Abstract
Zonal management of cereal–legume cover crop mixtures may help address weed and nitrogen management challenges common in organic reduced tillage systems. During a field study conducted over 3 years in Michigan, we evaluated the effects of cover crop management, tillage, and supplemental mulch on organically produced acorn squash (Cucurbita pepo). During the fall season before squash production, rye (Secale cereale L.) and vetch (Vicia villosa Roth) cover crop mixtures were sown in two distinct spatial arrangements: a “mixed planting,” in which seeds were sown in the same rows, and a “zonal planting,” in which vetch was planted only in the in-row zone and rye was planted only in the between-row zone of the subsequent squash crop. During the following spring season, cover crops were mowed, and four tillage and cover crop management combinations were established: full-width tillage with the mixed planting of rye–vetch (full-till mixed); strip-till with the same mixed planting (strip-till mixed); strip-till with the rye–vetch zonal planting (strip-till zonal); and strip-till with the zonal planting and additional rye mulch added between crop rows immediately after crop establishment (strip-till zonal plus rye). The strip-till mixed treatment resulted in yields equivalent to those of the full-till mixed treatment despite lower available nitrogen and greater early weed competition in some cases. Within strip-till treatments, zonal planting of rye–vetch provided no benefits relative to full-width planting (treatment 2 vs 3) and resulted in lower total cover crop biomass, a higher density of escaped weeds, and lower squash yields during 1 of 3 years. Supplemental rye mulch improved weed suppression and yields in strip-till zonal treatments and resulted in yields equivalent to those of the full-till mixed treatment in all years, but it provided no benefits relative to strip-till mixed. Our results demonstrate that strip-till organic squash production can produce yields equivalent to full-till production in Northern climates, but that zonal planting and supplemental mulch have limited benefits for addressing ongoing weed and nitrogen management challenges. Growers must weigh costs associated with these challenges against potential benefits for soil and pest regulating ecosystem services before adopting these agricultural conservation practices.
Despite the potential benefits of reduced tillage for improving soil health indicators, conserving moisture, and moderating temperature extremes, adoption among vegetable growers in northern climates has been limited by concerns regarding crop establishment, weeds, nitrogen (N) availability, and insect pests, particularly in organic production systems (Hoyt et al. 1994; Lowry and Brainard 2019a; Tillman et al. 2015). Weed and N management challenges of reduced tillage are particularly important in organic production systems in northern regions, where cover crop biomass for mulch is often insufficient for full-season weed suppression (Keene and Curran 2016; Mohler and Teasdale 1993; Ryan et al. 2011), and where reduced soil temperature caused by mulch have greater consequences for N availability and delayed maturity (Johnson and Hoyt 1999; Leavitt et al. 2011; Thorup-Kristensen 2006). Studies addressing these barriers suggest that the use of strip-tillage can often improve the establishment and growth of vegetable crops relative to no tillage, but that weed management and N availability can still be problematic relative to conventional full-width tillage (Brainard et al. 2013; Thomas et al. 2001; Thorup-Kristensen 2006; Übelhör et al. 2014).
Cucurbits, including pumpkins, winter squash, and zucchini (Cucurbita pepo), are economically important crops that are better suited for reduced tillage than many vegetables (Harrelson et al. 2008; O’Rourke and Petersen 2016). Their relatively large seed size and wide plant spacing facilitate establishment with minimal tillage, with reported benefits including improvements in soil conservation and moisture retention (O’Rourke and Petersen 2016), weed suppression (Harrelson et al. 2007; Rapp et al. 2004), fruit quality (Walters and Young 2008), promotion of beneficial insects (Appenfeller et al. 2022; Lewis et al. 2016; Shuler et al. 2005), and reductions in insect pests (Skidmore et al. 2019a, 2019b). However, reduced tillage of cucurbit crops has been reported by some studies to lower soil temperatures, reduce N availability, delay fruit maturation, and increase weed competition or management costs (Leavitt et al. 2011; Lilley and Sanchez 2016). When weeds are adequately controlled with herbicides, reduced tillage can result in equivalent or greater yields than full-tillage for pumpkins and squash (O’Rourke and Peterson 2016; Rapp et al. 2004; Walters et al. 2005). However, in organic cucurbit systems relying primarily on cover crop mulches for weed suppression, reduced tillage may reduce yields and profitability because of weed competition, poor crop establishment, or delayed maturity (Leavitt et al. 2011), especially when compared with full-tilled plasticulture production (Lilley and Sánchez 2016; Skidmore et al. 2019a, 2019b). The N availability may also be lower during the short-term in reduced tillage vegetable systems (Übelhör et al. 2014), resulting in lower yields or higher required N fertilizer rates for crops such as squash (Harrelson et al. 2008).
