Introduction
Ethofumesate is applied preplant or PRE at rates ranging from 1.12 to 4.37 kg ai ha–1 for control of monocotyledonous and dicotyledonous weeds in sugarbeet. Weed control following PRE application requires timely and adequate precipitation to activate herbicide into the weed seedling layer, because ethofumesate has low water solubility and is strongly adsorbed to soil (Shaner Reference Shaner2014; Schweitzer Reference Schweizer1975). Ethofumesate uptake by weeds occurs by shoot (coleoptile or hypocotyl) and root adsorption and is rapidly translocated to foliage of susceptible weed species, remaining as ethofumesate through adsorption and translocation (Duncan et al. Reference Duncan, Meggitt and Penner1982a; Eshel et al. Reference Eshel, Zimdahl and Schweizer1978). Several observations concluded that ethofumesate may affect surface waxes by inhibition of very-long-chain fatty acids, although the specific mechanism of herbicidal action is not fully known (Abulnaja et al. Reference Abulnaja, Tighe and Harwood1992; Devine et al. Reference Devine, Duke and Fedke1993). Ethofumesate soil-applied alone, soil-applied in mixtures, or mixed with glyphosate and applied POST controls agronomically and economically important weeds in sugarbeet across a range of environments (Dexter Reference Dexter1975; Ekins and Cronin Reference Ekins and Cronin1972; Peters et al. Reference Peters, Lueck and Radermacher2016a; Schweizer Reference Schweizer1979; Sullivan Reference Sullivan1973; Sullivan and Fagala Reference Sullivan and Fagala1970). Field research from Kansas and Colorado in 1970 reported that ‘NC 8438’ (ethofumesate) controlled green foxtail [Setaria viridis (L.) P. Beauv.], barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], redroot pigweed (Amaranthus retroflexus L.), and common lambsquarters (Chenopodium album L.) (Sullivan and Fagala Reference Sullivan and Fagala1970). Likewise, ethofumesate controlled common lambsquarters and Pennsylvania smartweed [Persicaria pensylvanica (L.) M. Gomez] in Ohio and redroot pigweed and Setaria species in Idaho (Ekins and Cronin Reference Ekins and Cronin1972).
One would anticipate a strong technical fit with ethofumesate in sugarbeet in Minnesota and eastern North Dakota, in that full-season residual activity of the herbicide (Elkins and Cronin Reference Ekins and Cronin1972) is well suited for prairie soils. Still, ethofumesate has not been widely used. Survey of weed control and production practices indicated that the percentage of soil-applied ethofumesate use ranged from 1% to 22% in 2005 to 2014, with average annual use of 9% during that period (Carlson et al. Reference Carlson, Peters, Khan and Boetel2015). Several factors may contribute to low adoption of ethofumesate. Ethofumesate is a relatively expensive treatment, especially at rates required for adequate weed control in Minnesota and eastern North Dakota soils (A. Dexter 2017, personal communication). Moreover, mechanical incorporation is often required to activate ethofumesate to achieve consistent weed control (Entz Reference Entz1982). Additionally, McAuliffe and Appleby (Reference McAuliffe and Appleby1981) reported significant chemical degradation losses when ethofumesate was applied directly to the surface of dry soils without an activating precipitation event or irrigation event within 4 d of application. Precipitation to activate soil-applied herbicides is inconsistent or limiting in May within the sugarbeet-producing region of Minnesota and eastern North Dakota. For example, average total May precipitation in Fargo, ND, is 66 mm, with daily precipitation totaling 6.4 mm or greater occurring only 3 d during the month (D. Ritchison 2017, personal communication). Growers typically use spring tillage, but the intended purpose is to create a smooth and firm seedbed at seeding depth to ensure successful stand establishment rather than herbicide incorporation (Cattanach Reference Cattanach1995; Smith et al. Reference Smith, Cattanach and Lamb1990). Finally, ethofumesate potentially reduces stand density of spring-seeded barley (Hordeum vulgare L.), oat (Avena sativa L.), or wheat (Triticum aestivum L.) seeded as a companion crop with sugarbeet on 55% of the sugarbeet acreage in 2017 (Peters et al. Reference Peters, Lueck, Metzger and Radermacher2016b; T. Grove, 2017, personal communication).
