Cover crop selection and manure placement for weed suppression and nitrogen use efficiency in a no-till organic corn system

Final Report for GNE11-025

Project Type: Graduate Student
Funds awarded in 2011: $14,986.00
Projected End Date: 12/31/2013
Grant Recipient: University of Maryland
Region: Northeast
State: Maryland
Graduate Student:
Faculty Advisor:
Dr. Ray Weil
University of Maryland
Expand All

Project Information

Summary:

The long-term profitability and sustainability of organic corn (Zea mays L.) production must include reliable weed and fertility management strategies that minimize production costs, conserve soil, and maximize nutrient use efficiency. This may be accomplished through integration of novel manure management practices (subsurface banding dry solids) with an optimal legume/grass cover crop mixture. The specific goals of this graduate student project are to 1) evaluate total aboveground cover crop biomass, nitrogen (N) content and species proportions in response to different hairy vetch (Vicia villosa Roth.)/cereal rye (Secale cereale L.) sown proportions, 2) determine the decomposition and N release of cover crop residues as affected by hairy vetch/cereal rye biomass proportions and pelletized poultry litter (PPL) application, PPL placement and tillage, 3) evaluate the effect of PPL application method on soil N spatial and temporal availability under different cover crop residues, and 4) determine the cover crop species proportions and PPL placement and rate that optimizes N availability and corn N use efficiency.

To address the research objectives, a field experiment was conducted in two fields at Beltsville, MD in 2011-2012 and 2012-2013. The experiment included a factorial of six fall-planted hairy vetch/cereal rye cover crop sown proportions (four different mixtures and two monocultures) and six spring PPL applications (four rates of sidedressed subsurface banded PPL, pre-plant broadcast PPL at a phosphorus (P)-based rate, and pre-plant tillage-incorporated PPL at a P-based rate) in a no-till corn system.

Although hairy vetch monocultures produced about half the biomass of cereal rye monocultures, mixtures usually produced similar or greater biomass levels than cereal rye monocultures. Cover crop N content was estimated to reach a maximum when hairy vetch made up around 50% or greater of total cover crop biomass. The proportion of mass and N remaining in decomposing cover crop residues during the corn growing season decreased as the proportion of hairy vetch biomass in the cover crop residue increased. Subsurface band manure application at a P-based rate did not affect the decomposition of surface mulches relative to no PPL. Greater soil inorganic N (IN; NO3-N + NH4+-N) was measured under the monoculture hairy vetch residue than under the monoculture cereal rye residue at emergence and fifth-leaf corn growth stages in both years. Soil IN concentrations under the mixtures were intermediate between soil IN concentrations under the two monocultures. Sidedress subsurface band PPL application provided similar or greater IN concentrations as the broadcast and incorporated treatments at fifth-leaf stage, and soil IN remained localized around the delivery location throughout the growing season.

Introduction:

Conventional no-till grain production and organic agriculture represent two of the most successful strategies to maintain profitability while improving long-term soil quality. This project is designed to address the needs of organic grain producers who desire to increase productivity in systems that combine the soil protecting capacity and reduced energy requirement of reduced tillage with the soil building potential of organic management.

Reduced-tillage soybean (Glycine max L. Merr.) production systems, where soybeans are no-till planted into a mat of cover crop residue flattened by a roller/crimper, have been successfully implemented in the eastern US from Pennsylvania to North Carolina (Mischler et al. 2010a, Smith et al. 2011). This system relies on highly persistent cereal rye cover crop residues to suppress weeds and a soybean cash crop that provides its own N; weeds are suppressed both physically by a thick cereal rye mulch and biogeochemically due to N being immobilized in the soil profile from a high carbon (C) input cover crop (Wells et al. 2013). However, inability to consistently control weeds and provide adequate N fertility has limited the success of similar cover crop-based organic reduced-till systems for corn (Zea mays L.) (Mischler et al. 2010b).