Various approaches have been evaluated to address weed and N management challenges in strip-till systems through adjustments in complementary cover cropping practices, including variations in the spatial arrangement of cover crops relative to future crop rows (Lowry and Brainard 2016; Maher et al. 2021). For example, several studies have evaluated the impact of segregated or zonal plantings of grass–legume cover crop mixtures in which the legume component is planted only in the in-row zone (where subsequent strip-tillage and crop planting will occur) and the grass component is sown only in the untilled between-row zone (Elhakeem et al. 2019; Gilley et al. 1997; Lowry and Brainard 2016, 2017, 2019b; Maher et al. 2021; Ofori and Stern 1987). The primary hypothesized benefits of these zonal plantings in reduced tillage systems include the following: improved N use efficiency because the N-fixing legumes are concentrated in the zone closest to early crop root growth; improved crop establishment because more recalcitrant grass residue is excluded from the zone where it might interfere with crop planting; enhanced weed suppression between crop rows because grass residues are generally more weed-suppressive than legumes; and lower total seed costs because more expensive legume seeds may be sown at a lower rate in segregated than in fully mixed plantings (Lowry and Brainard 2016). Zonal plantings may also improve the overall cover crop productivity or quality when interspecific competition between component species is greater than intraspecific competition (Elhakeem et al. 2019; Fujita et al. 1992; Lowry and Brainard 2016). However, observed benefits of zonal cover crop plantings for subsequent crops, including sweet corn (Lowry and Brainard 2016, 2017, 2019b) and cabbage (Maher et al. 2021), have been few and inconsistent. Zonal planting of rye–vetch reduced the ratio of carbon (C) to N of cover crop residue concentrated in the row, and increased soil inorganic N in the root zone of subsequent crops in some cases; however, it had little impact on the subsequent yield or weed suppression, partly because of the cover crop and N mixing across zones or the plasticity of crop and weed root N-foraging across zones (Lowry and Brainard 2017; Maher et al. 2021). Row spacing of winter squash is typically two- to three-times greater than that of other crops evaluated in zonal planting systems (Egel et al. 2019). We speculated that these wider spacing increases the likelihood of realizing weed and N management benefits from spatial segregation of residues through zonal planting.
This study focused on evaluating the effects of strip-tillage, zonal planting of cover crops, and supplemental mulch on cover crop productivity, N availability, weed density, and yield of organic winter squash (C. pepo). We hypothesized that N availability would be lower and weed density would be higher with strip-tilled treatments compared with conventional full-tilled treatments, but that the integration of spatially segregated cover crops and supplemental mulch would help overcome these constraints by increasing the total cover crop biomass and redistributing residue to favor crop N use efficiency and weed suppression.
Materials and Methods
Experimental design.
The study was conducted at the W.K. Kellogg Biological Station, Hickory Corners, MI (42.4058° N, 85.4023° W), from 2016 to 2019, in three separate fields. The fields were within 1000 m of each other and had the same soil type, which was a fine-to-coarse loam (mixed, mesic Typic Hapludalfs). For all three fields, squash was planted after an organic field corn–soybean–winter wheat rotation managed in accordance with typical organic grower practice. This included full tillage for all crops and mechanical cultivation during the growing season of field corn and soybean. After the wheat harvest, fields were disked and harrowed to incorporate crop residue and kill weeds before the establishment of experiments during early September of each year.
Experimental treatments.