Waterhemp is a dioecious Amaranthus species spreading from its epicenter in the flood plains of southern and western Illinois (Sauer Reference Sauer1957) to many regions in the Midwest (Horak and Peterson Reference Horak and Peterson1995; Hinz and Owen Reference Hinz and Owen1997; Steckel et al. Reference Steckel, Sprague and Hager2002). Glyphosate-resistant waterhemp populations were initially reported in Minnesota in 2007 and were reported across substantial sugarbeet acres in southwestern Minnesota in 2011 (Heap Reference Heap2018; Stachler and Luecke Reference Stachler and Luecke2011), as growers were slow to adopt alternative waterhemp control practices, increasing the incidence and severity of waterhemp infestation. Strategies to control glyphosate-resistant waterhemp biotypes included traditional soil-applied herbicides followed by POST herbicide mixtures with glyphosate. Treatments combining cycloate, ethofumesate, desmedipham, and phenmedipham with or followed by glyphosate improved waterhemp control compared to glyphosate alone (Stachler and Luecke Reference Stachler and Luecke2011). However, sugarbeet injury was greater compared to glyphosate alone in the Roundup Ready® sugarbeet system, especially in west central Minnesota fields with variable soil textures.
Waterhemp germinates in spring or summer, develops through the summer, and sets seed to complete its life cycle in fall (Radosevich et al. Reference Radosevich, Holt and Ghersa1997). However, waterhemp may emerge later in the growing season than other summer annual weeds, and the duration of emergence period is longer than most other annual broadleaf weeds. Hartzler et al. (Reference Hartzler, Buhler and Stoltenberg1999) reported that the emergence characteristics of waterhemp are distinctly different from two historically important Iowa weeds, giant foxtail (Setaria faberi Herrm.) and velvetleaf (Abutilon theophrasti Medik.). Waterhemp emergence was 5 to 25 d later than velvetleaf emergence, and period of emergence was 8 to 13 d longer than that of giant foxtail. Werle et al. (Reference Werle, Sandell, Buhler, Hartzler and Lindquist2014) reported that waterhemp and redroot pigweed germinated later [10% emergence at 230 growing degree days (GDD)] and for an extended period (766 GDDs accumulated between 10% and 90% emergence) than common ragweed (Ambrosia artemisiifolia L.) and kochia [Bassia scoparia (L.) A.J. Scott] (10% emergence at 19 GDD and 108 GDDs accumulated between 10% and 90% emergence), two additional important weeds in sugarbeet production areas in Minnesota and eastern North Dakota.
Waterhemp control has been shown to extend later into the growing season, with sequential (often referred to as layered) applications of soil-residual herbicides compared to single applications. Steckel et al. (Reference Steckel, Sprague and Hager2002) reported that acetochlor, dimethenamid-P, or S-metolachlor PRE at the 0.66× rate in corn (Zea mays L.) followed by (fb) acetochlor, dimethenamid-P, or S-metolachlor POST at the 0.34× rate improved common waterhemp control compared to PRE only treatments at the 1× rate. Aulakh and Jhala (Reference Aulakh and Jhala2015) reported that PRE application of sulfentrazone plus metribuzin fb glufosinate late POST in soybean [Glycine max (L.) Merr.] controlled common lambsquarters, waterhemp, eastern black nightshade (Solanum ptychanthum Dunal), and velvetleaf 69% to 78% at harvest. Broadleaf control improved to greater than 90% when acetochlor, pyroxasulfone, or S-metolachlor was mixed with glufosinate early POST following sulfentrazone + metribuzin PRE.
Single or layered application of chloroacetamide herbicides applied with glyphosate and ethofumesate POST improved waterhemp control in sugarbeet compared to three POST glyphosate and ethofumesate applications on approximately 14-d intervals (Peters et al. Reference Peters, Lueck and Radermacher2016a, Reference Peters, Lueck and Groen2017). However, chloroacetamide herbicides must be properly timed to sugarbeet growth stage and waterhemp emergence. Acetochlor, dimethenamid-P, and S-metolachlor are labeled for application at the two-leaf sugarbeet stage (Anonymous 2014, 2017, 2018), respectively. Sugarbeets seeded in early April reach the two-leaf stage on approximately May 10 in the southern Red River Valley and southwestern Minnesota or 5–10 d before waterhemp germination and emergence according to a waterhemp GDD model (Peters Reference Peters2016). A layered chloroacetamide application may follow 14–21 d after the two-leaf stage application or when sugarbeets are at the six- to eight-leaf sugarbeet stage. Glyphosate and ethofumesate at 0.14 kg ha–1 are usually mixed with chloroacetamide herbicides for control of emerged weeds. Layered application of chloroacetamide herbicides at reduced rates in combination with glyphosate and ethofumesate improved waterhemp control 16% when evaluated in July and August compared to a single application of chloroacetamide herbicides at full rates in experiments averaged across three locations in 2016 and 2017 in sugarbeet in Minnesota and eastern North Dakota. (Peters et al. Reference Peters, Lueck, Mettler and Groen2018).
