrid ‘7715’ (Sorghum Partners®, New Deal, TX) was no-till planted at 148,200 seeds ha−1 using a no-till drill (Model 1590; Deere and Co., Moline, IL) at all locations. The same seeding rate was used across all row widths, with drill slots being closed to accommodate the various row widths. Key operation dates and site characteristics are listed in Table 1. At grain sorghum planting and maturity, soil cores were collected from a 61-cm depth to assess nitrogen content in CC and NCC plots (Table 2). Daily rainfall events were recorded at weather stations located no more than 2.5 km from each site (Table 3). All locations received a broadcast application of 75 g ha−1 chlorantraniliprole (Prevathon®; E.I. du Pont e Nemours and Co., Wilmington, DE) during grain sorghum fill to control unwanted insects.
Table 3. Precipitation for each site-year during cover crop and grain sorghum growth and development.
a Thirty-year normals referenced from 1980 to 2010 for each location as recorded by the National Oceanic and Atmospheric Administration (Arguez et al. Reference Arguez, Durre, Applequist, Squires, Vose, Yin and Bilotta2010).
b Precipitation values reflect moisture that occurred during the growth and development of winter wheat cover crop.
c Abbreviation: WAP, weeks after sorghum planting.
d Values calculated from 30-yr normal precipitation from the planting date for each site-year through 8 WAP.
Herbicide Program and Row-Crop Cultivation Components
The herbicide program component consisted of the absence or presence of preplant, PRE, and 3 weeks after planting (WAP) POST applications to facilitate overlapping residual herbicides. The split preplant and PRE application consisted of a premix of 1,884 g ha−1 S-metolachlor, 707 g ha−1 atrazine, and 188 g ha−1 mesotrione (Lumax® EZ; Syngenta Crop Protection, LLC.); two-thirds of the total herbicide was applied 2 weeks before planting and the remainder applied immediately after planting. The POST application consisted of a tank mix of 43 g ha−1 pyrasulfotole and 245 g ha−1 bromoxynil (Huskie®; Bayer Crop Science, Research Triangle Park, NC), 280 g ha−1 dicamba (Clarity®; BASF Corp., Research Triangle Park, NC), 2,800 g ha−1 acetochlor and 1,389 g ha−1 atrazine (Degree Xtra®; Monsanto Co.), 2.5% vol/vol urea ammonium nitrate, plus 0.25% vol/vol nonionic surfactant (Activate Plus™, Winfield United, Bloomberg, MN).
The herbicide applications were made using a four-nozzle CO2-pressurized backpack sprayer calibrated to deliver 144 L ha−1 at 241 kPa using Turbo TeeJet® Induction 110015 nozzles (TeeJet Technologies, Springfield, IL) for the preplant and PRE applications and Air Induction Extended Range 110015 nozzles for the POST application. A three-shank row-crop cultivator (Buffalo Model 6200; Bison Industries Inc., Norfolk, NE) with 46-cm–wide sweeps was operated 5-cm deep at 6.4 km hr−1 approximately 2.5 WAP for the row-crop cultivation component.
Pigweed Height, Density, and Biomass Data Collection and Analysis
The average height of 10 plants per plot as well as density were recorded, and biomass harvested from representative 0.25-m2 areas between the center rows in each plot at 3 and 8 WAP. Biomass was oven dried at 65 C for 10 d and weighed. Data were analyzed using the Mixed Procedure in JMP Pro 14 (SAS Institute, Cary, NC), and means were separated using Fisher Protected LSD at α = 0.05. Pigweed height, density, and biomass data were assessed for basic assumptions of ANOVA. To meet these assumptions of ANOVA, Franklin County waterhemp density data collected 3 and 8 WAP in 2017 and 8 WAP in 2018 required natural log transformation, whereas waterhemp biomass data at 3 WAP in 2017 from Franklin County required square root transformation. All values were back transformed for discussion. When no site-by-year-by-treatment interactions were detected, site-year was considered a random effect with replication nested within the site-year (within a species). Contrasts of a single degree of freedom were applied to compare groups of treatments that excluded treatments with row-crop cultivation to assess the effects of NRW (i.e., 38 cm and 19 cm) and CC.
Results and Discussion
All treatments that included the herbicide program component resulted in excellent pigweed control (>97%; data not shown); therefore, data not containing this component were extracted and analyzed separately. No interactions were detected for Palmer amaranth height, density, or biomass for effects of site-year by treatment for Riley County in 2017 and 2018 and Reno County in 2017; therefore, data were pooled. Data for Reno County plots in 2018 were analyzed separately because there was a significant year-by-treatment interaction with all other site-years containing Palmer amaranth. Franklin County waterhemp data were analyzed separately for each site-year because significant year-by-treatment interactions were detected.