During the fall season before squash production, rye (Secale cereale L.) and vetch (Vicia villosa Roth) cover crop mixtures were sown in the following two distinct spatial arrangements: a mixed planting, in which seeds were sown in the same rows, and a zonal planting, in which vetch was planted only in the in-row zone and rye was planted only in the between-row zone of the subsequent squash crop (Fig. 1). During the following spring season, cover crops were mowed, and the following four tillage and cover crop management combinations were established (Fig. 1): full-width tillage with the mixed planting of rye–vetch (full-till mixed); strip-till with the same mixed planting (strip-till mixed); strip-till with the rye–vetch zonal planting (strip-till zonal); and strip-till with the zonal planting and additional rye mulch added between crop rows immediately after crop establishment (strip-till zonal plus rye). For all treatments, rye and vetch cover crops were sown with a grain drill in early September during the year before squash production. For mixed cover crop plantings (treatments 1 and 2), cover crops were sown at a recommended rate of 62.7 kg/ha rye and 22.4 kg/ha vetch, with both species planted in the same rows. For zonal plantings (treatments 3 and 4), seeds were planted in separate rows, with two rows of vetch alternated with six rows of rye (Fig. 1). This arrangement resulted in the ∼38-cm zone containing vetch that was subsequently strip-tilled and planted with squash, and a 114-cm zone that was left untilled with rye residue on the soil surface. Therefore, the in-row and between-row zones represented ∼25% and 75% of the total area, respectively. Cover crop seeding rates within these zones corresponded with monoculture rates equivalent to twice the seeding rates in mixture: 125.6 kg/ha of rye in the between-row zone and 44.8 kg/ha of vetch in the in-row zone. At the whole-plot level, these seeding rates for zonal plantings corresponded to 94.2 kg/ha of rye (125.6 × 0.75) and 11.2 kg/ha of vetch (44.8 × 0.25). Therefore, at the whole-plot level, zonal plantings had a 50% higher seeding rate of rye (94.2 vs 62.7 kg/ha) and a 50% lower seeding rate of vetch (11.2 vs 22.4 kg/ha) compared with mixed plantings. After squash planting, additional rye mulch harvested from neighboring fields was spread by hand between crop rows in the strip-till zonal plus rye treatment. Rye mulch biomass additions in this treatment were ∼3000, 2000, and 6000 kg/ha in 2017, 2018, and 2019, respectively. These additions were designed to bring the total aboveground rye–vetch biomass (including that produced in place) to ∼8000 kg/ha, a quantity considered necessary to provide adequate weed suppression for several agronomic crops (Mohler and Teasdale 1993).
Illustration of rye–vetch planting arrangement, tillage, and supplemental rye mulch for each experimental treatment.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Illustration of rye–vetch planting arrangement, tillage, and supplemental rye mulch for each experimental treatment.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Illustration of rye–vetch planting arrangement, tillage, and supplemental rye mulch for each experimental treatment.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
The four experimental treatments were replicated six times in 24 plots arranged in a randomized complete block design (Fig. 2). Plots measured 9.1 × 10.7 m, with each containing six rows of squash at a spacing of 152 cm. In-row spacing between squash plants was ∼60 cm.
Field layout and treatment design. Each year (2017–19), experiments were conducted in separate fields with plots arranged in a randomized complete block design with six replications. The annotated aerial view of the experiment on 8 Aug 2019 is shown. The inset is a magnified view of one of six replicates.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Field layout and treatment design. Each year (2017–19), experiments were conducted in separate fields with plots arranged in a randomized complete block design with six replications. The annotated aerial view of the experiment on 8 Aug 2019 is shown. The inset is a magnified view of one of six replicates.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Field layout and treatment design. Each year (2017–19), experiments were conducted in separate fields with plots arranged in a randomized complete block design with six replications. The annotated aerial view of the experiment on 8 Aug 2019 is shown. The inset is a magnified view of one of six replicates.
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Field operations and management.
Major field operations and corresponding dates are provided in Table 1. Cover crops were planted with a grain drill during early September the year before squash planting. The following spring, cover crops in the full-till mixed treatment were chisel-plowed and disk-killed in late April, and fields were cultivated two to three times before planting to manage emerging weeds and prepare the seedbed. Cover crops in all strip-till treatments (treatments 2–4) were allowed to grow until rye anthesis in late May or early June, and then mowed and allowed to decompose for 2 to 3 weeks before strip-tillage. Strip-tillage was accomplished with a one-row Unverferth 120 strip-tiller equipped with row cleaners, offset disks, and a rolling basket to create a tilled zone ∼30 cm wide and 30 cm deep. In strip-till zonal treatments, tillage was centered on the in-row zone where vetch had been planted, without disturbing the between-row zone where rye had been planted (Fig. 1).