A late spring may delay sugarbeet planting from mid-April until mid-May to early June in Minnesota and eastern North Dakota in some years. In this scenario, single or layered chloroacetamide application is not an effective weed management strategy, as waterhemp will emerge before sugarbeets reach the two-leaf stage or labeled timing for chloroacetamide POST application. Sugarbeet growers are advised to use cycloate or ethofumesate PPI or PRE. Additionally, growers may register with Syngenta (Syngenta Crop Protection LLC, Greensboro, NC) to use S-metolachlor (Dual Magnum®) PRE using a Section 24(c) Special Local Need label, whereby sugarbeet growers assume all risk of sugarbeet injury, sugarbeet root yield loss, and loss of sugarbeet crop at rates from 1.07 to 2.14 kg ha–1, depending on soil organic matter (OM) content and soil texture (Pusino et al. Reference Pusino, Liu and Gessa1992; Shaner et al. Reference Shaner, Brunk, Belles, Westra and Nissen2006).
Sugarbeet tolerance to soil-applied S-metolachlor has been inconsistent. Growth reduction of sugarbeet with S-metolachlor 2.25 kg ha–1 applied PPI averaged 6% and ranged from 0 to 14% in five environments between 1997 and 2002 (Dexter and Luecke Reference Dexter and Luecke2004). However, injury averaged 44% and ranged from 20% to 73% in 2003. Injury was attributed to cool soils that slowed sugarbeet emergence plus abundant precipitation patterns immediately following sugarbeet planting and before sugarbeet emergence.
Research investigated sugarbeet tolerance from S-metolachlor PRE at reduced rates. Lueck (Reference Lueck2017) reported that sugarbeet stand density (number of plants per 31-m row) decreased as S-metolachlor rate increased from 0.54 to 2.15 kg ha–1 across nine environments. However, stand density with S-metolachlor at 0.54 kg ha–1 across environments or S-metolachlor across rates in a high-OM soils cohort was the same as stand density with the untreated control when environments were grouped according to OM as described by Pusino et al. (Reference Pusino, Liu and Gessa1992) and Shaner et al. (Reference Shaner, Brunk, Belles, Westra and Nissen2006). Lueck (Reference Lueck2017) surmised that S-metolachlor was adsorbed more greatly by high organic matter soils across climatic conditions. S-metolachlor at rates greater than 0.54 kg ha–1 reduced sugarbeet stand density, especially with precipitation greater than 40 mm 7 d after seeding (Bollman and Sprague Reference Bollman and Sprague2008) in soil cohorts with less than 3.5% OM.
Our research indicates that S-metolachlor 0.54 kg ha–1 PRE fb layered chloroacetamide herbicides POST at reduced rates provides the best waterhemp control, especially when spring planting is delayed (Peters et al. Reference Peters, Lueck, Mettler and Groen2018). This waterhemp control management plan combining multiple reduced-rate treatments beginning PRE through POST is conceptually similar to the split application at reduced-rate strategy developed with desmedipham and phenmedipham in sugarbeet in the 1990s (Dexter Reference Dexter1994). However, some growers in Minnesota and eastern North Dakota are unwilling to use S-metolachlor PRE (at any rate) because of variable soil types and concerns with sugarbeet injury. They have inquired about ethofumesate at reduced rates PRE fb reduced rates of chloroacetamide herbicides. Tolerance of sugarbeet to ethofumesate is related to application rate and soil type (Schweizer Reference Schweizer1975). Sugarbeet tolerance to ethofumesate in Minnesota and eastern North Dakota is excellent at rates up to 4.37 kg ha–1. However, field and laboratory research concludes that ethofumesate interacts with POST herbicides, including increased absorption or efficacy with herbicides applied sequentially (Duncan et al. Reference Duncan, Meggitt and Penner1982b; Kniss and Odero Reference Kniss and Odero2013) or tank-mixed with ethofumesate (Eshel et al. Reference Eshel, Zimdahl and Schweizer1976). Ethofumesate applied PRE reduced epicuticular wax in cabbage (Brassica oleracea L.) (Leavitt et al. Reference Leavitt, Duncan, Penner and Meggitt1979) and onion (Allium cepa L.) leaves (Rubin et al. Reference Rubin, Adler, Varsano and Rabinowitch1986). Additionally, ethofumesate altered the structure of cuticular waxes, increasing transpiration losses from the leaf surfaces (Leavitt et al. Reference Leavitt, Duncan, Penner and Meggitt1979) and increasing uptake of herbicides that followed in sequence with ethofumesate (Devine Reference Devine, Duke and Fedke1993). Decrease in chain length and wax modification was attributed to herbicide concentration but occurred even at sublethal rates (Bolton and Harwood Reference Bolton and Harwood1976). Investigating crop response and evaluating potential sugarbeet tolerance risks from combining ethofumesate, S-metolachlor, and dimethenamid-P will benefit growers as they adopt additional waterhemp control strategies in sugarbeet. Therefore, the objective of this research was to determine if full or reduced rates of ethofumesate, S-metolachlor, and reduced-rate mixtures of ethofumesate = S-metolachlor applied PRE increase sugarbeet injury from dimethenamid-P applied POST.