Palmer Amaranth Density and Biomass Across 3 Site-Years (Riley and Reno Counties)
Palmer amaranth densities were similar (550 to 710 plants m−2) across all row widths with NCC at 3 WAP (Table 4). When a CC was added to the 76-cm row width, a 55% reduction in Palmer amaranth density was observed. Treatments with row-crop cultivation reduced density by 97% compared with SM. Row-crop cultivation and CC were effective components to integrate in terms of reducing the selection pressure on Palmer amaranth with POST herbicide applications; this demonstrated the importance of considering cultural and mechanical tactics when developing pigweed management strategies (Buhler Reference Buhler2002; Loux et al. Reference Loux, Dobbels, Bradley, Johnson, Young, Spaunhorst, Norsworthy, Palhano and Steckel2017). When NRW was combined with CC, density was reduced compared with SM. Contrasts revealed that differences in Palmer amaranth density could not be attributed to NRW at 3 WAP. Treatments containing CC resulted in a 50% reduction in Palmer amaranth density across all row widths (Table 4). It is important to note that the control offered by CC was achieved with a dual-purpose winter wheat, which is commonly used for forage (i.e., biomass producing) qualities; it is unclear if results would have differed if a single-purpose wheat cultivar had been selected.
Table 4. Influence of grain sorghum row width, winter wheat cover crop, and row-crop cultivation on Palmer amaranth density and biomass averaged across site-years in Riley County during 2017 and 2018 and Reno County during 2017 in the absence of the herbicide program component.
a Abbreviations: –, no CC or RC was present in the treatment; CC, winter wheat cover crop; NCC, no cover crop; NS, not significant; RC, row-crop cultivation; WAP, weeks after planting.
b Means followed by the same letter within a column are not statistically different according to Fisher Protected LSD (α = 0.05).
c Height data for means and contrasts were NS and are not shown.
d The76-cm row width without CC or RC is described as standard management in the text.
e All contrasts were conducted in the absence of RC-containing treatments.
f P = 0.05 to 0.01.
g P = 0.01 to 0.0001.
h P = 0.1 to 0.05.
i P < 0.0001.
Biomass was reduced only with the 19-cm row width compared with the 76- and 38-cm row widths and NCC treatments at 3 WAP (Table 4). Compared with SM, combining CC with row-crop cultivation reduced biomass by 97%, whereas row-crop cultivation alone reduced weed biomass by 83%. The combination of NRW plus CC reduced Palmer amaranth biomass compared with SM, but NRW alone did not. When pooled across all combinations, contrasts found that NRW reduced biomass compared with the 76-cm row width. The use of a CC resulted in a 52% biomass reduction compared with NCC across all row widths by 3 WAP in these 3 site-years (Table 4).
Palmer amaranth densities were similar by 8 WAP in all NRW plus NCC and all 76-cm row width treatments (excluding row-crop cultivation-containing treatments) (Table 4). This indicated that CC or NRW as stand-alone tactics did not reduce late-season densities. Densities after row-crop cultivation or NRW plus CC treatments (≤208 plants m−2) were reduced from SM (431 plants m−2) by 8 WAP. That each component on its own was not enough to provide suppression illustrated the importance of applying a systems approach that uses cultural and mechanical strategies in addition to an herbicide program (Beckie Reference Beckie2006; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012; Owen Reference Owen2016; Owen et al. Reference Owen, Beckie, Leeson, Norsworthy and Steckel2014). Generally, row-crop cultivation has been associated with increasing late-season weed emergence because of soil disturbance (Forcella and Lindstrom Reference Forcella and Lindstrom1988). In some environments, as in this study, increased emergence did not occur; therefore, timely row-crop cultivation could be used to control early-season weeds without causing additional emergence, especially in dryland cropping systems. Contrasts indicated that no reduction in density could be attributed to NRW, but the addition of CC across all row widths reduced density by 50% (Table 4).
Palmer amaranth biomass was reduced with the CC plus 76-cm row width or 38-cm NCC treatments compared with SM by 8 WAP. Both 19-cm row width treatments (CC or NCC) produced a similar level of biomass to SM (Table 4). Treatments including CC or row-crop cultivation reduced Palmer amaranth biomass compared with SM. Contrasts revealed that NRW provided 27% and 46% reductions in late-season biomass with 38- and 19-cm row widths, respectively, compared with the 76-cm row width, and the addition of CC provided a 37% reduction in late-season biomass compared with NCC treatments across all row widths (Table 4). Butts et al. (Reference Butts, Norsworthy, Kruger, Sandell, Young, Steckel, Loux, Bradley, Conley, Stoltenberg, Arriaga and Davis2016) found similar results in soybean in terms of a benefit from NRW for Palmer amaranth late-season biomass suppression.
