Despite the benefits of reduced tillage for fuel savings and soil health, adoption by organic farmers has been limited due in part to the challenges created for weed and nitrogen management. The objective of our proposed research was to improve weed suppression and nutrient management in organic vegetable production by using a novel approach of alternating cover crop strips in zone tillage systems.
In the first year of the study we tested various cover crop spatial arrangements in combination with strip-tillage. N-fixing hairy vetch (Vicia villosa Roth) was planted in a strip directly in-line with future sweet corn rows (IR) to supply N directly to the crop. This was compared to the standard practice of planting vetch across the whole plot. Cereal rye (Secale cereale L.) was planted between crop rows and mowed and left on the soil surface to provide a mulch for weed suppression. Additionally, white clover (Trifolium repens L.) was frost-seeded in the between-row area to act as a living mulch for additional weed suppression and increased N. We found that while white clover emergence in the spring was high, cereal rye suppression of the white clover living mulch resulted in little white clover survival into the summer. In the preliminary trial, we did not see any significant differences due to hairy vetch arrangement, most likely due to poor vetch biomass.
In 2012 and 2013, we excluded the white clover living mulch to further examine how cereal rye and hairy vetch spatial arrangement impacts cover crop growth, as well as soil N, weeds, and sweet corn yields. Treatments in the second year consisted of tillage (conventional vs. zone till), cover crop spatial arrangement (segregated vs. mixed), and weed management (high vs. low). In both years, rye biomass was lower in the segregated spatial arrangement, but there was no significant difference in vetch biomass based on cover crop spatial arrangement. In 2012, total rye and vetch biomass was high, and when used as a mulch in the between-row of zone tillage, the rye and vetch mulch effectively suppressed weed growth. This resulted in greater sweet corn yields within zone tillage under low weed management. Under high weed management, in 2012 there was no significant difference in yields based on cover crop spatial arrangement or tillage. In 2013, we had lower cover crop biomass compared to 2012. Winter annual weeds established within the cover crop and were not terminated when mowed, resulting in greater weed biomass within the between-row of zone tillage. In 2013, zone tillage resulted in lower sweet corn yields in zone tillage under high weed management. In both 2012 and 2013 we saw a greater proportion of early season inorganic N availability within the crop row compared to between-row when rye and vetch were segregated strips. Strip intercropping of rye and vetch showed promise for targeting N availability to the crop, and future work will examine whether this resulted in increased N uptake by the crop.
Reduced tillage systems can provide a number of agronomic benefits including improved soil health, decreased fuel and labor costs, and soil water conservation. However, organic farmers rely heavily on tillage for weed control and increased nitrogen availability. The goal of this project is to evaluate whether these challenges may be alleviated through strip intercropping of a grass and legume cover crop within a zone tillage (ZT) system.
Zone (or strip) tillage is a form of conservation tillage that isolates soil disturbance to a narrow strip directly in row with crop establishment. This divides the cropping system into two distinct yet adjacent zones: tilled and untilled. The tillage zone enhances soil aeration and creates a finer seedbed into which the crop will be planted (Licht and Al-Kaisi, 2005). The untilled zone between-row (BR) maintains some of the benefits of no till, such as improved water infiltration, organic matter retention, and decreased erosion (Johnson and Hoyt, 1999). Zone-tillage enables a greater level of control over organic matter turnover. Incorporating crop residue in-row (IR) enables faster decomposition and mineralization of organic matter to increase nutrient availability to the crop. The between-row zone is left undisturbed, and when combined with a preceding cover crop, residue is preserved on the soil surface to provide a number of ecosystem services throughout the growing season, such as suppressing weed germination, reducing soil erosion and conserving soil moisture (Mohler and Teasdale, 1993; Unger and Vigil, 1998).
Grass and legumes are two functionally diverse classes of cover crops that provide unique ecosystem services to the cropping system. Cereal grass cover crops are capable of providing a large amount of biomass to be used as a mulch for weed suppression, however the carbon rich residues can immobolize N and reduce yields due to N deficiency. Legume cover crops are capable of providing large quantities of N via N fixation, but provide insufficient biomass for weed control. By segregating legumes and cereal grass cover crops into distinct zones (or strips), their functions can be targeted efficiently to maximize benefits to the crop.
Cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) are commonly used winter cover crops in northern climates, often planted in a uniform mixture. When arranged as a strip intercrop, hairy vetch is planted directly in line with the future cash crop to concentrate the N-rich residue in close proximity to the crop rooting zone. Cereal rye is planted between future cash crop rows to reduce the problem of N tie-up as well as potential interference with cash crop establishment. When combined with zone tillage, hairy vetch is incorporated prior to sweet corn planting, thereby reducing the potential for hairy vetch reemergence as a weed during the cropping season. Rye residue planted in the between-row area is mowed and left on the soil surface to reduce weed emergence and growth. We also proposed combining cereal rye and hairy vetch segregated strips with a white clover living mulch to provide N and additional weed suppression during the sweet corn growing season.
The objectives of this project were to evaluate the effect of vetch and rye alternating strips and zone-tillage on:
- Weed suppression and community composition within the in- and between-row environments.
- Cover crop contribution of N and soil N-dynamics.
- Yield and quality of organic sweet corn.
- Soil quality by utilizing short-term indicators of changes in soil health.
In summer 2011, we conducted a preliminary trial at the Kellogg Biological Station in Hickory Corners, MI to look at the effects of segregating rye and hairy vetch into strips, along with frost seeding a white clover living mulch into cereal rye, on sweet corn yield, weed emergence, and N dynamics. Treatments were arranged in a randomized complete block design and consisted of various cover crop species and spatial arrangements combined with strip tillage.
We used the information gathered from the 2010-2011 preliminary trial to further examine how cereal rye and hairy vetch spatial arrangement can impact cover crop growth, as well as soil N, weeds, and sweet corn yields. Treatments were organized in a split-split plot design with four replicates. Main plots were tillage (conventional vs. strip till), split plots were cover crop spatial arrangement (segregated vs. mixed, with a no cover crop contol), and split-split plots were weed management (high vs. low). Segregated cover crop strips consisted of two rows of rye alternated with two rows of hairy vetch (row spacing 7.5 inches). This was compared to the traditional practice of seeding a uniform mixed biculture of rye and hairy vetch. In both cover crop spatial arrangements we used the same overall seeding rate: 56 lbs of rye and 22 lbs of hairy vetch per acre. High weed management subplots were hand-weeded approximately weekly, while low weed management subplots were weeded only twice over the whole growing season. Variables examined included N availability, weed emergence and biomass, and sweet corn yields. To monitor N availability over the course of the summer, soil samples were taken approximately every 10-14 days, dried, ground, and a KCl extraction was performed to measure inorganic N. Seeds of common lambsquarters (Chenopodium album) were broadcasted over low weed management subplots to increase uniformity of the seedbank. Lambsquarters emergence was counted over the summer, and biomass was collected at three time points: right before each of the two weeding events, as well as at harvest. No additional nitrogen was added in this study.
Preliminary Trial Results:
In spring 2011 we frost seeded clover into cereal rye within the BR zone. White clover emergence in the spring was high, but cereal rye suppression of the white clover living mulch resulted in little white clover survival into the summer. After the first year resulting in little clover survival, we abandoned this idea and decided to just focus our efforts on examining the rye and vetch system.
Due to poor vetch biomass production in the 2010-2011 preliminary trial, we did not see significant differences in N availability or yields of sweet corn when vetch was segregated within the IR, compared to seeded across the entire plot. We also did not see significant differences in emergence of either common lambsquarters or giant foxtail. However, we were successful at terminating both the rye and hairy vetch cover crops by mowing. Additionally, not seeding rye within the IR zone increased strip tillage performance by removing the heavy rye residue and dense roots from the tillage zone. N availability was much higher in the IR of all ST treatments compared to the BR. We speculate this difference may have been enhanced by the lack of rye residue and N immobilization in the IR zone.
Main Trial Results: Rye and Vetch Spatial Arrangements and Zone Tillage
Cover crop biomass:
In both 2012 and 2013, there was no significant difference in hairy vetch biomass due to cover crop spatial arrangement (see Fig. 1). However, in both years we found significantly lower rye biomass when rye and vetch were segregated into strips. In 2013, we had lower rye and hairy vetch biomass compared to 2012 regardless of cover crop spatial arrangement. Unusually warm growing conditions in the spring of 2012 probably accounted for this difference.