Materials and methods
Field experiments were conducted in Minnesota and eastern North Dakota in 2014, 2015, and 2016. Location-year combinations (herein referred to as an environment) were Amenia-2014, Belgrade-2015, Crookston-2015, Amenia-2015, and Amenia-2016. The experiment was a randomized complete block design with four to six replications depending on environment. Experiments evaluated sugarbeet tolerance to PRE applications of ethofumesate at 1.68 and 4.37 kg ha–1, PRE applications of S-metolachlor at 0.80 and 1.60 kg ha–1, and PRE applications of ethofumesate at 0.80 kg ha–1 + S-metolachlor at 1.68 kg ha–1 alone or fb a POST application of dimethenamid-P at 0.95 kg ha–1. A nontreated control was nested in the design for comparison. Detailed soil descriptions for each environment can be found in Table 1. Herbicide rates for sugarbeet were consistent with label recommendations for soil texture and OM content. Herbicide formulations and application rates are listed in Table 2. All treatments within an environment were applied as a single PRE application date or the same PRE application date followed by a POST application. All PRE herbicide applications were made within 1 d following sugarbeet seeding. POST applications were made at the two- to four-leaf sugarbeet growth stage. All treatments were applied using a bicycle wheel sprayer with a shielded boom to reduce particle drift at 159 L ha–1 spray solution through 8002 XR flat-fan nozzles (TeeJet Technologies, Glendale Heights, IL) spaced 51 cm apart and pressurized with CO2 at 207 kPa to the center four rows of each plot. Sugarbeet was planted approximately 3 cm deep at 152,000 (±1,000) seeds ha–1 after fall chisel plowing and a single pass with a field cultivator with rolling baskets in spring. Individual plots were 3.4 m by 9.1 m and contained six rows on 56-cm spacing. Entire trial sites were kept weed-free with applications of glyphosate 1.27 kg ae ha–1. Diseases and insects were controlled season-long at each environment. Precipitation data were collected from nearby weather stations operated by the North Dakota Agricultural Weather Network, Community Collaborative Rain, Snow and Hail Network, and the University of Minnesota Experiment Station.
Table 1. Soil descriptions for environments in 2014, 2015, and 2016.
Table 2. Herbicides, herbicide rates, and application timing for the experiments.a
a Abbreviations: fb, followed by.
b Bayer Crop Science, Research Triangle Park, NC.
c Syngenta Crop Protection, Greensboro, NC.
d BASF Corp., Research Triangle Park, NC.
Sugarbeet tolerance was evaluated by counting sugarbeet at the six-leaf stage and before harvest and by assessing visible sugarbeet injury between 7 and 13 d after treatment (DAT) (hereafter referred to as 10 DAT) and between 17 and 29 DAT (hereafter referred to as 23 DAT). Evaluation was a visual estimate of percentage injury ranging from 0% (no injury) to 100% (all plants completely eliminated) relative to the untreated check rows between individual plots. At harvest, sugarbeet was defoliated and harvested mechanically from the center two rows of each plot and weighed. A 10-kg sample was collected from each plot and analyzed for sucrose content and sugar loss to molasses by American Crystal Sugar Company (East Grand Forks, ND). Root yield (kg ha–1), purity (%), and recoverable sucrose (kg ha–1) were calculated using Equations (1–3).
$${\rm{Root\ yield}}\left( {{\rm{kg}}/{\rm{ha}}} \right) = {{{\rm{harvested\ plot\ weight}}\left( {{\rm{kg}}} \right)} \over {{\rm{hectare\ area\ of\ harvested\ plot}}}}$$
(1)
$${\rm{Purity}}\left( {\rm{\% }} \right) = {{{\rm{\% \ sucrose\ content}} - {\rm{\% \ sugar\ loss\ to\ molasses}}} \over {{\rm{\% \ sucrose\ content}}}} \times 100$$
(2)
$$\eqalignno{	{{Recoverable\ sucrose}}\left( {{\rm{kg}}/{\rm{ha}}} \right) \cr 	= \left( {{{\left[ {\left( {{\rm{\% \ purity}}/{\rm{}}100} \right)\ \% \ {\rm{sucrose\ content}}} \right]} \over {100}}} \right) \times {\rm{root\ yield} \hskip 25pt (3)}$$
(3)
Data were subjected to ANOVA using the MIXED procedure in SAS 9.4 (SAS Institute, Cary, NC) to test for treatment effects and interactions using the appropriate expected mean square values as recommended by McIntosh (Reference McIntosh1983). Each location-year combination was considered an environment at random from a population as suggested by Blouin et al. (Reference Blouin, Webster and Bond2011). Environments, replications, and all interactions containing these effects were designated random effects in the model; herbicide treatment and application timing were designated as fixed effects. Significantly different treatment means were separated using t-tests when data were found to be significantly different at the P ≤ 0.05 level. Single degree-of-freedom contrasts were used to compare the effect of herbicide, herbicide rate, and application timing on sugarbeet density at the six-leaf stage and at preharvest, sugarbeet visible injury 10 and 23 d after POST treatment, and sugarbeet root yield, percent sucrose content, and recoverable sucrose ha–1 averaged across five environments.
