The use of cover crops is an integral part of sustainable field cropping systems, with well-documented benefits to soil health, pathogen suppression, and sustaining crop yields. However, cover crops have been utilized to a lesser extent in year-round, intensive high tunnel cropping systems. This is due to both biophysical constraints in these systems, such as short windows for non-cash crops, as well as economic constraints, such as potential lost revenues from cash crops sacrificed by growing cover (non-cash) crops. This situation presents a set of trade-offs common to sustainable agriculture systems – balancing production with other ecosystem services enhanced by conservation practices that take lands temporarily out of cash crop production. High tunnel systems offer a compelling example of such trade-offs. These unheated greenhouses are passively heated and ventilated by physical manipulation of the plastic covering the structure. These highly productive, predominantly soil-based systems have been growing in popularity due to cost share programs and the growth of year-round market opportunities. In the Southeast, tunnels afford producers the ability to extend warm-season crops approximately one month earlier and later, and allow production of cool-season crops throughout the winter months. Although this may increase the availability of locally produced foods, the intensity of these production systems leave little time for integrating cover crops and/or fallow periods typically associated with sustainable agriculture practices. We approach this project from an interdisciplinary, systems-oriented perspective via three main objectives. Objective 1 is a survey of high tunnel growers in the southern region to identify current practices and potential temporal windows in which cover crops may be integrated. Objective 2 is focused on the trade-offs discussed above through evaluating the economic costs and ecosystem service benefits of cover crops in tomato high tunnel rotations. This multi-state field experiment will integrate cover crop treatments into a spring-planted tomato production system on research farms in Kentucky and Tennessee. Ecosystem services evaluated include soil parameters associated with soil health, nitrogen leaching, weed seed bank management, and yields. Economic parameters will include input costs and returns in both standard and cover crop-based rotations, as well as opportunity costs in cover crop rotations. Objective 3 is focused on identifying novel cover crops for high tunnels. Specifically, we will evaluate a suite of warm- and cool-season cover crops for rapid biomass production, weed suppression, and other desirable traits identified by our participating farmers. This multi-state, on-farm trial will include three farms each in Georgia, Kentucky, and Tennessee. Climatic data collected at individual sites will be used in cover crop modeling activities to improve our understanding of cover crop response to the high tunnel environment, so that results and recommendations may be extended throughout the southern region. Project results will be communicated to the academic community through publication of peer-reviewed journal articles and presentation at academic conferences, to the grower community via outreach with the multi-state Center for Crop Diversification (http://www.uky.edu/Ag/CCD/) and UT Organic and Sustainable Crop Production Program (http://organics.tennessee.edu), presentations at state and regional producer conferences, and on-farm field days with participating growers.
We approach this project from an interdisciplinary, systems-oriented perspective via three main objectives.
Objective 1: Characterize high tunnel cropping systems and existing cover crop use in high tunnels in the Southern SARE Region through a high tunnel grower survey. Early project efforts include development of a survey to gather management practice data for high tunnels to be used to characterize management techniques regionally, establish baseline practices, and include market information that is in high demand in local and regional food systems including early and late season pricing for tunnel-produced crops.
Objective 2: Evaluating the ecosystem services and economic costs of using cover crops in high tunnel crop rotations. This two-year, two-site study will be conducted on the University of Kentucky Horticulture Research Farm and the University of Tennessee East Tennessee Research and Education Center. Incorporating winter cover crops into tomato production systems, this study will compare economic and ecological costs and benefits of cover crop treatments with a continuously cropped (year-round) tomato-lettuce rotation. The cover crop treatments include a cool season grass (alone), a cool season legume (alone), and a cool season grass-legume mixture (together). Cool season cover crops were selected based on two years of previous experience with winter cover crops in high tunnels in Kentucky (Jacobsen, unpublished data) where they demonstrated tolerance to late sowing, had adequate but not excessive biomass production, and were effectively terminated with mowing and tillage.
Broadly, the ecosystem services evaluated include parameters associated with soil health, nitrogen leaching losses, weed seed bank dynamics, and yields. Economic parameters evaluated will include costs such as inputs (fertility, irrigation, water, seed, etc.), opportunity costs lost from not growing a cash crop, fertility (nitrogen) credits from the cover crop, labor requirements, and gross returns from yields.