Major field operations by treatment and date, 2016–19.
During all 3 years, acorn squash (C. pepo var. turbinata) was planted during mid to late June. In 2017 and 2018, squash cultivar Honey Bear was direct-seeded. However, in 2018, after substantial feeding from 13-lined ground squirrels (Ictidomys tridecemlineatus), surviving squash plants were removed by hand and 3-week-old seedlings of Honey Bear and Taybelle cultivars that were available in limited quantities from other experiments were transplanted. In 2019, 3-week-old seedlings were transplanted on 27 Jun.
Fertilization consisted of soybean meal (7–1–1 N–P–K, non-GMO soybean meal; Zeeland Farm Services Inc., Zeeland, MI) broadcast at 1400 kg/ha before preplant strip-tillage or cultivation each year for all treatments. This rate was intended to meet crop phosphorus (P) and potassium (K) demands based on the soil test nutrient analysis, and to provide most crop N requirements after potential legume cover crop N inputs based on the assumption that ∼75% of the 98 kg N/ha applied in the soybean meal would be available during the year of application (Carlson et al. 2020; Sexton and Jemison 2011; Sullivan et al. 2019). During all years, insect arthropod pests were monitored, and their densities were found to be below the typical thresholds that would have justified pesticide application. The abundances of beneficial arthropod species, including pollinators, parasites, and epigeal natural enemies, were also monitored within all plots using detailed sampling methods and results reported by Appenfeller et al. (2022). To evaluate possible treatment effects on disease, no protective fungicides were applied; however, the disease incidence was low during all years except 2019, when late-season powdery mildew (Erysiphe cichoracearum) occurred in all plots regardless of treatment.
To help manage weeds, all plots were mechanically cultivated with different tools and timings used for the full-till treatments compared with the strip-till treatments (Table 1). Between-row weeds were mechanically cultivated as needed with a rolling cultivator (Model CS; Hillside Cultivator Co, Lititz, PA) for strip-till treatments and with standard “s-tine” cultivating sweeps for the full-till treatments. For strip-till treatments, between-row cultivation occurred two to three times and was restricted to the ∼30-cm band adjacent to the in-row zone, which consistently had the most weeds. In-row weeds in all treatments were managed with two to three mechanical cultivation events with finger weeders. In 2017 and 2018, 25-cm-diameter finger weeders were mounted on a spring-loaded floating arm attached to a rear-mounted manually steerable toolbar (Argus; KULT-Kress LLC, Gordonville, PA). In 2019, finger weeders were belly-mounted under a cultivating tractor to a spring-loaded floating arm (Tilmor LLC, Dalton, OH). Finger weeders were calibrated to maximize weed mortality with minimal crop damage with tips overlapping by ∼2 cm, moderate down-pressure, and a working speed of 3 to 4 kph. After cultivation, plots were hand-weeded to minimize late-season competition and seed production from weeds escaping cultivation.
Cover crop and soil sampling.
Weed evaluation.
Winter weed biomass during cover crop growth from the same quadrants was evaluated at the same time as the cover crop biomass evaluation. All weeds from each plot were clipped at the soil surface, combined from both quadrants in each plot zone, dried, and weighed. Weed density after squash planting and early cultivation events, but before hand-weeding, was evaluated by species in two 50-cm × 50-cm quadrants in each plot on 7 Jul 2017, 25 Jul 2018, and 11 Jul 2019.
Squash growth and yield.
