Introduction
Horseweed is a facultative, winter annual weed species that thrives across a wide geography and in many different habitats. It is native to North America, where it grows along roadsides, railways, and in crop production fields with reduced or no-tillage (Weaver Reference Weaver2001). Horseweed plants can produce up to 200,000 seeds, which are capable of wind dispersal into the planetary boundary layer and can travel more than 500 km, due to an attached pappus (Bhowmik and Bekech Reference Bhowmik and Bekech1993; Shields et al. Reference Shields, Dauer, VanGessel and Neumann2006; Weaver Reference Weaver2001). Horseweed emergence can be difficult to predict because it is not correlated with soil temperature, air temperature, or rainfall (Main et al. Reference Main, Steckel, Hayes and Mueller2006). Emergence has been observed throughout the growing season; however, peak emergence occurs in May and in late August to early September in the North Central Region (Buhler and Owen Reference Buhler and Owen1997; Tozzi and Van Acker Reference Tozzi and Van Acker2014). Horseweed seeds are nondormant and readily germinate on the soil surface, making horseweed especially difficult to manage in no-tillage situations (Buhler and Owen Reference Buhler and Owen1997).
Horseweed is considered one of the top 10 most common and troublesome weeds in broadleaf crops in the United States and Canada (Van Wychen Reference Van Wychen2016). Soybean yields were reduced 83% when horseweed was left uncontrolled (Bruce and Kells Reference Bruce and Kells1990). Yield loss has intensified due to the widespread occurrence of herbicide-resistant populations and lack of effective management options. Currently, 18 countries have confirmed horseweed populations resistant to at least one herbicide site of action (Heap Reference Heap2020). Horseweed was the first weed with confirmed resistance to glyphosate (WSSA Group 9) in the United States (VanGessel Reference VanGessel2001). Since this discovery, glyphosate-resistant (GR) horseweed populations have been confirmed in 25 states and populations are often resistant to more than one herbicide site of action (Heap Reference Heap2020). In Michigan, horseweed with resistance to acetolactate synthase inhibitors (WSSA Group 2), triazine herbicides (WSSA Group 5), diuron (WSSA Group 7), and paraquat (WSSA Group 22) has also been confirmed (Heap Reference Heap2020). The spread of herbicide-resistant horseweed populations, in addition to potential resistances to other herbicide sites of action, makes using additional management strategies necessary.
Cover crops are being adopted rapidly for the ecosystem services they provide to the soil and as a weed suppression tool. Cover crops suppress weeds via resource competition and space capture while living, and by creating a mulch layer on the soil surface after termination (Mirsky et al. Reference Mirsky, Ryan, Teasdale, Curran, Reberg-Horton, Spargo, Wells and Moyer2013; Teasdale et al. Reference Teasdale, Brandsaeter, Calegari, Neto, Upadhyaya and Blackshaw2007). In addition, cover crops reduce weed emergence by inhibiting seed germination through the release of allelochemicals (Kelton et al. Reference Kelton, Price, Mosjidis and Price2012). However, allelopathic suppression is short lived and generally occurs within 20 d after cover-crop decomposition (Chou and Patrick Reference Chou and Patrick1976). The mulch left after termination suppresses weeds by modifying light quantity and quality, changing soil surface temperature, and creating a physical barrier to seedling emergence (Teasdale and Mohler Reference Teasdale and Mohler1993). In addition, high levels of mulch also create a physical barrier for emerging weed seedlings, which inhibits upward movement of seedlings and downward penetration of light (Mirsky et al. Reference Mirsky, Ryan, Teasdale, Curran, Reberg-Horton, Spargo, Wells and Moyer2013). However, cover-crop residues often do not persist long enough to provide season-long weed suppression in long-season grain crops, because of residue degradation (Osipitan et al. Reference Osipitan, Dille, Assefa and Knezevic2018).
Fall-planted cover crops suppress winter annual weeds in the fall as well as the following spring (Teasdale Reference Teasdale1996). This is critical for GR horseweed suppression because horseweed emerges in the fall or early spring. Among fall-planted cover crops, cereal rye and winter wheat are two of the most commonly grown cover crops, prior to soybeans (CTIC 2017). Cornelius and Bradley (Reference Cornelius and Bradley2017) reported 70% and 50% reduction in winter annual weed emergence from cereal rye and winter wheat, respectively. Recent studies reported that fall-planted cover crops reduced horseweed density before cover crop termination in the spring (Pittman et al. Reference Pittman, Barney and Flessner2019; Wallace et al. Reference Wallace, Curran and Mortensen2019). Wallace et al. (Reference Wallace, Curran and Mortensen2019) observed that cereal rye, alone or in mixtures, reduced horseweed size and improved horseweed size uniformity at burndown. Late-season horseweed suppression by fall-planted cover crops is less consistent. Davis et al. (Reference Davis, Gibson, Bauman, Weller and Johnson2007) observed similar reductions in horseweed density by a winter wheat cover crop compared with spring-applied residual herbicides 1 mo after burndown in 1 of 4 yr. In contrast, Wallace et al. (Reference Wallace, Curran and Mortensen2019) reported cereal rye terminated 10 d before soybean planting did not reduce horseweed density at the time of POST herbicide application.