Developing a successful reduced-tillage organic corn production system will require an integrated, multi-tactical approach to weed and fertility management. Hairy vetch can provide substantial levels of N to a corn crop; however, the residue provides inferior season-long weed suppression compared to hairy vetch/cereal rye mixtures since legume residues generally decompose more rapidly than those of grasses (Teasdale and Abdul-Baki 1998). Using a hairy vetch/cereal rye mixture may increase weed suppression compared to a hairy vetch monoculture, but such mixtures often reduce the release rate and total available N from the residue (Clark et al. 1997, Rosecrance 2000). Subsurface banding of PPL prior to the period of rapid N uptake by corn might compensate for insufficient N from the cover crop mixture, while keeping N below the zone of cover crop decomposition and weed emergence.  

Past work has demonstrated increased weed suppression and N use efficiency from subsurface banded compared to broadcast manures (Everaarts 1992, Tewolde et al. 2009). In addition, research has shown that cover crop mulch can be an effective weed control tactic when high biomass levels (8000 kg/ha) are achieved (Mohler and Teasdale 1993). Furthermore, the development of roller-crimper technology has made it possible to terminate a cover crop without the use of herbicides, forming a weed-suppressive mulch through which a large seeded crop can be planted (Ashford and Reeves 2003, Mirsky et al. 2009). However, technology to subsurface band dry fertilizer solids (e.g. poultry litter) has not previously been available. Recent development of this technology represents a milestone in the integrated management of weeds and fertility in reduced-tillage organic grain production, but the efficacy of this technology to reduce weed competition and allow more efficient crop N use has not yet been fully tested, particularly in combination with legume N sources. Research on this technology has focused primarily on nutrient losses resulting from subsurface banded poultry litter application as compared to surface application (Pote et al. 2009, Watts et al. 2011, Pote and Meisinger 2014) and crop yield and quality response in conventional row-crop and pasture systems (Warren et al. 2008, Tewolde et al. 2009, Pote et al. 2011, Adeli et al. 2012). This project will expand upon previous knowledge of the benefits of subsurface banded fertilizer and high-biomass residues for weed control and efficient crop N use by integrating these two tactics into a high-residue no-till organic corn production system using recently developed equipment.

Project Objectives:

Cover crop mixture performance

Evaluate total aboveground cover crop biomass, N content and species biomass proportions in response to different hairy vetch and cereal rye sown proportions.

Cover crop residue decomposition

Characterize C and N dynamics over time in decomposing cover crop residues composed of a range of hairy vetch/cereal rye biomass proportions to determine the cover crop mixture that results in the most persistent weed suppressive mulch.

Determine the effects of PPL application (0 vs. 67 kg plant available N/ha), PPL placement (broadcast vs. subsurface banded) and tillage on the rate of residue decomposition and N release for residues of varying composition.

Soil N

Characterize the spatial distribution of soil IN in the subsurface band PPL treatment compared to no PPL, broadcast PPL and incorporated PPL across a gradient of cover crop residue biomass proportions over time.

Crop and weed N uptake

Determine the efficiency at which N is taken up and the relative sufficiency of N for corn growth in plots with cover crop residues of varying composition and subjected to no PPL, subsurface banded, broadcast and incorporated PPL

Note: Corn and weed N uptake data have been collected but were not summarized at the time of writing this report.




Cooperators

Click linked name(s) to expand
  • Dr. Steven Mirsky
  • Dr. Ray Weil

Research

Materials and methods:

Field management

A field experiment was conducted at the Beltsville Agricultural Research Center from fall 2011 to fall 2013. The experiment included a factorial of six hairy vetch/cereal rye cover crop sown proportions and six PPL treatments (four rates of sidedressed subsurface banded PPL, pre-plant broadcast PPL at a P-based rate, and pre-plant tillage-incorporated PPL at a P-based rate) in a no-till corn system. The experiment was established on North Farm, which has been managed organically since 2000 and certified by the Maryland Department of Agriculture according to National Organic Program requirements since 2003 (39.03 N, 76.93 W) and South Farm, which is under conventional agricultural management (39.02 N, 76.94 W). Soils on North Farm are classified as fine-loamy, mixed, active, mesic Fluvaquentic Endoaquepts (Hatboro series) and soils on South Farm are classified as fine-loamy, mixed, active, mesic Fluvaquentic Dystrudepts (Codorus series). The second and first year experiments were conducted in adjacent fields within the same farms. Three replicates were planted on each site-year. The site-years in this report are labeled as “North Farm” (“NF”) or “South Farm” (“SF”) followed by the year in which the cover crops were sampled (2012 or 2013).