Acorn squash was harvested in late September during each year. At harvest, fruit was separated into marketable and nonmarketable categories, counted, and weighed. In 2017 and 2018, the whole dry shoot biomass was estimated based on a random subsample of three whole plants clipped at the soil surface, with all fruit removed and dried. In 2019, foliar decay caused by powdery mildew damage was extensive at harvest; therefore, earlier drone images were used to evaluate the shoot development and N content. Images were collected from 18 m above ground on 5 Aug 2019, using a Phantom 4 Pro quadcopter (DJI Technology Co., Ltd., Shenzhen, China) with an integrated 20-megapixel natural color (red, green, blue) camera and a RedEdge-MX multispectral sensor (MicaSense, Inc., Seattle, WA). Orthomosaics were produced using Pix4Dmapper photogrammetry software (Pix4D Inc., Prilly, Switzerland). Experimental plot boundaries were digitized with a 1-m inward buffer using QGIS (version 3.2) (QGIS.org 2021). Two vegetation indices were calculated from the multispectral data, soil-adjusted vegetation index (Huete 1988), and normalized difference red edge index (NDRE) (Barnes et al. 2000). Pixels within each plot were classified as either vegetation or background based on threshold soil-adjusted vegetation index values reported by Otsu (1979). Then, the vegetation fraction was calculated (vegetation pixels divided by total pixels) as a measure of relative plant growth analogous to shoot biomass. The mean NDRE index values for plots were used as an integrated measure of both plant cover and spectral quality (greenness). Previous studies have demonstrated strong correlations between the NDRE and crop N status (Kanke et al. 2012).
Statistical analysis.
The fixed effects of the cropping system (four tillage × cover crop treatments) on cover crop biomass, weed density, soil N, crop biomass, remote sensing-derived vegetation fraction and NDRE index, and crop yield were evaluated using generalized linear mixed model procedures of SAS (PROC MIXED version 9.4; SAS Institute, Cary, NC). Treatment was a fixed effect and replicate was a random effect. Because of weather-related year × treatment interactions for the cover crop, weed, N, and crop responses, years were analyzed separately. Data were evaluated to determine assumptions of normality and equal variance, and weed abundance data were square-root-transformed as necessary to improve model assumptions. When treatment effects were significant, means were separated using Tukey’s honestly significant difference (α = 0.05).
Results
Cover crop biomass.
The total cover crop biomass based on the whole plot varied by treatment during all three years, ranging from as little as 1000 to 3000 kg/ha for the early-ended plantings in the full-till mixed treatment to 6000 to 7000 kg/ha in the strip-till mixed and zonal treatments in 2017 and 2018 (Fig. 3). For mixed plantings, delayed termination (treatment 1 vs 2) (Fig. 1), resulted in greater rye, vetch, and total biomass during all 3 years (Fig. 3). Within strip-till treatments, zonal planting (treatment 2 vs 3) (Fig. 1) reduced vetch biomass on a whole-plot basis during all 3 years, but it had no effect (2017), increased (2018), or reduced (2019) the whole-plot rye biomass (Fig. 3). Total cover crop shoot biomass was either unaffected (2018) or reduced (2017 and 2019) in zonal compared with mixed plantings.
Mean ± SEM whole-plot shoot biomass of cover crops in (A) 2017, (B) 2018, and (C) 2019. In strip-till treatments, the whole-plot biomass represents the weighted average of biomass samples taken from in-row and between-row zones (see Eq. [1]). Significant pairwise differences among treatments are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Mean ± SEM whole-plot shoot biomass of cover crops in (A) 2017, (B) 2018, and (C) 2019. In strip-till treatments, the whole-plot biomass represents the weighted average of biomass samples taken from in-row and between-row zones (see Eq. [1]). Significant pairwise differences among treatments are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Mean ± SEM whole-plot shoot biomass of cover crops in (A) 2017, (B) 2018, and (C) 2019. In strip-till treatments, the whole-plot biomass represents the weighted average of biomass samples taken from in-row and between-row zones (see Eq. [1]). Significant pairwise differences among treatments are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
The effect of zonal planting (treatment 2 vs 3) (Fig. 1) on the cover crop shoot biomass within zones (between-row vs in-row) varied by year and species (Fig. 4). As expected, within zonal plantings, the between-row zone had lower vetch biomass but greater rye biomass and total biomass compared with the in-row zone. Compared with the mixed plantings, zonal plantings had greater between-row rye biomass during 2 of 3 years but greater in-row vetch biomass in 2019. Zonal planting reduced the total in-row cover crop biomass by 60% to 70% compared with mixed plantings during all 3 years, but there were inconsistent effects on the total between-row cover crop biomass (Fig. 4).