Results and discussion
Field growing conditions
Sugarbeet planting dates ranged between April 16 and May 17 across environments (Table 3), as is typical for sugarbeet production in eastern North Dakota and Minnesota. Sugarbeet are usually planted in rows spaced 56 cm apart to a depth of approximately 3 cm, with 11.4-cm spacing within the row or a density of 265 seeds (±10 seeds) per 31-m row (M. Metzger, 2018 personal communication). Seed attrition occurs after germination and before emergence, usually as a result of environmental and edaphic factors (Cattanach Reference Cattanach1995; Smith et al. Reference Smith, Cattanach and Lamb1990; Campbell and Enz Reference Campbell and Enz1991). Emergence ranging from 181 to 206 plants per 31 m and preharvest density ranging from 172 to 197 plants per 31 m are considered ideal. Precipitation following seeding was near the 30-yr average in three environments: Amenia-2014, Belgrade-2015, and Crookston-2015. Precipitation was greater than normal at Amenia-2016 and less than normal at Amenia-2017 (Table 4). Average overall density at the six-leaf sugarbeet stage in the untreated control across environments was 205 sugarbeet plants per 31-m row and ranged from 189 to 220 (Table 5). Thus, differences in density in this experiment probably were directly due to treatment or an interaction of environment and treatment rather than environmental factors.
Table 3. Planting dates, average stand density, and harvest dates, across environments.
Table 4. Sugarbeet planting date, days from planting (DAP) to first precipitation, cumulative precipitation, DAP to sugarbeet emergence, and average air temperature from planting to emergence by environment.
a Thirty-year average precipitation in May and June was 155 mm at Amenia, 205 mm at Belgrade, and 162 mm at Crookston.
b Number of days to first precipitation event totaling greater than 6.4 mm.
c Climatic data at Amenia were collected by the North Dakota Agricultural Weather Network; Belgrade climate data were collected by a local observer, Community Collaborative Rain, Snow and Hail Network; Crookston climate data were collected by the University of Minnesota Experiment Station.
d Sugarbeet days to emergence predicted by growing degree days accumulation and verified by visual observation.
e Average daily air temperature during the interval between planting and sugarbeet emergence.
Table 5. Sugarbeet plant density in response to herbicide treatments and environment at six-leaf stage. Includes average density, density as a percent of untreated control, and standard deviation of the mean averaged across environments.
a Abbreviations: fb, followed by; NS, not significant; SD, standard deviation.
b Significance at P < 0.05 and P < 0.01 levels denoted by * and **, respectively.
Sugarbeet stand density
Sugarbeet stand density at the six-leaf stage was influenced by herbicide treatment in certain environments (Table 5). There were no statistical differences between herbicides, herbicide rate, and/or application timing at three environments: Amenia-2015, Belgrade-2015, and Amenia-2016. Single degree-of-freedom comparisons were incomplete in the Amenia-2014 environment, as ethofumesate at 1.68 kg ha–1 was added to the experimental design beginning in 2015 at that location. Herbicide treatments affected sugarbeet density at the six-leaf stage at Crookston-2015 (Table 5). Single degree-of-freedom contrasts averaged across all PRE-only treatments (193 sugarbeet in 31-m row) and compared with PRE fb dimethenamid-P treatments (183 sugarbeet in 31-m row) was significant (P = 0.0010). Single degree-of-freedom contrasts comparing PRE-only treatments found that differences in stand densities were more strongly related to S-metolachlor PRE (P = 0.0002) than ethofumesate PRE (P = 0.0422), although both contrasts were significant. Single degree-of-freedom contrast comparing S-metolachlor PRE rate (0.80 or 1.60 kg ha–1 alone or fb dimethenamid-P) was highly significant (P < 0.0001) and significant across all environments (P = 0.0178). Numerically, S-metolachlor at 1.68 kg ha–1 had 18 fewer sugarbeet plants in a 31-m row than S-metolachlor 0.8 kg ha–1 in the Crookston-2015 environment.