Palmer Amaranth Height, Density, and Biomass in Reno County, 2018
Palmer amaranth was not present at 3 WAP because of the lack of moisture to stimulate emergence (Table 3), but all treatments were still implemented per protocol (i.e., row-crop cultivation at 2.5 WAP). Palmer amaranth height was only reduced in the 19-cm row width plus CC and row-crop cultivation plus CC treatments, compared with SM, by 8 WAP (Table 5). Contrasts indicated that NRW had no effect on height at 8 WAP; however, CC treatment reduced height compared with NCC over all row widths.
Table 5. Influence of grain sorghum row width, winter wheat cover crop, and row-crop cultivation on Palmer amaranth height, density, and biomass at 8 WAP in Reno County in 2018 in the absence of the herbicide program component.
a Abbreviations: –, no CC or RC was present in the treatment; CC, winter wheat cover crop; NCC, no cover crop; NS, not significant; RC, row-crop cultivation; WAP, weeks after planting.
b 3 WAP data are not included as Palmer amaranth was not emerged.
c Means followed by the same letter within a column are not statistically different according to Fisher Protected LSD (α = 0.05).
d 76-cm row width without CC or RC is described as standard management in the text.
e All contrasts were conducted in the absence of RC-containing treatments.
f P = 0.01 to 0.0001.
g P = 0.05 to 0.01.
h P = 0.1 to 0.05.
Palmer amaranth densities were greater in the 76-cm row width plus CC and row-crop cultivation plus CC treatments than the SM 8 WAP (Table 5). Typically, CC is expected to decrease weed density; it is possible that the density increase for CC-containing treatments was caused by moisture retained at the soil surface, along with other abiotic conditions occurring in this site-year (Wells et al. Reference Wells, Reberg-Horton and Mirsky2014). The CC at Reno County in 2018 also could have been disadvantaged because of dry conditions; 94 mm of precipitation fell during CC growth as compared with the 179 mm normally received during this time (Table 3). Subsequently, less CC aboveground biomass was produced at grain sorghum planting (2,580 kg ha−1) compared with all other site-years (Table 2). The condition of the CC was also likely further degraded by the 134 mm of precipitation that occurred during 5 to 6 WAP (Table 3). The row-crop cultivation treatment did not change Palmer amaranth density compared with SM, which corresponded to observations in other site-years, indicating the soil disturbance from row-crop cultivation did not stimulate more weed emergence. Densities in NRW or CC plus NRW treatments did not differ from SM. Although the 38-cm row width and 19-cm row width plus CC treatments did not differ from SM, the 19-cm row width with NCC treatment reduced density, compared with the 19-cm row width plus CC treatment. This indicated the addition of CC to this row width was counterproductive. Contrasts indicated the 38- and 19-cm row widths reduced weed density by 47% and 68%, respectively, compared with the 76-cm row width. When CC and NCC treatments across all row widths were compared, use of a CC resulted in a 170% increase in weed density compared with the NCC treatments. Thus, the use of a CC may not consistently result in lower weed densities (Table 5).
Palmer amaranth biomass was less in the CC plus 19-cm row width treatment and in response to row-crop cultivation compared with SM 8 at WAP in Reno County in 2018 (Table 5). Similar levels of biomass were observed for the row-crop cultivation and 19-cm row width plus CC treatments, whereas all other treatments did not differ from SM. Contrasts revealed no differences in biomass between the 76- and 38-cm row widths, whereas the 19-cm row width provided greater suppression compared with the 76- and 38-cm row widths. CC also provided a 40% reduction in late-season biomass compared with the NCC treatments across all row widths. This could be due to increased crop-weed competitiveness, which has been associated with the use of CCs (Teasdale Reference Teasdale1996). The weed biomass data conflict with the density data in this site-year: CC reduced biomass but increased density, likely as a function of the plasticity of Palmer amaranth (Horak and Loughin Reference Horak and Loughin2000) (i.e., fewer but larger weeds) in this dryland environment (Table 5).
Waterhemp Height, Density, and Biomass in Franklin County, 2017
Waterhemp was shorter in treatments containing row-crop cultivation, CC, or NRW plus CC, compared with SM at 3 WAP (Table 6). No differences in waterhemp height were observed with NRW in the absence of CC, as compared with SM. Waterhemp density in all treatments did not differ from SM at 3 WAP. Contrasts indicated no overall effects on waterhemp density as a result of NRW or CC (Table 6).