In several studies, researchers reported weed suppression by cereal cover crops improves with increasing cover-crop biomass (Finney et al. Reference Finney, White and Kaye2016; Ryan et al. Reference Ryan, Mirsky, Mortensen, Teasdale and Curran2011b; Smith et al. Reference Smith, Reberg-Horton, Place, Meijer, Arellano and Mueller2011). Cereal rye generally produces more biomass than does winter wheat (Bauer and Reeves Reference Bauer and Reeves1999; Cornelius and Bradley Reference Cornelius and Bradley2017; Duiker Reference Duiker2014; Kaspar and Bakker Reference Kaspar and Bakker2015; McCormick et al. Reference McCormick, Sule, Barker and Beuerlein2006; Price et al. Reference Price, Reeves and Patterson2006; Reeves et al. Reference Reeves, Price and Patterson2005). Adjusting seeding rate is a method to potentially increase cereal cover-crop biomass accumulation. However, many previous studies reported adjusting the seeding rate had little effect on biomass accumulation of cereals, because of tillering as a means of compensation (Brennan et al. Reference Brennan, Boyd, Smith and Foster2009; Masiunas et al. Reference Masiunas, Weston and Weller1995; Ryan et al. Reference Ryan, Curan, Grantham, Hunsberger, Mirsky, Mortensen, Nord and Wilson2011a; Webster et al. Reference Webster, Simmons, Culpepper, Grey, Bridges and Scully2016). Delaying cover-crop termination is another method to increase biomass accumulation. A study comparing incremental delays in spring termination found cereal rye biomass increased 37% with each 10-d delay (Mirsky et al. Reference Mirsky, Curran, Mortensen, Ryan and Shumway2011).
To offset delayed planting in no-till systems, because of cooler and wetter soil conditions, many growers have begun “planting green,” whereby cover-crop termination is delayed until shortly after planting of the main crop (CTIC 2017). By delaying cover-crop termination 4 to 30 d, compared with early termination, planting green provided 94% to 181% greater cover-crop biomass production in Pennsylvania (Reed et al. Reference Reed, Karsten, Curran, Tooker and Duiker2019). Soil moisture at planting may be reduced if cover crops are allowed more time to grow and accumulate more biomass (Mirsky et al. Reference Mirsky, Curran, Mortensen, Ryan and Shumway2011). Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) reported planting green provided 7% to 24% drier and 0.7 C to 2.4 C cooler soil conditions at soybean planting, and increased soil moisture during dry periods after planting in Pennsylvania. However, Liebl et al. (Reference Liebl, Simmons, Wax and Stoller1992) found delaying cereal rye termination until planting in Illinois resulted in up to a 45% reduction in soybean stand and subsequent yield loss compared with conventional management. In contrast, Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) reported no stand reductions or effect on yield from planting green compared with early cover-crop termination. More information on the effectiveness of planting green as a weed suppression tool is needed.
Challenges in GR horseweed management require additional strategies. Fall-planted cereal cover crops may improve horseweed management; however, little is known about planting green for weed suppression. The objective of this research was to evaluate the effects of fall-planted cereal cover crops for management of GR horseweed. We compared GR horseweed suppression by winter wheat and cereal rye seeded at two recommended seeding rates. We also examined how termination time affected cover-crop biomass, residue persistence, and GR horseweed suppression.
Materials and Methods
Field experiments were conducted in commercial fields in Isabella County, MI, in 2018 (43.61°N, 84.88°W) and 2019 (43.63°N, 84.9812°W) and at the Michigan State University (MSU) Agronomy Farm in East Lansing, MI, in 2019 (42.69°N, 84.49°W). Sites were selected on the basis of GR horseweed escapes the previous season. The soil series in Isabella County were a Selfridge sand (loamy, mixed, active, mesic Aquic Arenic Hapludalfs) with pH of 6.4 and 2.2% organic matter in 2018 and a Wasepi loamy sand (coarse-loamy, mixed, semiactive, mesic Aquollic Hapludalfs) with pH of 5.2 and 2.2% organic matter in 2019. Soils at MSU were Conover loam (fine-loamy, mixed, active, mesic Aquic Hapludalfs) with pH of 5.7 and 3.0% organic matter.