Mean ± SEM cover crop shoot biomass within zones (between-row and in-row) for strip-till treatments in (A) 2017, (B) 2018, and (C) 2019. For the zonal planting, the cover crop biomass within each zone (between-row and in-row) is presented separately. For the mixed planting, cover crops were planted uniformly across the plots; therefore, biomass did not differ by zone. Significant pairwise differences are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Mean ± SEM cover crop shoot biomass within zones (between-row and in-row) for strip-till treatments in (A) 2017, (B) 2018, and (C) 2019. For the zonal planting, the cover crop biomass within each zone (between-row and in-row) is presented separately. For the mixed planting, cover crops were planted uniformly across the plots; therefore, biomass did not differ by zone. Significant pairwise differences are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Mean ± SEM cover crop shoot biomass within zones (between-row and in-row) for strip-till treatments in (A) 2017, (B) 2018, and (C) 2019. For the zonal planting, the cover crop biomass within each zone (between-row and in-row) is presented separately. For the mixed planting, cover crops were planted uniformly across the plots; therefore, biomass did not differ by zone. Significant pairwise differences are indicated by different capital letters for total biomass, by small black letters for vetch biomass, and by small white letters for rye biomass (P < 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Soil inorganic nitrogen.
Early cover crop termination and tillage in the full-till mixed treatment resulted in two- to three-times greater concentrations of preplant soil inorganic N relative to strip-till treatments in 2017 and 2018, but no difference was observed in 2019 (Table 2). At the time of soil sampling, the cover crops had been ended for 2 to 3 weeks in strip-till treatments, but strip-tillage had not yet been performed. Despite differences in the cover crop biomass between zones (Fig. 4), zonal planting only resulted in detectable differences in the preplant soil inorganic N concentration between zones during 1 of 3 years (Table 2). In 2019, on average, the zonal planting treatments had a 44% higher in-row soil N concentration compared to the between-row soil N concentration before strip-tillage.
Mean ± SEM preplant soil inorganic nitrogen (N) concentration (mg NO3−-N + NH4+-N kg−1 soil) across four tillage and cover crop treatments and two row locations. Pairwise differences among treatments are indicated by different lowercase letters within columns. When a significant treatment × location interaction was present (2019 only), pairwise differences between locations (BR vs IR) within treatments are indicated by different uppercase letters across rows. BR = between row; IR = in row.
Weeds.
Weed biomass at the time of cover crop termination was less than 300 kg/ha in all treatments during all years and was unaffected by the planting spatial arrangement (data not shown). Weed density after squash establishment and early mechanical cultivation (but before hand-weeding), was higher in strip-till mixed and strip-till zonal treatments relative to the full-till mixed treatment in five of six cases (Fig. 5). Dominant species present in strip-till treatments in 2017 and 2018 included mostly winter annuals and perennials (Trifolium pratense, Anthemis arvensis, Rumex crispus, Conyza canadensis). However, in 2019, the summer annual grass species, including Setaria faberii, Echinochloa crus-gali, and Digitaria sanguinalis, were dominant. Zonal planting of cover crops had no detectable effect on weed density (Fig. 5; strip-till zonal vs strip-till mixed), but it resulted in higher densities of several competitive weeds, including an approximately three-fold increase in T. pratense density in 2017 (P < 0.01) and a two-fold increase in A. arvensis density in 2018 (P = 0.03). The addition of supplemental rye mulch had no effect on the total weed density in 2017 or 2018, but it reduced the weed density by 76% in 2019, largely through its effects on summer annual grasses (P = 0.03).
Weed density. Mean ± SEM density of Trifolium pratense, Anthemis arvensis, Rumex crispus, Conyza canadensis, and various grass species (Setaria faberii, Echinochloa crus-gali, and Digitaria sanguinalis) in (A) 2017, (B) 2018, and (C) 2019. Significant pairwise differences in total weed density between treatments are indicated by different letters (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Weed density. Mean ± SEM density of Trifolium pratense, Anthemis arvensis, Rumex crispus, Conyza canadensis, and various grass species (Setaria faberii, Echinochloa crus-gali, and Digitaria sanguinalis) in (A) 2017, (B) 2018, and (C) 2019. Significant pairwise differences in total weed density between treatments are indicated by different letters (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Weed density. Mean ± SEM density of Trifolium pratense, Anthemis arvensis, Rumex crispus, Conyza canadensis, and various grass species (Setaria faberii, Echinochloa crus-gali, and Digitaria sanguinalis) in (A) 2017, (B) 2018, and (C) 2019. Significant pairwise differences in total weed density between treatments are indicated by different letters (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Crop growth and yield.