Table 6. Influence of grain sorghum row width, winter wheat cover crop, and row-crop cultivation on waterhemp height, density, and biomass at 3 and 8 WAP in Franklin County during 2017 in the absence of the herbicide program component.
a Abbreviations: –, no CC or RC was present in the treatment; CC, winter wheat cover crop; NCC, no cover crop; NS, not significant; RC, row-crop cultivation; WAP, weeks after planting.
b Means followed by the same letter within a column are not statistically different according to Fisher Protected LSD (α = 0.05).
c 76-cm row width without CC or RC is described as standard management in the text.
d All contrasts were conducted in the absence of RC-containing treatments.
e P = 0.05 to 0.01.
f P = 0.1 to 0.05.
g P < 0.0001.
h P = 0.01 to 0.0001.
Waterhemp biomass was less with row-crop cultivation and with CC plus 19-cm row width treatments than SM at 3 WAP (Table 6). For this site-year, a combination of cultural tactics or mechanical control was required to reduce waterhemp biomass, albeit when data were pooled across row widths, CC provided a 53% reduction in biomass compared with NCC treatments (Table 6).
Late-season waterhemp was shorter compared with SM for all CC-containing treatments at 8 WAP (Table 6). Row-crop cultivation plus CC reduced waterhemp height compared with row-crop cultivation alone, indicating that some level of suppression was achieved with the addition of CC. When row-crop cultivation is used, producers should consider the use of a CC or retain previous crop residues for benefits outside of weed management (e.g., soil conservation, soil moisture retention) (Hartzler et al. Reference Hartzler, Van Kooten, Stoltenberg, Hall and Fawcett1993; Keene and Curran Reference Keene and Curran2016). Waterhemp was not shorter with NRW in the absence of CC compared with SM 8 WAP. Furthermore, contrasts revealed a 29% reduction in height with CC compared with the NCC treatments (Table 6).
At 8 WAP, reductions in waterhemp density from SM were only observed in row-crop cultivation treatments (Table 6). The general lack of difference between treatments was likely due to the waterhemp emergence pattern in this specific environment. For example, the majority of waterhemp may have emerged earlier in the season, prior to the row-crop cultivation 2.5 WAP, and thus a significant proportion of emerged waterhemp was controlled. Fewer waterhemp emerged late, thereby having a reduced density at 8 WAP, and CC likely provided the grain sorghum crop other competitive advantages.
Reductions in waterhemp biomass were observed for all CC or row-crop cultivation treatments as compared with SM at 8 WAP (Table 6). Biomass in treatments containing NRW in the absence of CC did not differ from SM, though contrasts indicated that 38- and 19-cm row widths contributed 49% and 46% reductions in biomass by 8 WAP, compared with the 76-cm row widths. The use of NRW provided a similar benefit in soybean in terms of suppressing late-season waterhemp biomass (Butts et al. Reference Butts, Norsworthy, Kruger, Sandell, Young, Steckel, Loux, Bradley, Conley, Stoltenberg, Arriaga and Davis2016). CC also provided waterhemp suppression, with a 64% reduction in biomass compared with NCC-containing treatments (Table 6).
Waterhemp Density in Franklin County, 2018
No differences in waterhemp height or biomass were detected at either observation time in Franklin County in 2018 (data not shown). At 3 and 8 WAP, waterhemp density was less than with SM in those treatments that contained row-crop cultivation (Table 7). All treatments, regardless of the presence of CC or NRW, resulted in similar densities. This could have been due to the lack of moisture from planting through 8 WAP (73 mm, compared with the 200 mm normally received during this period) (Table 3). In the absence of a CC, the 19-cm row width reduced density at 8 WAP, compared with the 38-cm row width. The general lack of difference between NRW and CC indicate both components were ineffective at providing waterhemp suppression in this site-year. Although reduced CC biomass or soil nitrogen availability could have contributed to the lack of differences in this site-year, similar biomass and soil nitrogen levels were found (Table 2), which indicated that environmental factors (e.g., rainfall, soil moisture, thermal amplitude at the soil surface) contributed to the lack of differences.
Table 7. Influence of grain sorghum row width, winter wheat cover crop, and row-crop cultivation on waterhemp density at 3 and 8 WAP in Franklin County in 2018 in the absence of the herbicide program component.
a Abbreviations: –, no CC or row-crop cultivation was present in the treatment; CC, winter wheat cover crop; NCC, no cover crop; NS, not significant; row-crop cultivation, row-crop cultivation; WAP, weeks after planting.
b Means followed by the same letter within a column are not statistically different according to Fisher Protected LSD (α = 0.05).
c 3 WAP height and biomass and 8 WAP height and biomass means and contrasts were found to be NS and are not shown.
d 76-cm row width without CC or row-crop cultivation is described as standard management in the text.
e All contrasts were conducted in the absence of row-crop cultivation-containing treatments.
f P = 0.1 to 0.05.