In 2018, the experiment was established as a split-plot, randomized complete block design with three replications. In 2019, the experiment was established as a split-split plot randomized complete block design with three replications. Each plot measured 3 m wide by 9 m long. The main plot factor was cover crop, the subplot factor was cover-crop termination timing, and the sub-subplot factor in 2019 was POST herbicide. The main plots consisted of the following five cover-crop factors: (1) winter wheat seeded at a low rate of 67 kg ha−1 (WWL), (2) winter wheat seeded at a high rate of 135 kg ha−1 (WWH), (3) cereal rye seeded at a low rate of 67 kg ha−1 (CRL), (4) cereal rye seeded at a high rate of 135 kg ha−1 (CRH), and (5) no cover-crop control (NC). The subplots consisted of two cover-crop termination timings: either 1 wk before soybean planting (i.e., early termination) or 1 wk after soybean planting (i.e., planting green). Cover crops were terminated by applying glyphosate (Roundup PowerMAX; Bayer CropScience, St. Louis, MO) at 1.27 kg ae ha−1 plus ammonium sulfate (Actamaster; Loveland Products, Inc., Greeley, CO) at 2% w w−1. The sub-subplot factors in 2019 were (1) an effective POST herbicide application or (2) a noneffective POST herbicide application. The effective POST herbicide application consisted of glyphosate at 1.27 kg ae ha−1 plus dicamba (XtendiMax; Bayer CropScience, St. Louis, MO) at 0.56 kg ae ha−1 with a drift-reduction agent (Intact, Bayer CropScience, St. Louis, MO) at 0.5% w w−1. The noneffective POST herbicide application consisted of glyphosate at 1.27 kg ae ha−1 plus ammonium sulfate at 2% w w−1.
Main plots of ‘Wheeler’ cereal rye and ‘Sunburst’ winter wheat were sown in 19-cm rows using a no-till drill (Great Plains, Salina, KS) the fall before data collection. Dates for all field operations are listed in Table 1. Cover crops were terminated and subplots were established 1 wk before (i.e., early termination) or 1 wk after (i.e., planting green) planting soybean the following spring. GR and dicamba-resistant soybean ‘AG 26X8’ (Bayer CropScience, St. Louis, MO) was planted in 76-cm rows at a seeding rate of 383,000 seeds ha−1. POST herbicide applications were made approximately 5 wk after soybean planting (WAP) when emerged horseweed was approximately 10 cm tall. In 2018, the effective POST herbicide application was applied to all plots. In 2019, POST herbicide application was a sub-subplot factor and was established at this time. All herbicide applications were made using a tractor-mounted, compressed air sprayer calibrated to deliver 177 L ha−1 at 207 kPa of pressure through 11003 TTI nozzles (TeeJet Technologies, Spraying Systems Co., Wheaton, IL).
Table 1. Cover crop seeding and termination, soybean planting, POST herbicide application, and soybean harvest dates for the three experimental locations.
a Abbreviation: MSU, Michigan State University.
Data Collection
Two permanent 0.25 m2 quadrats were established in each no-cover plot to measure horseweed emergence throughout the growing season. Newly emerged horseweed plants were counted and removed from each quadrat weekly beginning on May 1, 2018, at the Isabella Co. site and April 15 at the 2019 sites.
Before early termination, aboveground cover-crop biomass, weed density, biomass were collected from two randomly placed 0.25 m2 quadrats in each early-termination plot. Measurements were collected using the same method in each planting green plot at the time of planting-green termination, approximately 2 wk later. In addition to horseweed, annual bluegrass (Poa annua L.), common chickweed [Stellaria media (L.) Vill.], shepherd’s purse [Capsella bursa-pastoris (L.) Medik.], and dandelion (Taraxacum officinale F. H. Wigg.) were present at the time of early-termination subplot establishment at all site-years. Spring whitlowgrass (Draba verna L.) and white campion (Silene latifolia Poir.) were also present at the 2019 locations. Common lambsquarters (Chenopodium album L.) had emerged by the time of planting-green termination at all site-years.
Cover-crop subsamples of biomass were analyzed for C:N ratios by A&L Great Lakes Laboratories, Inc. (Fort Wayne, IN) using a TruMac CNS Macro Analyzer (LECO Corporation, St. Joseph, MI). Horseweed density and biomass were collected from two random 0.25 m2 quadrats in all plots at the time of POST herbicide application and again prior to soybean harvest. At soybean harvest, fall-emerged horseweed rosettes were segregated from fully mature horseweed plants. Biomass samples were dried for approximately 7 d at 65 C and weighed.
Soil moisture was measured at the time of soybean planting with a Field Scout TDR 300 Soil Moisture Meter (FieldScout, Spectrum Technologies, Aurora, IL) by collecting five measurements per plot at a depth of 7.6 cm. When soybean reached the VE growth stage, percent groundcover was measured using line transects (Laflen et al. Reference Laflen, Amaemiya and Hintz1981) laid diagonally across each plot. The presence of cover-crop residue, horseweed, other weed, or no vegetation was recorded at every 30-cm point along a 9-m transect. Soybean populations were also assessed in all plots at this time. Soybean was harvested using a small-plot research combine (Massey-Ferguson 8XP, AGCO, Duluth, GA) equipped with a 1.5-m header. Yields were adjusted to 13% moisture.