Average sugarbeet stand density was reduced by harvest compared to the six-leaf evaluation across environments (Table 6). This observation is not unusual in sugarbeet production. Sugarbeet mortality occurs primarily from soilborne diseases such as Rhizoctonia root and crown root and Aphanomyces root rot. Loss of stand also occurs from sugarbeet plants spaced too close together (doubles), that do not grow and are not collected by harvest equipment. Sugarbeet density at harvest was least at Amenia-2014 and Amenia-2016 and greatest at Belgrade-2015 and Amenia-2015. Although herbicide treatments resulted in reduced sugarbeet density, differences were not statistically different. Amenia-2016 was an exception, where PRE herbicides fb dimethenamid-P reduced density by 14 sugarbeet plants compared with PRE-only herbicides alone (P = 0.0153). Single degree-of-freedom contrasts comparing PRE herbicides indicated that S-metolachlor rate partially explained density differences (P = 0.0475).
Table 6. Sugarbeet plant density in response to herbicide treatments and environment at preharvest. Includes average density, density as a percent of untreated control, and standard deviation of the mean averaged across environments.
a Abbreviations: fb, followed by; NS, not significant; SD, standard deviation.
b Significance at P < 0.05 and P < 0.01 levels denoted by * and **, respectively
Herbicide treatment affected sugarbeet stand density at Crookston-2015 the most. Soil and climatic features occurred in Crookston-2015 that did not occur in the other environments. Soil OM was 2.6% at Crookston-2015, the lowest in the experimental area across environments. Average 24-h air temperature between planting and sugarbeet emergence was coldest (9.1 C), and the experimental area received the most precipitation from planting to 14 d after seeding (57.7 mm). Consequently, sugarbeet took a relatively long 17 d from seeding to emergence. Herbicide treatment also affected sugarbeet density at Amenia-2014 (numeric treatment means only). Planting date at Amenia-2014 was the latest, and neither soil OM nor climatic variables could explain sugarbeet stand loss.
Sugarbeet injury from S-metolachlor was previously reported by other researchers. Dexter and Luecke (Reference Dexter and Luecke2004) reported that sugarbeet injury in 2003 was greater than injury observed in the previous 11 yr of research (29 environments). The researchers did not identify a soil or climatic variable clearly linked to sugarbeet injury in 2003 but concluded that cold air temperatures (days from planting to two-leaf stage) and abundant precipitation may have contributed to injury.
Sugarbeet stand density and density reduction due to herbicide treatment were compared using numeric averages across environments. Density was measured by comparing number of sugarbeet in herbicide treatment and untreated control and averaged across environments (Tables 5 and 6). Stand density at both measurement intervals was least with S-metolachlor 1.60 kg ha–1 fb dimethenamid-P. Variation in sugarbeet density across environments and estimated as standard deviation of the mean ranged from 10.4 to 31.1 at the six-leaf stage and from 24.7 to 35.6 at harvest (Tables 5 and 6) and was greatest with S-metolachlor 1.60 kg ha–1 alone or S-metolachlor 1.60 kg ha–1 fb dimethenamid-P at the six-leaf stage and S-metolachlor 1.60 kg ha–1 alone or S-metolachlor 0.8 at kg ha–1 + ethofumesate at 1.68 kg ha–1 fb dimethenamid-P, preharvest. These results demonstrate that stand loss is not a treatment effect that will occur generally across environments. Rather, stand loss is an outcome from treatment that occurs in environmental conditions we do not fully understand. For example, sugarbeet density with herbicide treatments was greater than 93% when averaged across application timing. However, standard deviation of the sample ranged from 0.6 to 6.4 across environments (Figure 1), indicating that treatment interacted differently with features of the environment at each location.
Figure 1. Comparison of sugarbeet density as a percent of untreated control (bar graph) and standard deviation of a sample mean (error bars), six-leaf stage, averaged across application timing and five environments. Etho, ethofumesate; S-meto, S-metolachlor.
Sugarbeet stature reduction
Sugarbeet visible stature reduction was evaluated 7–13 DAT (hereafter referred to as 10 DAT) and 17 to 29 DAT (hereafter referred to as 23 DAT) across environments (Tables 7 and 8). Stature reduction ranged numerically from 0 to 32%, 10 DAT and from 0 to 23%, 23 DAT. Injury was symptomology associated with chloroacetamide herbicides in sugarbeet including plant-to-plant variation in color and size of the foliage. Visual chlorosis occurred in moderately injured sugarbeet. Growth reduction was the more severe injury response relative to chlorosis especially 10 DAT. Although herbicide treatments caused stature reduction at each environment, treatment differences measured by single degree-of-freedom contrasts were environment dependent and were greatest at Amenia-2015 and Crookston-2015.
Table 7. Sugarbeet visible stature reduction in response to herbicide treatment and environments 7 to 13 d after treatment (DAT).a
a Abbreviations: fb, followed by; NS, not significant.
b Significance at P < 0.05 and P < 0.01 levels denoted by * and **, respectively.
Table 8. Sugarbeet visible stature reduction in response to herbicide treatments and environments 17 to 29 d after treatment.a
a Abbreviations: fb, followed by; NS, not significant.
b Significance at P < 0.05 and P < 0.01 levels denoted by * and **, respectively.