Soybeans were uniformly seeded in spring 2011 and 2012 and incorporated by disking in late summer prior to cover crop establishment. The fields were cultivated and cultimulched to prepare a cover crop seedbed. The fields received 67 kg K2O/ha prior to cover crop planting. The hairy vetch/cereal rye sown proportions included: 1.0/0, 0.8/0.2, 0.6/0.4, 0.4/0.6, 0.2/0.8, and 0/1.0. The proportions of hairy vetch and cereal rye represent proportions of their monoculture seeding rates, which were 34 kg/ha and 168 kg/ha, respectively (Table 1). The two cover crop species were planted in separate, sequential passes using a grain drill, forming 19 cm rows. Hairy vetch seed was inoculated with Rhizobia. The cover crops were planted on October 7, 2011 and September 17, 2012 on North Farm and on October 10, 2011 and September 25, 2012 on South Farm.

Cover crops were terminated on May 17, 2012 and May 31, 2013 on North Farm and on May 29, 2012 and May 22, 2013 on South Farm using a roller/crimper. Hairy vetch was in full flower to early pod and cereal rye was in the soft dough stage, except on South Farm in 2013 when hairy vetch was 50% flowering and cereal rye was in the milk stage. Pelletized poultry litter treatments were applied in random strips perpendicular to cover crop treatments. Each unique combination of cover crop sown proportion x PPL treatment x rep formed a plot. To establish the tillage-incorporated plots, cover crops were flail-mowed and disked 7-10 days prior to rolling the no-till treatments.

Pelletized poultry litter was obtained from Perdue Agricycle, LLC (Seaford, DE). To pelletize poultry litter, raw broiler litter was first heated and pasteurized, removing moisture. The dried material was ground to a powder and moisture was returned as the material passed through a pelletizing mill. Pelletized poultry litter was broadcast using a Stoltzfus boom drop fertilizer spreader (W-Chain Sower spreader, Morgantown, PA) at 3.4 Mg/ha just prior to corn planting in the pre-plant broadcast and incorporated treatments. These rates were estimated to provide 67 kg plant-available N/ha and the approximate amount of P removed by a corn crop, with plant-available N calculated as:

                                    PAN = 0.9(NH4+-N)+0.5(organic N)

where PAN is the concentration of N estimated to become available in one growing season, NH4+-N and organic N are the concentrations of NH4+-N and organic N in the PPL. Fifty percent of organic N was assumed to mineralize during the first season of application according to University of Maryland recommendations for raw broiler litter. Poultry litter pellets were assumed to have a similar proportion of mineralizable N as raw broiler poultry litter during a single growing season according to Spargo et al. (unpublished). The NH4+-N availability factor was based on NH3-N losses expected from immediate incorporation or subsurface band application.