Precipitation and temperature data were obtained throughout the growing season from the Michigan Automated Weather Network (http://www.agweather.geo.msu.edu/mawn/; Michigan State University, East Lansing, MI) stations located in Mecosta, Mount Pleasant, and East Lansing, MI, for Isabella Co. in 2018 (hereafter, Isabella 2018), Isabella Co. in 2019 (Isabella 2019), and MSU in 2019 (MSU 2019), respectively (Table 2). Daily soil temperature was collected using HOBO Pendant® Temperature/Alarm Data Loggers (Onset Computer Corp., Bourne, MA) placed in the soil at a depth of 2 cm.
Table 2. Monthly and 5-yr average precipitation at Isabella County, MI, in 2018 and 2019 and MSU in 2019. a
a Michigan Automated Weather Network (http://www.agweather.geo.msu.edu/mawn/), Michigan State University, East Lansing, MI.
b Abbreviations: MSU, Michigan State University; NA, not applicable.
c Precipitation up to early cover-crop termination.
d Precipitation up to soybean planting.
e Precipitation up to planting green cover-crop termination.
f The harvest month does not include rainfall after harvest.
g Total precipitation is rainfall total from planting until cover-crop termination, not including precipitation in December, January, February, and March and the total rainfall from soybean planting until harvest.
Statistical Analysis
Data analysis was conducted using lmer in R v. 3.6.2 (R Development Core Team 2019). The statistical model consisted of site-year (individual year and location), cover treatment, and termination time as fixed effects and replication nested in site-year and the interaction between cover treatment and replication nested within site-year as the random effects. Replications were used as an error term for testing the effect of site-year, and data were combined over site-year when the interaction of site-year and cover treatment or termination time was not significant. The cover treatment by replication interaction was used as an error term to test the effect of cover treatment. Also, preplanned contrasts were performed to compare cover-crop species pooled over cover-crop seeding rates and cover-crop seeding rates pooled over cover-crop species. Data for horseweed density and biomass at harvest and soybean yield were analyzed separately by POST herbicide treatments. Normality assumption was checked by examining histogram and normal probability plots of the residuals. Unequal variance assumption was assessed by visual inspection of the side-by-side box plots of the residuals followed by the Levene test for unequal variances. In cases of marked deviations from normality, the data were log-transformed and additional analyses were performed using the transformed data. For all experiments, treatment means were separated using Fisher protected LSD at α ≤ 0.05.
Results and Discussion
Horseweed Emergence
Horseweed began emerging on April 25 at the 2019 sites and May 14 at Isabella 2018 (Figure 1). Peak horseweed emergence occurred 2 to 3 wk before early termination at all 3 site-years and all horseweed plants exhibited a summer annual life cycle. The period of peak horseweed emergence was earlier at the MSU site in 2019 compared with the Isabella County locations (Figure 1). Greater than 80% of total emergence occurred during 1 wk at each of these locations. Peak emergence occurred when 50 to 100 growing degree days (GDDs) (base, 10 C) were accumulated at each site-year (Figure 2). However, horseweed continued to emerge at each location throughout the spring and early summer. Late emergence in June at Isabella 2018 occurred in the weeks after heavy rainfall (Figure 3). Similarly, late emergence occurred at Isabella 2019 and MSU 2019 following rain events (Figures 4 and 5). Our data suggest that initial emergence depends on accumulated GDDs and adequate soil moisture, whereas late emergence follows rainfall events. Horseweed emerged in September and October at each site; these seedlings were thought to be from newly shed seed.
Figure 1. Cumulative horseweed emergence as a percent of the seasonal total by date at the Isabella Co. 2018, Isabella Co. 2019, and MSU 2019 sites.
Figure 2. Cumulative horseweed emergence as a percent of the seasonal total by growing degree day (base 10 C) at the Isabella Co. 2018, Isabella Co. 2019, and MSU 2019 sites.
Figure 3. Weekly horseweed emergence and precipitation at the Isabella Co. 2018 site.
Figure 4. Weekly horseweed emergence and precipitation at the Isabella Co. 2019 site.
Figure 5. Weekly horseweed emergence and precipitation at the MSU 2019 site.