Sweet corn yields and nitrogen availability under high weed management:
In 2012, under high weed management, sweet corn yields were not affected by either cover crop spatial arrangement or tillage (see Fig. 2). In 2013, there were significantly lower yields in ZT compared to CT. In 2012 and 2013, ZT combined with segregated rye and vetch strips resulted in a greater proportion of inorganic N concentrated within the crop row compared to the mixed rye-vetch and full-width conventional tillage (see Fig. 3). However, we saw lower season long N availability in ZT compared to CT in both the IR and BR of no corn subplots (see Fig. 4).
Sweet corn yields and weed biomass under low weed management:
In 2012, under low weed management we found greater sweet corn yields in ZT compared to CT (See Fig. 5). Higher yields were likely due to suppression of weed emergence and biomass by the rye and vetch mulch maintained between crop rows within ZT. In 2013 we had lower cover crop biomass compared to 2012, which allowed winter annual weeds such as red clover and corn chamomile to establish (see Fig. 6). The winter annual weeds were not terminated when cover crops were mowed, and led to significantly greater weed biomass and lower sweet corn yields under low weed management within ZT compared to CT. In both 2012 and 2013 we did not see a significant difference in weed biomass, emergence, or sweet corn yields based on cover crop spatial arrangement (data not shown).
Educational & Outreach Activities
Results of this research were presented at: 1)farmer field days at the Kellogg Biological Station in 2012 and 2013, 2) the organic session at the 2012 and 2013 Great Lakes Fruit and Vegetable Expo, 3) the 2013 Midwest Organic and Sustainable Education Service, and 4) the 2013 Michigan Organic Reprting Poster Session, 4) the 2013 annual meeting of tri-societes (ASA, CSA, SSSA), and 5) 2014 annual meeting of the Weed Science Society of America. Publications are currently in preparation.
Our results from suggest that segregating cover crops into strips may be a viable option to address some of the challenges of cover crops and reduced tillage in organic systems, such as reducing cereal rye N tie up and interference with crop establishment. We found a greater proportion of inorganic N concentrataed within the crop row when rye and vetch were strip intercropped. Results in 2012 have shown that hairy vetch can produce sufficient biomass and N to meet crop demand when segregated in cover crop strips. Additionally, we found that zone tillage in combination with a cover crop mulch can effectively reduce weed competition, as well as reduce time spent on weed management. Effective weed suppression in reduced till systems is dependent on cover crop biomass, and when cover crop biomass is not sufficient weeds may establish and suppress the crop.
Prior to advocating for farmer adoption of zone tillage and stripped intercropping of cover crops, there are a few existing challenges that need to be addressed. First, there is a lack of reduced tillage equipment readily available to diverse small-scale vegetable farms. For example, many small-diversified vegetable growers are planting on a wide range of row spacings and it may be uneconomical to purchase equipment on a fixed row spacing like a multi-row strip-tiller. The high residue mulch can make cultivation required to control escaped weeds difficult. We are currently conducting the next phase of research to address these challenges by testing new high residue cultivation methods for control of escaped weeds, as well as attempting to identify reduced tillage equipment suitable for farms of multiple scales. We are also currently conducting a survey of organic vegetable and field crop growers to gain a better understanding of their attitudes towards reduced tillage, and to identify constraints to adoption.
Areas needing additional study
Future research within this project will examine differences in crop N uptake, as well as N movement through the cropping system by looking at how tillage and cover crop placement effect N mineralization, leaching, and denitrification, as well as sweet corn root growth. We plan on conducting one more year of this study.
The next phase of this research will be to develop integrated weed management practices for organic reduced-tillage systems. Cover crop mulches have been shown to effectively suppress weeds in some cases, but are often insufficient to provide complete weed control. Integrated strategies that utilize an ecological approach are needed to enhance weed management in organic systems (Barberi, 2002), especially where RT is used (Brainard et al. 2013). High-residue cultivators can be used in combination with cover crop residue to suppress weeds in RT systems. However, these cultivators cannot readily control IR weeds without damaging the crop. In ZT systems, where cover crop residue is excluded from the IR-zone, specialized cultivation tools developed for standard tillage systems may be utilized, but this approach has not been previously evaluated.