The incorporated treatment was spaded (Imants 27sx, Reusel, Netherlands) to 20 cm and cultimulched just before planting. Corn was planted the same day as the cover crops were rolled, except on North Farm in 2013 when it was planted June 21, 2013. Corn was planted at 84,000 seeds/ha in 76-cm rows (North Farm 2012: Blue River 53R57, South Farm 2012: TA 657-13VP, North Farm 2013: Blue River 51B57, South Farm 2013: TA 522-22DP). All corn received ~10 kg plant-available N/ha as starter PPL.  In the subsurface banded treatments, PPL was sidedressed to approximately 10 cm depth using a prototype subsurface band applicator at the eighth-leaf growth stage on July 3, 2012 and fifth-leaf growth stage on July 17, 2013 on North Farm and at the fifth-leaf growth stage on June 27, 2012 and June 25, 2013 on South Farm. North Farm received subsurface banded PPL rates of 0, 1.8, 3.4 and 6.8 Mg/ha; South Farm received rates of 0, 3.4, 6.8 and 14 Mg/ha. On North Farm, weeds were managed by two-three rotary hoe operations and two-three between row cultivations in the incorporated plots, or two high-residue cultivation operations in the no-till plots. On South Farm, the field received glyphosate applications on June 1 and June 21, 2012 and on May 31 and June 26, 2013 at 2.8 kg active ingredient/ha. North Farm was irrigated with approximately 2.5 cm on July 5, 2012; South Farm received 2.5 cm on July 10, August 1, and September 29, 2012 and 4 cm on June 27, July 29, and September 8, 2013.

Cover crop biomass and decomposition sampling

Aboveground cover crop biomass was collected by clipping all material within a 76 cm x 67 cm frame immediately after termination in four to six locations within each rep of each cover crop sown proportion. The biomass samples were separated by species, oven dried (60°C), weighed dry and ground to pass a 1.0-mm screen. The ground samples were analyzed for C and N concentration by total combustion analysis (Thermo Delta V interfaced to Temperature Conversion Elemental Analyzer, Thermo Scientific, Waltham, MA).

The litter bag decomposition study was conducted on North Farm in 2012 and South Farm in 2013. Nylon mesh “litter bags” bags (30 x 30 cm  dimensions, 1 mm mesh size) were filled with a dry weight of hairy vetch and/or cereal rye shoots corresponding to an average biomass level for the site-year based on samples collected in the incorporated plots just prior to mowing. The 2012 and 2013 litter bag targeted biomass levels were 8000 and 9000 kg aboveground dry matter/ha, respectively. Pure hairy vetch and cereal rye biomass was harvested from experimental border areas. Partially decomposed hairy vetch material near the soil surface was avoided during biomass collection. Samples of the material were weighed fresh and dry for moisture content determination. Moisture contents for the hairy vetch and cereal rye were used to determine the fresh weights required to achieve the total targeted dry weight biomass level and the following hairy vetch/cereal rye biomass proportions: 0/1.0, 0.25/0.75, 0.5/0.5, 0.75/0.25, and 1.0/0. The fresh material was cut to 10 cm lengths (the approximate length of flail-mowed residues) and placed in litter bags at specific fresh weights. Six bags were prepared for each cover crop mixture proportion in each rep. The bags were buried at a slight angle at 20 cm depth in mixtures with roughly corresponding cover crop proportions (sown proportions of 0/1.0, 0.2/0.8, 0.6/0.4, 0.8/0.2 and 1.0/0 hairy vetch/cereal rye) immediately after mowing and disking the incorporated plots.

Litter bags for the no-till plots were prepared in the same way just prior to rolling, only the material was cut to 25 cm to lay flat in the bag. Litter bags in the broadcast PPL treatment received PPL within the bag at the same rate as applied to the larger plots. The no-till litter bags were placed in the plots with roughly corresponding cover crop proportions (sown proportions of 0/1.0, 0.2/0.8, 0.6/0.4, 0.8/0.2 and 1.0/0 hairy vetch/cereal rye) and designated as either broadcast, subsurface banded (at 67 kg plant-available N/ha) or no PPL application immediately after corn planting. Existing residue in the area occupied by the litter bags was clipped and removed, and the bags were pinned in each corner to ensure good contact with the soil surface. Soil temperature readings were taken at 5 cm depth in the 0/1.0, 0.6/0.4 and 1.0/0 hairy vetch/cereal rye mixtures with subsurface banded PPL and at 20 cm depth in the same mixtures with incorporated PPL using sensors connected to data loggers from corn planting through corn harvest (Decagon, 5TE soil moisture, temperature, EC sensors, Pullman, WA). 