Cover-Crop Response Traits
Cover-crop biomass averaged over cover species at early termination was 1,240, 560, and 1,000 kg ha−1 for Isabella 2018, 2019, and MSU 2019, respectively (Table 3). Cover crops were sown approximately 3 wk earlier at the Isabella 2018 site; accumulated GDDs between cover-crop planting and early termination were 541, 315, and 326 (base, 4.4 C) for Isabella 2018, 2019, and MSU 2019, respectively (Table 1). Precipitation before early termination at Isabella 2018 was two times greater than at Isabella 2019 and MSU 2019 (Table 2). Soil-type differences between the 2019 sites may have contributed to differences in cover-crop biomass. At Isabella 2019, sandy soils with a lower water-holding capacity may have limited cover-crop growth. The MSU 2019 soil type was a loam, which generally has greater nitrogen availability because of the higher clay and silt content as well as higher soil organic matter (Hassink Reference Hassink1994), allowing greater biomass accumulation even though cover-crop planting was relatively late. Previous studies in Michigan and Iowa reported similar ranges (800 to 2,900 kg ha−1) of aboveground cereal rye biomass (Hill Reference Hill2014; Jahanzad et al. Reference Jahanzad, Barker, Hashemi, Eaton, Sadeghpour and Weis2016; Rogers Reference Rogers2017; Snapp et al. Reference Snapp, Swinton, Labarta, Mutch, Black, Leep and O’Neil2005).
Table 3. Interaction effect of termination time and cover-crop treatment, P values, contrasts in cover-crop dry biomass, C:N ratios, and GDDs accumulated at the time of cover-crop termination.
a Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
b Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; GDD, growing degree day; MSU, Michigan State University; NA, not applicable; NS, not significant; WWH, winter wheat, high; WWL, winter wheat, low.
c Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
d The main effect of termination time was significant for cover-crop biomass at the Isabella Co. site in 2018 and 2019; planting-green cover-crop biomass was greater than at early termination.
e Contrasts comparing cover-crop species pooled over seeding rate and termination time.
f Contrasts comparing seeding rates pooled over cover-crop species and termination time.
g GDDs (base, 4.4 C) accumulated from the timing of planting until termination.
Delaying termination 15 to 20 d by planting green resulted in an additional 227, 171, and 222 GDDs at Isabella 2018, 2019, and MSU 2019, respectively. Cover-crop biomass increased by 154% to 614% by delaying termination; average cover biomass at the time of planting-green termination was 3,870, 1,970, and 3,720 kg ha−1 at Isabella 2018, 2019, and MSU 2019, respectively (Table 3). Mirsky et al. (Reference Mirsky, Curran, Mortensen, Ryan and Shumway2011) reported 10-d incremental delays in termination resulted in 37% higher cereal rye biomass with each delay. Lower initial biomass and fewer GDDs between termination timings resulted in a smaller increase in cover biomass at Isabella 2019 compared with the other site-years. The planting green biomass produced at Isabella 2018 and MSU 2019 was similar to that reported by Hayden et al. (Reference Hayden, Brainard, Henshaw and Ngouajio2012), who found cereal rye produced 3,300 to 5,870 kg ha−1 when planted in early September and terminated in mid to late May in Michigan. At MSU 2019, there was an interaction between cover-crop treatment and termination time. In general, cover-crop biomass was higher for planting-green treatments. However, CRH terminated early produced similar biomass to WWL with the planting-green termination. Combined over termination time and seeding rate, cereal rye produced 31% and 53% more biomass than winter wheat at Isabella 2018 and MSU 2019, respectively (Table 3). Other researchers have reported more biomass with cereal rye compared with winter wheat (Cornelius and Bradley Reference Cornelius and Bradley2017; Haramoto Reference Haramoto2019). However, winter wheat benefited from delayed termination more than cereal rye at MSU 2019, increasing biomass 452% and 216%, respectively. Seeding rate had no effect on cover crop biomass, which is likely due to cereal cover crops compensating for lower seeding rates by tillering (Masiunas et al. Reference Masiunas, Weston and Weller1995).
When terminated early, cereal rye and winter wheat were at Feekes stage 6 at Isabella 2018 and Feekes stage 5 at the 2019 sites. Cover crops at the planting-green termination time reached Feekes stage 10.4 at Isabella 2019 and Feekes stage 10.5 at Isabella 2018 and MSU 2019. At Isabella 2018, cover-crop treatment and termination time did not affect the C:N ratio of the harvested cover crop biomass (Table 3). However, in 2019, all planting-green cover-crop treatments had a C:N ratio at or greater than 24:1, whereas the ratios in early-terminated cover crops were below 24:1 at the 2019 sites (Table 3). Cereal rye terminated in mid-April had a C:N ratio of 22:1 in Iowa (Jahanzad et al. Reference Jahanzad, Barker, Hashemi, Eaton, Sadeghpour and Weis2016). Delaying termination by 2 wk increased biomass and lignin content in cereal rye and winter wheat and increased the C:N ratio in our research. The ideal C:N ratio for microbes is 24:1; cover residues with C:N ratios greater than 24:1 decompose more slowly compared with residues with C:N ratios less than 24:1 (Jahanzad et al. Reference Jahanzad, Barker, Hashemi, Eaton, Sadeghpour and Weis2016; Odhiambo and Bomke Reference Odhiambo and Bomke2001; USDA NRCS 2011), providing potentially a longer period of weed suppression. Cereal rye generally had a higher C:N ratio compared with winter wheat, which is similar to previous findings (Ashford and Reeves Reference Ashford and Reeves2003).