No treatment effects on acorn squash yields were detected in 2017 or 2019 (Fig. 6). In 2018, yields of both squash cultivars were 40% to 50% lower in the strip-till zonal treatment compared with the full-till mixed treatment, but no differences between full-till mixed and other strip-till treatments were detected. Squash plant growth, as estimated by the shoot dry biomass in 2017 and 2018 and by the vegetation fraction in 2019, followed a similar pattern (Table 3). No significant differences were detected in 2017 or 2019; however, in 2018, the shoot biomass was 43% lower in the strip-till zonal treatment compared with the full-till mixed treatment. Despite there being no significant differences in yield in 2019 (Fig. 6), the full-till mixed treatment had a significantly higher NDRE vegetation index than that of all strip-till treatments in 2019.
Crop yield. Mean ± SEM yield of C. pepo in (A) 2017, (B) 2018, and (C) 2019. Bars with different shades indicate the two cultivars that were used in the experiments. Significant pairwise differences between treatments are indicated by different letters within the same cultivar (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Crop yield. Mean ± SEM yield of C. pepo in (A) 2017, (B) 2018, and (C) 2019. Bars with different shades indicate the two cultivars that were used in the experiments. Significant pairwise differences between treatments are indicated by different letters within the same cultivar (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Crop yield. Mean ± SEM yield of C. pepo in (A) 2017, (B) 2018, and (C) 2019. Bars with different shades indicate the two cultivars that were used in the experiments. Significant pairwise differences between treatments are indicated by different letters within the same cultivar (α = 0.05).
Citation: HortScience 58, 2; 10.21273/HORTSCI16863-22
Mean ± SEM acorn squash (‘Honey Bear’) dry weight (g per three plants) at harvest (2017, 2018), remotely sensed crop vegetation fraction (Veg Frac.), and normalized difference red edge (NDRE) vegetation index in Aug 2019 across four tillage and cover crop treatments. Differences among treatments are indicated by different letters (P < 0.05).
Discussion
As anticipated, strip-tillage after standard mixed rye–vetch cover crops resulted in challenges with both N and weed management, consistent with previous studies. Observed reductions in the potential N availability in strip-till relative to full-till (Table 2) treatments were expected because of the larger amounts of higher C:N rye biomass in the later-terminated cover crop mixtures (Fig. 3) and reduced mineralization from organic residues and fertilizer left on the surface instead of being incorporated (Dou et al. 1994; Grandy and Robertson 2006). Higher observed weed densities in strip-till treatments compared with full-till treatments (Fig. 5) were also expected and consistent with those of several previous studies with similar levels of rye–vetch residue (Maher et al. 2021). In strip-till mixed treatments, the rye–vetch biomass ranged from ∼4000 to 6500 kg/ha, which was consistent with the results of previous studies that examined rye–vetch productivity in northern climates (Keene and Curran 2016; Mohler and Teasdale 1993; Ryan et al. 2011); however, it was considerably less than the 8000 kg/ha of rye–vetch mulch considered necessary to effectively suppress weeds in agronomic crops such as corn or soybeans (Mirsky et al. 2013; Teasdale and Mohler 1993; Teasdale and Mohler 2000).