Single degree-of-freedom contrast indicated that visible stature reduction 10 DAT with PRE herbicide treatments followed by dimethenamid-P was greater than injury with PRE treatments alone, at Amenia-15 and Crookston-2015, highly significant (P ≤ 0.0001 and P ≤ 0.0001, respectively), and were highly significant across environments (P ≤ 0.0001) (Table 7). Stature reduction was greatest with ethofumesate fb dimethenamid-P (P = 0.0005; P ≤ 0.0001, and P = 0.0006) at Amenia-2016, Crookston-2015, and across environments, respectively, or ethofumesate + S-metolachlor PRE fb dimethenamid-P (P = 0.0011; P = 0.0001, and P = 0.0005) at Amenia-2016, Crookston-2015, and across environments, respectively. Stature reduction with ethofumesate occurred with the labeled and reduced labeled rates except at Amenia-2015 (P = 0.037), where ethofumesate 4.37 kg ha–1 caused greater visible injury than ethofumesate at 1.68 kg ha–1. Stature reduction with S-metolachlor PRE fb dimethenamid-P was the same as S-metolachlor PRE alone. However, S-metolachlor at 1.6 kg ha–1 reduced sugarbeet stature compared with S-metolachlor at 0.8 kg ha–1 (P = 0.01) across environments.
Sugarbeet recovered rapidly from early-season growth inhibition. Sugarbeet injury 23 DAT ranged from 0 to 23% across treatments and averaged from 1% to 11% across environments (Table 8). As with evaluation 10 DAT, single degree-of-freedom contrasts indicated greater stature reduction with PRE herbicide treatments fb dimethenamid-P than with PRE herbicide treatments alone (P = 0.0419; P = 0.0311, and P = 0.0334 at Amenia-2015, Crookston-2015, and across environments, respectively. Stature reduction 23 DAT was mostly explained by the single degree-of-freedom contrast comparing S-metolachlor at the 1.60 kg ha–1 rate with S-metolachlor at the 0.80 kg ha–1 rate (P = 0.0012) across environments.
Ethofumesate PRE has a history of safe use in sugarbeet when rate is adjusted for soil texture (Dexter Reference Dexter1975; Ekins and Cronin Reference Ekins and Cronin1972; Schweizer Reference Schweizer1975,Reference Schweizer1979; Sullivan Reference Sullivan1973; Sullivan and Fagala Reference Sullivan and Fagala1970). Likewise, dimethenamid-P POST alone at 0.72 to 1.43 kg ha–1 has been safely applied to sugarbeet at the two- to four-leaf stage (Rice et al. Reference Rice, Ransom and Ishida2002; Bollman and Sprague Reference Bollman and Sprague2008; Peters et al. Reference Peters, Lueck and Groen2017, Reference Peters, Lueck, Mettler and Groen2018). However, previous research did not evaluate ethofumesate followed by dimethenamid-P as a weed control treatment. Sugarbeet stature reduction was observed when ethofumesate + desmedipham POST followed ethofumesate and trichloroacetic acid (TCA) PRE (Duncan et al. Reference Duncan, Meggitt and Penner1982b). Duncan et al. (Reference Duncan, Meggitt and Penner1982b) reported that ethofumesate applications reduced deposition of major wax components, resulting in increased absorption of foliar-applied herbicides following ethofumesate PRE. Injury from herbicide combinations occurred with other herbicide chemicals. Dexter (Reference Dexter1994) reported that sugarbeet treated with soil-applied EPTC and cycloate were more susceptible to injury from desmedipham than sugarbeet not treated with soil-applied herbicide. The authors did not indicate if sugarbeet injury was attributed to reduced wax in the cuticle or increased absorption but recommended PRE herbicide should be considered when selecting desmedipham rate POST to reduce the risk of excessive sugarbeet injury.
Sugarbeet stature reduction with herbicide treatments occurred primarily at the Amenia-2015 and Crookston-2015 environments, although visible injury was observed at every location in this experiment. Significance of the treatment effects at the Amenia-2015 and Crookston-2015 environments influenced stature reduction across all environments. Ethofumesate or ethofumesate + S-metolachlor PRE fb dimethenamid-P and S-metolachlor at 1.6 kg ha–1 PRE reduced sugarbeet stature 10 DAT across environments compared to ethofumesate or ethofumesate + S-metolachlor PRE alone and S-metolachlor at 0.8 kg ha–1 PRE. These herbicide treatments may cause significant sugarbeet stature reduction injury that should be considered when planning a weed management system, even though sugarbeet injury was less at 23 DAT.