Cover-crop treatment had no effect on daily soil temperature fluctuations. At Isabella 2018, soil temperature fluctuated less in planting-green treatments compared with early-termination treatments over 5 d around the time of soybean planting (data not shown). In general, soil temperatures within termination times were similar and differences never exceeded 2 C.
Cover crops had no effect on soil moisture at soybean planting, regardless of termination time, at the Isabella sites. In contrast, soil moisture at MSU 2019 followed a trend of increasing soil moisture with increased cover-crop biomass. Pooled over termination time, soil moisture was 2.6%, 2.9%, and 3.7% higher in WWH, CRL, and CRH plots, respectively, compared with NC controls (Table 4). However, soil moisture was 1.8% lower when planting green into cover crops compared with early-terminated covers. Soil moisture at planting can be reduced if cover crops are allowed more time to grow and accumulate biomass (Mirsky et al. Reference Mirsky, Curran, Mortensen, Ryan and Shumway2011). At the Isabella sites, low precipitation between terminations and coarse soil texture allowed water movement through the soil regardless of cover residue. The MSU 2019 site received more rainfall and had a loam soil. The presence of early-terminated cover-crop residue likely led to less surface evaporation and greater soil moisture retention by the loam soil. In contrast, evapotranspiration by planting green cover crops resulted in drier soils at MSU 2019. Similarly, Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) reported drier soils when planting green compared with terminating cover crops before planting soybeans.
Table 4. Soil moisture at 7.6-cm depth measured at the time of soybean planting and cover-crop groundcover at soybean growth stage VE.
a Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; NA, not applicable; WWH, winter wheat, high; WWL, winter wheat, low.
b Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
c Soil moisture reported as volumetric water content and measured as described in the text.
d Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
Groundcover was measured when soybean reached the VE growth stage (approximately 3 WAP) to determine if greater cover-crop biomass at termination and a higher C:N ratio would result in more persistent groundcover to suppress weeds, including GR horseweed, later in the season. Pooled over cover treatment, early-terminated and planting-green cover crops provided 31% and 56% groundcover, respectively (Table 4). Averaged over termination time, cereal rye and winter wheat provided 47% to 50% and 34% to 43% groundcover, respectively. In addition, cereal rye often lodged, creating more groundcover compared with winter wheat which remained upright (John A. Schramski). The mulch layer formed by cover residue provides weed suppression (Mirsky et al. Reference Mirsky, Ryan, Teasdale, Curran, Reberg-Horton, Spargo, Wells and Moyer2013), suggesting the potential for greater GR horseweed suppression in the cereal rye planting-green treatments.
Horseweed Suppression at Cover-Crop Termination
Horseweed density at the early termination timing varied greatly between site-years; densities in the NC plots were 1,845, 715, and 82 plants m−2 at Isabella 2018, 2019, and MSU 2019, respectively. Horseweed densities in NC plots were lower at the planting-green termination time with 748 and 251 plants m−2 at Isabella 2018 and 2019, respectively. In contrast, relatively low numbers of horseweed plants and other weed presence resulted in much greater horseweed density by the time of the planting-green termination at MSU 2019. At Isabella 2018, all cover-crop treatments, with exception of WWH, reduced horseweed density 46% to 56% compared with NC (Table 5). Similarly, previous studies have found cover crops reduced horseweed density 80% at the time of termination compared with NC (Wallace et al. Reference Wallace, Curran and Mortensen2019; Pittman et al. Reference Pittman, Barney and Flessner2019). At Isabella 2019, horseweed densities in the planting-green treatments were 66% lower compared with the early termination treatments (Table 5). However, cover crops did not reduce horseweed density compared with the NC at this site. Horseweed density differences between termination timings was likely attributed to intraspecies competition. At MSU 2019, neither cover treatment nor termination time had an effect on horseweed density.
Table 5. Main effects of cover treatment and termination time, and P values for horseweed density and biomass at the time of cover-crop termination.
a Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; NA, not applicable; WWH, winter wheat, high; WWL, winter wheat, low.
b Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
c Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
d The interaction effect for cover treatment and time was significant. Means followed by the same capital letter are not statistically different at α ≤ 0.05.
e Competition from other weeds between termination times is believed to have reduced biomass between terminations.