Practical Implications for Management
The herbicide program component provided the most effective pigweed control in contrast to the cultural and mechanical components considered. The success of this program was likely due to the use of overlapping residuals, multiple effective sites of action in each application, and the timeliness of all applications. This herbicide program achieved excellent pigweed control (>97%) across all systems, and this type of approach would slow the development of herbicide resistance (Godar and Stahlman Reference Godar and Stahlman2015; Meyer et al. Reference Meyer, Norsworthy, Young, Steckel, Bradley, Johnson, Loux, Davis, Kruger, Barapour, Ikley, Spaunhorst and Butts2015; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012; Reddy et al. Reference Reddy, Stahlman, Geier, Thompson, Currie, Schlegel, Olson and Lally2013; Sarangi and Jhala Reference Sarangi and Jhala2018; Steckel et al. Reference Steckel, Sprague and Hager2002), albeit herbicide resistance will eventually develop in the absence of integrated strategies, even with a robust herbicide program (Shaner Reference Shaner2014).
The integration of other mechanical and cultural tactics must be considered to extend the life of the limited, effective herbicide options currently available in grain sorghum (Stahlman and Wicks Reference Stahlman, Wicks, Smith and Frederiksen2000; Thompson et al. Reference Thompson, Dille, Peterson, Ciampitti and Prasad2017). Row-crop cultivation was the most effective component outside of the herbicide program and provided 79% reduction in pigweed density by 3 WAP when implemented at 2.5 WAP. Greater success of the row-crop cultivation component would be possible when implemented in fields with lower pigweed densities than those in this study (Buhler et al. Reference Buhler, Gunsolus and Ralston1992; Dieleman et al. Reference Dieleman, Mortensen and Martin1999). This mechanical tactic could substantially reduce the selection pressure on pigweed imposed by POST herbicides and should be used when row widths are wide enough to accommodate row-crop cultivation equipment and soil conservation plans allow (Buhler Reference Buhler2002). Even though the integration of CC with row-crop cultivation did not consistently improve weed control, it may facilitate soil conservation (Buhler Reference Buhler1995; Keene and Curran Reference Keene and Curran2016). Ultimately, more consideration must be given to integrate row-crop cultivation with herbicides to improve the long-term control offered by the system and to control weeds within the row (Buhler Reference Buhler1995; VanGessel et al. Reference VanGessel, Schweizer, Wilson, Wiles and Westra1998).
Pigweed control with the CC component had mixed results. Although the treatment in half the site-years (specifically, Riley County and Reno County during 2017) (Table 4) resulted in approximately 50% reductions in Palmer amaranth density and biomass in both early- and late-season observations, there was no change in or greater densities of Palmer amaranth and waterhemp in the other site-years. Although this demonstrates the potential benefit of CC as a strategy to reduce the selection pressure on pigweed by herbicides and to limit seedbank replenishment, more research is needed to understand other agronomic practices (e.g., termination timing, species selection) to improve the consistency of CC performance in dryland cropping systems featuring grain sorghum.
Pigweed control 3 WAP was not influenced by NRW, and NRW would not reduce the selection pressure on pigweed from POST herbicide applications. Limited early-season benefit from NRW has been reported in grain sorghum (Besancon et al. Reference Besancon, Heiniger, Weisz and Everman2017) and other crops (Bradley Reference Bradley2006; Butts et al. Reference Butts, Norsworthy, Kruger, Sandell, Young, Steckel, Loux, Bradley, Conley, Stoltenberg, Arriaga and Davis2016; Norsworthy and Oliveira Reference Norsworthy and Oliveira2004). Our research demonstrated NRW could result in reduced pigweed biomass at 8 WAP, which would limit pigweed seed production. As result of this research, integrating the use of NRW or row-crop cultivation together with an herbicide program will achieve consistent control of pigweed and reduce the risk of evolving herbicide resistance.
Acknowledgements
The authors appreciate undergraduate researchers Peter P. Bergkamp and Dakota W. Came, former graduate student Garrison J. Gundy, Assistant Scientist Cathy Minihan, and the experiment field staff for their assistance with this project. The research received no specific grant from any funding agency, commercial, or not-for-profit sectors. No conflicts of interest have been declared.