One litter bag was collected from each plot at each of six approximate sampling times: cover crop termination, crop emergence, and growth stages third-leaf, eighth-leaf, silking and physiological maturity. The litter bag contents were oven-dried and ground to pass a 1 mm sieve. A subsample of the ground material was ashed for 4 hours at 550° C to determine the proportion of soil present in each bag. The ground litter bag contents were analyzed for C and N concentration by total combustion analysis (Leco CHN analyzer, St. Joseph, MI). Soil collected from the outside of the litter bags in the incorporated and no-till plots was also analyzed for C and N concentrations. The residue mass and N content in each bag was adjusted for soil contamination using the proportion of ash in each litter bag, and the soil’s N concentration.

Soil sampling

On November 21, 2011 and October 12, 2012, soil samples were collected in each rep to a depth of 30 cm for soil IN analysis and routine soil testing. The soil samples were air-dried, passed through a 2 mm sieve, extracted for IN using 1 M KCl and analyzed using colorimetric flow injection analysis (Seal AQ2 Automated Discrete Analyzer, Mequon, WI). Soil samples were analyzed for pH, cation exchange capacity and nutrient concentration using Mehlich III extractant at A&L Eastern Labs (Richmod, VA).

On South Farm in 2012 and 2013, soil samples were collected at corn emergence (June 7-8, 2012 and June 4-5, 2013), fifth-leaf post-sidedress (June 28-29, 2012 and June 26-27, 2013), silking (July 31-August 1, 2012 and July 18, 23, and 24, 2013), milk (August 16, 2012 and August 7, 2013) and physiological maturity (October 1 and 4, 2012 and September 10-11, 2013) in the monoculture hairy vetch, monoculture cereal rye and 0.6/0.4 hairy vetch/cereal rye sown proportion with no PPL, broadcast, incorporated and 67 kg plant-available N/hasubsurface banded PPL. In all cover crop residue treatments and all PPL treatments except subsurface band, 2 cm diameter soil cores were taken at 0, 20 and 38 cm from the interrow center and split at four depths (0-5, 5-10, 10-20, 20-30 cm). In the plots receiving subsurface banded PPL, soil cores were taken at 0, 5, 10, 25, and 38 cm from the band and split at five depths (0-5, 5-10, 10-15, 15-20, 20-30 cm). In each plot, four parallel transects perpendicular to the corn rows were sampled in this way, with cores from the same depth and distance from band or interrow composited.The soil samples were air-dried, passed through a 2 mm sieve, extracted for IN by 1 M KCl and analyzed by colorimetric flow injection analysis in 2012 (Technicon Autoanalyzer II, Technicon Instruments, Tarrytown, NY) or discrete analysis in 2013 (Seal AQ2 Automated Discrete Analyzer, Mequon, WI).

Corn and weed sampling

Corn biomass samples were collected in all treatments at fifth-leaf stage, silking and maturity. At all of the sampling events except maturity, six to eight corn plants were collected from each plot, weighed dry and ground to pass a 1 mm screen. The samples were analyzed for N concentration by total combustion analysis (Elementar Vario Max N/C analyzer, Mt. Laurel, NJ). At maturity, populations were counted for a representative 6-m section of corn row located in the center of each plot. At maturity, corn plants were removed from within a 3-m section in the center of each plot was and weighed fresh. A six-plant subsample, including ears, was chipped and weighed before and after drying. Moisture content was calculated and used to determine the dry weight of the full 3-m section. The six-plant subsample was ground to pass a 1-mm sieve, homogenized and analyzed for C and N concentration by total combustion analysis. For grain yield determination in 2012, corn ears were removed and weighed from all plants within the 6-m section. Six representative ears were weighed fresh and oven-dry. The moisture content and proportion of grain mass of the subsample were used to estimate the grain yield of the full 6-m section. For grain yield determination in 2013, a small-plot combine was used to measure yield and moisture content for a 9 to 12-m section in each plot.