Sugarbeet root yield, sucrose content, and recoverable sucrose
Sugarbeet root yield did not differ significantly across environments, so data were combined over environments. Herbicides applied PRE, POST, or PRE fb POST did not affect sugarbeet root yield, sucrose content, or recoverable sucrose (Table 9), even though herbicide treatments reduced early-season sugarbeet density in some environments and tended to reduce stature across all environments. However, reduced rates of S-metolachlor + ethofumesate fb dimethenamid-P tended to reduce root yield and recoverable sugar. Sugarbeet root yield and recoverable sucrose from S-metolachlor 0.8 kg ha–1 + ethofumesate 1.68 kg ha–1 PRE fb dimethenamid-P numerically was less at four of five environments compared to S-metolachlor + ethofumesate PRE alone.
Table 9. Sugarbeet root yield, sucrose content, and recoverable sucrose in response to herbicide treatment, averaged across five environments.a
a Abbreviations: fb, followed by.
a Root yield reported in megagrams (Mg) ha–1; 1 Mg = 1,000 kg = one metric ton.
Sugarbeet recovery from early-season stature reduction caused by soil-applied herbicide treatments has been reported by other researchers. Smith and Schweizer (Reference Smith and Schweizer1983) reported that sugarbeet can recover from stature reduction caused by herbicides applied PRE and POST in spring and early summer and yield similarly to weed-free treatments. Likewise, Bollman and Sprague (Reference Bollman and Sprague2007) reported that sugarbeet overcame injury caused by PRE herbicides applied under different tillage regimes and closed canopy and produced recoverable sucrose the same as untreated control comparisons. Sugarbeet also compensate for stand loss. Khan and Hakk (Reference Khan and Hakk2016) reported no differences in root yield or recoverable sucrose among plant densities ranging from 100 to 250 sugarbeet plants per 31-m row at 56-cm spacing between rows. However, individual sugarbeet size and weight from treatments at 50 sugarbeet plants per 30-m row (1.86 kg) was greater than average mean root weight of individual sugarbeet treatments from 150 and 200 sugarbeet plants per 30-m row (0.88 kg). That being said, sugarbeet stand density loss or stature reduction delay row closure and presumably increase the likelihood of late-season weed germination and emergence that may indirectly affect root yield and recoverable sucrose (Wilson Reference Wilson1999).
Our research concludes that sugarbeet growers need to take precautions before using PRE and POST soil residual herbicides in a weed management system, even though sugarbeet tolerance generally has been acceptable with soil-residual herbicides applied singly. Our research supports the use of S-metolachlor PRE but at rates up to 0.80 kg ha–1 when dimethenamid-P at 0.95 kg ha–1 or greater follows in a weed control system. S-metolachlor at 1.60 kg ha–1 followed by dimethenamid-P reduced sugarbeet stand density in some environments. Unfortunately, we do not understand the environmental trigger causing sugarbeet stand loss from S-metolachlor at rates greater than 0.80 kg ha–1 when dimethenamid-P follows.
Our research suggests caution when using ethofumesate or ethofumesate + metolachlor PRE fb dimethenamid-P POST at 0.95 kg ha–1 or greater. Sugarbeet density loss and stature reduction generally was negligible with ethofumesate, ethofumesate + S-metolachlor PRE or with dimethenamid-P POST alone. However, stature reduction was consistently observed across environments, especially at 10 DAT, when dimethenamid-P at 0.95 kg ha–1 POST followed ethofumesate or ethofumesate + S-metolachlor. These observations of greater phytotoxicity are consistent with research documenting increased POST herbicide uptake when following (Devine et al. Reference Devine, Duke and Fedke1993; Duncan et al. Reference Duncan, Meggitt and Penner1982b; Rubin et al. Reference Rubin, Adler, Varsano and Rabinowitch1986) or when tank-mixed with ethofumesate (Eshel et al. Reference Eshel, Zimdahl and Schweizer1976). Whether observed increased absorption results in increased phytotoxicity probably depends on the herbicide mode of action or environmental conditions (Kniss and Odero Reference Kniss and Odero2013; Rubin et al. Reference Rubin, Adler, Varsano and Rabinowitch1986).
Dimethenamid-P followed ethofumesate, S-metolachlor, or ethofumesate + S-metolachlor in these experiments. Sugarbeet tolerance from other chloroacetamide herbicides following ethofumesate, S-metolachlor, or ethofumesate + S-metolachlor was not evaluated in these experiments. Likewise, sugarbeet tolerance from dimethenamid-P or other chloroacetamide herbicides split-applied at reduced rates and following ethofumesate, S-metolachlor, or ethofumesate + S-metolachlor was not evaluated in these experiments.
Author ORCIDs
Peters Thomas J. https://orcid.org/0000-0003-0184-7513
Acknowledgments
No conflicts of interest have been declared. Financial support for this research was provided by the Sugarbeet Research and Education Board, using check-off dollars from growers representing American Crystal Sugar Cooperative, Minn-Dak Farmers Cooperative, and Southern Minnesota Beet Sugar Cooperative.