Objective 3: Identifying novel cover crops that fit the unique “niche” of high tunnel production systems. This objective consists of a) A replicated study to evaluate novel cover crop candidates at the UK Horticulture Research Farm in Years 1 and 2 of the projects, and b) On-farm trials with nine participating farmers (three per state) in Years 2 and 3 of the project.
Cover crop selection will be based on the unique temporal and microclimate conditions of high tunnels occurring in each of these seasons, and further refined with participating grower input. As day length can make different species flower at different times, the north-south gradient between the Kentucky, Tennessee, and Georgia sites will be helpful in elucidating growing degree day vs. day length effects, improving the applicability of this study across broader areas of the Southeast. Our participating growers have identified several management challenges they are keen to work with cover crops to manage, including knot nematode suppression, compaction mitigation, and enhancing soil organic matter and overall soil health. In addition to participating grower feedback, the baseline practices from our survey results (Objective 1) may identify additional opportunities for cover crop selection. These survey results will be incorporated in Year 2 of the evaluation study.
Educational & Outreach Activities
Two of our participating farmers in Georgia have begun on-farm trials of cover crops for their high tunnels. Species were selected based on management goals for increasing the sustainability of their high tunnel systems (soil health, nitrogen credits, nematocidal effects). The farmers completed their initial cool-season trial, and are beginning their warm season trial in Summer 2017. Observational data were taken and consulting with the farmers were conducted in Fall 2017. Plant biomass and microclimate data will also be taken from Summer 2017, onward. The participating farmers in Tennessee and Kentucky will begin their cool season cover trials in Fall 2017. Participating farmers and researchers have been communicating through annual meetings, with good discussions of cover crop goals, temporal windows, and management challenges to ensure the entire project team has a working understanding of each farmer’s high tunnel system and their goals for sustainable management.
Project outcomes will be derived from our activities associated with our three main objectives. Progress to date is described below:
Objective 1: Characterize high tunnel cropping systems and existing cover crop use in high tunnels in the Southern SARE Region through a high tunnel grower survey. Currently we have drafted a survey for dissemination in early 2018, and collated a large network of contact for electronic and paper dissemination through tabling at regional grower conferences in 2016/2017. Care has been taken to balance production-oriented questions that are relevant to the natural science portion of this project (e.g. cover crop temporal windows, tunnel crop species, prior experience with cover crops, etc), with marketing and production information. In-depth interviews with six high tunnel producers in Kentucky to generate broader narratives about the roles of high tunnels for overall farm economics, production practices, and technical and policy challenges to sustainable high tunnel management.
Objective 2: Evaluating the ecosystem services and economic costs of using cover crops in high tunnel crop rotations. At the time of this writing, we are approximately 8 months into this two-year, two-site study. Winter cover crops were successfully established at both the Kentucky and Tennessee sites, and soil, plant, and weed sampling and experimental management timelines are on-track. Soil health and winter crop/cover crop plant samples are currently being analyzed in the lab. We will continue to track input, output (yield), and labor data for the remainder of the study. Plant petiole nitrate analysis, used to track nitrogen sufficiency during the growing season (post-cover crop) is being conducted weekly on the summer tomato crop, and fertilizer input and associated labor costs tracked accordingly.
Objective 3: Identifying novel cover crops that fit the unique “niche” of high tunnel production systems. In year one of the project, we conducted an initial screening of 12 warm-season and 11 cool-season novel cover crops to determine minimum viable plot size, hone experimental protocol, and determine some basic physiological tolerances to guide cover crop species selection. We identified several promising candidates, as well as screened out a number of candidates that experienced stand establishment or temperature tolerance issues. Examples of stand-out selections from our qualitative observations, as well as cover crop/weed biomass samples include (but are not limited to) the following warm season accessions: teosinte, sunn hemp, foxtail millet, iron clay pea, and Japanese millet. Promising cool season accessions include (but are not limited to): black oats, festulolium, and triticale. At the time of this writing, we are beginning our full warm season trial with refined protocol for biomass and weed sampling, and accessions informed by our initial screening and participating grower input. The full trial will be conducted at the University of Kentucky Horticulture Research Farm, and repeated for 2 years. Selected species will also be used in on-farm trials in each of the 3 participating states (Kentucky, Tennessee, Georgia) to obtain grower experience on these cover crops, as well as additional temperature and biomass data for growth model development in year 3 of the project.