Similar to density, horseweed biomass at cover-crop termination varied among site-years and termination times. Pooled over termination time, cover crops reduced horseweed biomass 41% to 89% compared with the NC at Isabella 2019 (Table 5). These results are similar to those of Hayden et al. (Reference Hayden, Brainard, Henshaw and Ngouajio2012), who reported cereal rye reduced winter annual weed biomass 95% to 97% in Michigan. There was an interaction between cover treatment and termination time at MSU 2019 (P = 0.0033). With the exception of WWH terminated early, cover crops reduced horseweed biomass 81% to 88% compared with the NC terminated at the same respective time (Table 5). However, horseweed biomass in winter wheat terminated when planting green was similar to that of NC terminated early. Horseweed experienced rapid growth between terminations at this site-year and biomass increased 173% in the NC plots. As a result, the main effect of cover treatment was masked at MSU 2019.
Horseweed suppression varied by termination time at Isabella 2018. When combined over termination time, only WWL provided horseweed suppression compared with NC at Isabella 2018 (Table 5). When horseweed biomass data were analyzed separately by termination time, the early-termination cover-crop treatments reduced horseweed biomass by greater than 59% compared with the NC (data not shown). Applications of glyphosate were made to terminate cover crops and control other weeds. Thus, delaying termination by planting green allowed a longer period of growth for cover crops as well as other weed species. In NC plots, dandelion and common chickweed biomass increased by greater than 200% between the early and planting green terminations at Isabella 2018 (data not shown). Horseweed biomass in the NC plots at the planting-green termination was 82% lower compared with early termination at this site-year. Thus, dandelion and chickweed provided horseweed competition when cover crops were absent and termination was delayed. In addition, when planting green, cover crops reduced other weed biomass 88% compared with the NC plots at Isabella 2018 (data not shown). These findings suggest weed diversity was reduced at this site-year when cereal rye and winter wheat were left to compete with weeds. More diverse weed communities are less competitive and less prone to dominance by herbicide-resistant species, such as horseweed (Storkey and Never Reference Storkey and Neve2018). Therefore, cover-crop presence may increase the overall competitiveness of the weed community in certain scenarios.
With the exception of the planting-green treatment at Isabella 2018, we observed horseweed biomass reduction at the time of termination from cereal cover crops. Similarly, Wallace et al. (Reference Wallace, Curran and Mortensen2019) reported cereal rye alone or in mixtures reduced horseweed size and improved size uniformity at burndown. The effectiveness of the auxinic herbicides 2,4-D and dicamba is less consistent on large horseweed plants (Keeling et al. Reference Keeling, Henniger and Abernathy1989; Kruger et al. Reference Kruger, Davis, Weller and Johnson2010; Wiese et al. Reference Wiese, Salisbury and Bean1995). The ability of cereal rye and winter wheat to reduce biomass at termination could provide growers greater GR horseweed control at the time of burndown herbicide application.
Horseweed Suppression After Termination
POST herbicide applications were made approximately 5 WAP. At this time, cover treatment and termination time had no effect on horseweed density (Table 6). However, planting green into cover crops reduced horseweed biomass at the time of POST herbicide application 46% to 93% compared with NC (Table 6). Wallace et al. (Reference Wallace, Curran and Mortensen2019) reported no reduction in horseweed density at the time of POST herbicide application when using a cereal rye cover crop. In contrast, Mirsky et al. (Reference Mirsky, Curran, Mortensen, Ryan and Shumway2011) observed greater suppression of weeds 8 WAP from greater biomass-producing cereal rye varieties, earlier planting dates, and later termination dates, and Norsworthy (Reference Norsworthy2004) reported a 68% and 21% reduction in weed biomass 3 wk after corn emergence from cereal rye and winter wheat cover crops, respectively, compared with the NC. Reducing horseweed size at the time of POST herbicide application may improve herbicide effectiveness when managing GR horseweed.
Table 6. Interaction effect of termination time and cover treatment interaction, and P values for horseweed density and biomass at the time of POST herbicide application.
a Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
b Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; WWH, winter wheat, high; WWL, winter wheat, low.
c Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
Horseweed density and biomass collected before soybean harvest were separated by rosettes and inflorescent plants expected to produce viable seed. Horseweed emerging in July and August typically overwinter as rosettes and do not contribute to the seed bank that growing season (Loux et al. Reference Loux, Stachler, Johnson, Nice, Davis and Nordby2006). For this reason, only inflorescent horseweed density and biomass data are presented. At Isabella 2018, an effective POST herbicide application was made to all plots and no horseweed were present at the time of soybean harvest (Table 7). In 2019 sites, neither cover-crop treatment nor termination time affected horseweed biomass at soybean harvest when an effective POST herbicide application was made at both locations.