Weed biomass was collected in all treatments at the second-leaf (July 2, 2013) or third-leaf (incorporated – June 5, no-till – June 11, 2012), fifth-leaf (June 19-21, 2012; July 16, 2013), eighth-leaf (July 22, 2013) and silking (July 25-26, 2012; August 20-22, 2013) growth stages on North Farm. At all growth stages except silking, weeds within a 76 x 67 cm frame or 76 x 133 cm frame were clipped at the base, dried, weighed, ground and analyzed for N concentration. At silking, a 76 cm x 133 cm frame was used, and the weeds were separated by species, counted and weighed. All species from each plot were ground and analyzed for N concentration together.

Statistical analysis: cover crop biomass and N content

Cover crop biomass in response to sown proportions was modeled using plant competition functions developed by de Wit (1960).

                                 yv=yvvPvkv/((Pvkv)+Pr)
                                            
yr = yrrPrkr/((Prkr)+Pv)
                                 ytotal = yv + yr

where yv is the hairy vetch biomass, yvv is the hairy vetch monoculture biomass, kv is the Relative Crowding Coefficient (RCC) of hairy vetch with respect to cereal rye, Pv is the proportion of the monoculture hairy vetch seeding rate, yr is the cereal rye monoculture biomass, kr is the RCC of cereal rye with respect to hairy vetch and Pr is the proportion of the monoculture cereal rye seeding rate. The RCC is a measure of an individual species’ aggressiveness: the greater the RCC value, the better a species has done in mixture (Williams and McCarthy 2001). An RCC value greater than one indicates that the species performed better when grown with the opposing species than when grown with itself. The product of two species’ RCCs provides a measure of overyielding: values greater than one indicate that mixture biomass exceeded the weighted average of monoculture biomass (Hall 1974). The term “overyielding” does not imply that the mixture produced greater absolute biomass than both monocultures (this phenomenon is termed “transgressive overyielding”). Relative Crowding Coefficients have been shown to vary in response to monoculture densities, so interpretation of this index is limited to the monoculture seeding rates used in this study (Connolly et al. 1986, Snaydon 1991).

Model fitting was performed for each site-year independently using the nlme function in R, with rep included as a random effect (R Development Core Team 2014). We used mean monoculture biomass values to parameterize  and  as constants in the model. In addition to the de Wit analysis, total cover crop biomass at each seeding rate proportion was compared to monoculture biomass using a linear mixed model (R function lme) with rep as a random effect. Means were calculated using lsmeans and contrasts were performed using glht in R package multcomp (R Development Core Team 2014). An exponential correlation structure, using corExp with sampling location x and y coordinates as spatial covariates, was included in models when it improved model fit. In non-linear (nlme) models that included spatial correlation, rep was assigned as a fixed effect rather than random effect to achieve model convergence. The RCCs and RCC products were evaluated to see whether their 90% and 95% confidence intervals overlapped with one.

The N content of each cover crop species for each sample was computed as the product of the species tissue N concentration and biomass. Total N content for each sample was computed as the sum of each species’ N content. The de Wit model did not accurately predict the effect of sown proportion on cover crop N content. Instead, linear or quadratic equations were used to model hairy vetch and cereal rye N content and total aboveground N content of cover crops as a function of hairy vetch biomass proportion. The hairy vetch biomass proportion was calculated as the proportion of hairy vetch biomass out of the total biomass collected in each sample at the time of cover crop termination. Regression was performed only over the range of hairy vetch proportions that were observed in the field to avoid making predictions in regions of no data. The lme function in R was used for model fitting, with rep included as a random effect and monoculture N contents included as constants (R Development Core Team 2014). An exponential correlation structure, using corExp with x and y coordinates as spatial covariates, was included in models when it improved model fit.

Land Equivalent Ratio (LER) was used to evaluate mixture advantage in terms of biomass production and N content. This index represents the efficiency of the mixture to use resources in the environment relative to monoculture. 