Contrary to expectations, zonal planting of rye–vetch cover crops provided little or no benefit for N management or weed management in strip-till treatments (strip-till mixed vs strip-till zonal). Zonal planting was hypothesized to increase the total cover crop productivity, improve the efficiency of crop N utilization by concentrating low in-row C:N legume residues, and improve weed suppression by concentrating high between-row C:N rye residues (Lowry and Brainard 2016, 2017; Maher et al. 2021). However, compared with mixed planting, zonal planting reduced vetch biomass at the whole-plot level during all 3 years (Fig. 3) and resulted in lower total cover crop productivity during 2 of 3 years (Fig. 3). Because zonal plantings had lower vetch seeding rates at the whole-plot level, reductions in whole-plot vetch biomass are not surprising. However, the anticipated benefits of concentrating vetch biomass in the in-row zone were not realized; compared with the mixed planting, zonal plantings resulted in increased in-row vetch biomass (Fig. 4) and soil N availability (Table 3) only in 2019. Furthermore, although zonal planting increased rye biomass between rows during 2 of 3 years (Fig. 4), this did not translate to expected reductions in weed density (Fig. 5). Previous studies that evaluated zonal planting of rye–vetch mixtures in crops grown in relatively narrow rows (76 cm) also showed inconsistent effects on N availability and weed suppression, partly because of the mixing of shoot tissue during flail mowing and strip-tillage and the sprawling growth of vetch (Lowry and Brainard 2017; Maher et al. 2021). We anticipated that the wider row spacing (152 cm) of winter squash would allow for more defined separation of cover crop shoot residues between zones and result in larger impacts of zonal plantings on the efficiency of N and weed management. However, these potential benefits were not observed. In addition to the inconsistent effects of zonal planting on cover crop distribution across zones, the expansive lateral root growth of winter squash (Weaver and Bruner 1927) may have limited the potential to derive benefits from zonal planting despite the greater distance between rows than that of most other vegetables.
Supplemental rye mulch placed between crop rows to attain ∼8000 kg/ha of residue was expected to improve weed suppression and crop growth with strip-tillage. However, supplemental rye mulch provided a detectable improvement in weed suppression only in 2019 (Fig. 5). The supplemental mulch effect was likely greatest in 2019 because of low cover crop biomass production that year (Fig. 3) and the prevalence of summer annual grass weeds (Fig. 5), which are likely more sensitive to spring applications of mulch than larger established winter annual and perennial species. These results suggest that the suppression of overwintering weed species (e.g., R. crispus and T. pratense) in organic reduced tillage vegetable systems require more than the 8000 kg⋅ha−1 of total mulch residue in the zonal strip-till plus rye treatment. Although heavy mulch applications may be justifiable for small-scale growers of high-value organic crops such as onions (Brown et al. 2019), they are likely impractical for the larger-scale production of lower-value crops. Beyond mulching, organic growers may improve the potential success of reduced tillage production through longer-term ecological strategies that target weak points in the life cycle of winter annual and perennial weeds. For example, because problematic winter annual and perennial species are better adapted to rotations involving winter wheat than those with strictly summer annual crops (Brainard et al. 2008), our system may have performed better in crop rotations dominated by summer annual vegetables. Additional strategies aimed at improving cover crop biomass production and weed suppression—including adjustments in cover crop cultivars, planting methods, and fertilization strategies—may be helpful for improving the performance of organic reduced tillage systems (Mirsky et al. 2013).
Despite challenges with both weeds and N availability in strip-till treatments, crop growth and yields were not detectably reduced compared with the full-till mixed treatment, with the exception of the strip-till zonal treatment in 2018. Comparable yields in strip-till may be explained by the fact that supplemental N sources (soymeal) and weed management practices (finger-weeding and hand-weeding) minimized the potential negative effects of weeds and N deficiency on crop growth. Costs associated with these practices—especially for hand-weeding—must be weighed against potential benefits to evaluate their potential economic impact. Alternatively, comparable yields of strip-till and full-till treatments may have been attributable to benefits from strip-till unrelated to weeds or fertility that influenced yield. For example, previous studies have shown short-term benefits of surface mulch in strip-till, such as improving soil moisture retention, reducing temperature extremes, and regulating insect or disease pests (Appenfeller et al. 2022; Haramoto and Brainard 2012; Lowry and Brainard 2017; Mochizuki et al. 2007; Power et al. 1986; Quinn et al. 2016; Rivers et al. 2017; Tamburini et al. 2016). Maintenance of short-term crop yields in strip-till treatments compared with full-till treatments is essential for grower adoption and realization of benefits associated with long-term soil health. However, our results are consistent with those of previous studies that documented the continued need to adjust weed and soil management practices to compensate for N and weed constraints under strip-till. In organic production systems, these adjustments are likely to include increased sources of N fertility (more productive legume cover crops or supplemental N fertilization), integration of novel cultural and mechanical weed management strategies, and continued implementation of longer-term ecological approaches aimed at balancing weed and soil management tradeoffs. Although these approaches may be practical and economically justifiable in some cases, they may also entail increased costs that need to be assessed relative to both environmental and economic benefits before widespread adoption can be expected.
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