Table 7. Main effects of cover treatment and termination time, and P values for inflorescent horseweed biomass and density at the time of soybean harvest when an effective POST herbicide (glyphosate plus dicamba) was applied.
a Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; WWH, winter wheat, high; WWL, winter wheat, low.
b Inflorescent horseweed were not present at Isabella Co. in 2018; therefore, only horseweed biomass data from 2019 are presented.
c Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
d Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
In 2019, sub-subplots were created to measure the ability of cereal rye and winter wheat to suppress horseweed in the absence of an effective POST herbicide application. Consequently, horseweed density and biomass were greater in these plots. Cover crops did not reduce horseweed density compared with the NC when averaged over seeding rates and termination time (Table 8). However, horseweed biomass was 69% lower in the planting-green treatments compared with early-termination timing at MSU 2019. Similarly, biomass was reduced 86% in the planting-green treatments compared with early-termination treatments at Isabella 2019. However, because of variability in the biomass collected in planting-green treatments at Isabella 2019, a significant difference was not detected between termination times. A meta-analysis found that cover-crop residues often do not persist long enough to provided weed suppression throughout the season in long-season crops (Osipitan et al. Reference Osipitan, Dille, Assefa and Knezevic2018). Our data for 2 of 3 site-years generally supported this.
Table 8. Main effects of cover treatment and termination time, and P values for inflorescent horseweed biomass and density at the time of soybean harvest when a noneffective POST herbicide (glyphosate only) was applied.
a Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; WWH, winter wheat, high; WWL, winter wheat, low.
b Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
c Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
Soybean Establishment and Yield
Pooled over site-year and termination time, cover crops had no effect on soybean stand establishment compared with the NC (Table 9). However, soybean stands were higher in WWL compared with other cover-crop treatments, and soybean stand was similar between the early-terminated and planting-green treatments (Table 9). Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) observed no stand reductions from planting soybean into green cover crops. However, earlier research documented 10% to 35% soybean stand reductions following cereal rye cover crops (Liebl et al. Reference Liebl, Simmons, Wax and Stoller1992; Moore et al. Reference Moore, Gillespie and Swanton1994; Reddy Reference Reddy2001).
Table 9. Main effects of cover treatment and termination time, and P values for soybean stand and yield for plots treated with and without a POST application of dicamba.
a Abbreviations: CRH, cereal rye, high; CRL, cereal rye, low; MSU, Michigan State University; WWH, winter wheat, high; WWL, winter wheat, low.
b Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
c Isabella 2019 and MSU 2019 received POST herbicide applications of No POST (glyphosate only) or POST (glyphosate + dicamba; Isabella 2018 received a POST (glyphosate + dicamba) on all plots.
d Winter wheat and cereal rye were seeded at 67 and 135 kg ha−1 for the low and high seeding rates, respectively.
Cover-crop treatment had no effect on soybean yield at any site-year (Table 9). Termination time affected soybean yield under two circumstances. At Isabella 2018, all plots received an effective POST herbicide application, and soybean yield in planting-green covers was 30% greater than in early-terminated covers. When a noneffective POST herbicide was applied at Isabella 2019, soybean in the planting-green cover-crop treatments yielded 108% more than early-terminated cover crops (Table 9). Early-season weed suppression by cover crops before the time when a POST herbicide application would have been applied resulted in higher yields at these locations. Termination time differences were not detected at Isabella 2019 with a noneffective POST herbicide or at MSU 2019, regardless of POST herbicide. The effective POST herbicide application at Isabella 2019 made up for horseweed competition differences early in the season. Soybean yield was greater at MSU 2019 and horseweed competition was relatively low, resulting in no differences among cover-crop treatments or termination times. Similarly, Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) reported no soybean yield difference between termination times. Our findings suggest using cereal rye and winter wheat terminated at either time results in similar or greater soybean yield compared with NC.
In conclusion, cereal rye and winter wheat effectively suppressed GR horseweed early in the season in a no-till soybean system. At the times of cover-crop termination and POST herbicide application, cover crops suppressed horseweed by reducing biomass through resource competition rather than affecting horseweed emergence. Cereal rye produced more biomass and provided more groundcover than did winter wheat. However, horseweed suppression was similar between cover species and cover seeding rates throughout the season. Delaying cover-crop termination by planting green increased cover crop biomass, groundcover, and residue persistence. This ultimately led to greater horseweed suppression through the time of POST herbicide application. However, cereal cover crops alone were not effective at controlling horseweed until soybean harvest. More research is needed to explore how effective herbicides can be integrated with the practice of planting green into cover crops. Planting green into cover crops reduced soil moisture at planting at 1 site-year and did not negatively affect soybean stand. Soybean yield was greater when planting green compared with early-terminated cover crops at 2 site-years, likely due to greater early-season horseweed suppression. Cereal rye and winter wheat cover crops provide growers an additional strategy for GR horseweed management. Delaying cover-crop termination by planting green provides additional horseweed suppression through the time of a POST herbicide application.
Acknowledgements
We thank the Michigan Soybean Promotion Committee and Project GREEEN for supporting this research, and Gary Powell and Brian Stiles for their technical assistance. No conflicts of interest have been declared.