                                   LER= yvr/(yvvPv)
                                   LER= yrv/(yrrPr)
                                   LERm = PvLERv + PrLERr

where LERv, LERr, and LERm are LERs of hairy vetch, cereal rye and total mixture, respectively, yvr is the biomass or N content of hairy vetch in mixture with cereal rye, and yrv is the biomass or N content of cereal rye in mixture with hairy vetch. Hairy vetch or cereal rye LERs greater than one indicate that the species performed better in mixture than in monoculture. Mixture LERs greater than one indicate overyielding (Willey 1979).  Land Equivalent Ratios were calculated for each sown proportion and analyzed by site-year using the lme function in R (R Development Core Team 2014). Rep was included as a random effect, and exponential spatial correlation was added when it improved the model fit. Contrasts were performed using the glht function in R package multcomp to determine whether mean LERs were significantly different than one.

Statistical analysis: cover crop decomposition

The proportions of mass and N remaining were calculated for each litter bag by dividing the ash-free mass and N in each bag by the ash-free mass and N content of the bag within the corresponding treatment and rep that was collected immediately after bag preparation. The proportions of mass and N remaining for each cover crop and manure treatment were modeled over soil growing degree days (gdd), calculated by summing the mean daily soil temperatures above 0°C (Honeycutt et al. 1988). The soil gdd were based on average soil temperatures for the no-till plots and incorporated plots containing the temperature sensors. An asymptotic exponential decay function was used to model proportion of mass and N remaining over soil gdd. This model is based on the concept that plant litter contains both readily decomposable components that disappear rapidly and resistant components that remain:

                                  y = Pe-kt + (1-P)

where y is the proportion mass or N remaining at a given time, t (in units of soil gdd), P is the proportion of mass or N lost, and k is the exponential decay constant. P and k were estimated parameters. Model fitting was done separately for each cover crop and manure treatment in each site-year using the nlme function in R (nlme package, R Development Core Team 2014).  A random rep effect on the decay constant was included in the models.

Half-lives (the soil gdd required for 50% of original mass or N to be lost) were calculated using the proportional loss and decay constants estimated by exponential decay model fitting as:

                                 t1/2 = (ln(0.5-(1-P))/P)/-k

Statistical analysis: soil inorganic N concentration

We modeled soil IN movement from the two major sources of IN: poultry litter and cover crop residue. First, Euclidean distance from IN source was calculated using the Distance Formula:

                                 d = √(x2+y2)

where d is the Euclidean distance from the IN source, x is the horizontal distance from the IN source, and y is the vertical distance from the N source. The IN source was considered to be the soil surface for broadcast and no PPL treatments, and the PPL band for subsurface band treatment.

The effect of Euclidean distance from the IN source on the natural log of soil IN at the corn emergence, fifth-leaf, silking, milk and maturity growth stages was modeled using a random-walk model (normal curve) for diffusion/dispersion (Appelo and Postma 2010).

                                 y = a + (x/(s√2π))e(-d^2/2s^2)

where y is the natural log of IN concentration at a given Euclidean distance, d from the band, x is the height of the curve’s peak, a is the background soil IN concentration, and s is a shape coefficient controlling the width of the peak. To generate spatial soil predictions, non-linear modeling was performed separately for each rep using the random walk equation. The model residuals for each rep were evaluated for additional trends. For example, for the subsurface band treatment, random walk model residuals were fitted to a second random walk model as a function of distance from soil surface to account for cover crop IN in addition to PPL band IN. Semivariograms were produced from random walk model residuals using the variogram function (package gstat, R Development Core Team 2014) to evaluate spatial autocorrelation. The semivariograms did not produce detectable trends and could not be accurately modeled with spherical, exponential or Gaussian models. Therefore, inverse distance-weighted interpolation was used predict variability not accounted for by the random walk models instead of kriging. The back-transformed predicted values from each treatment and sampling date were plotted on a 38 cm wide x 30 cm deep grid (1 cm spacing), forming a profile half the width of a corn interrow. The predictions generated from the random walk models and inverse distance-weighted interpolation were used to calculate means of soil IN for each replicate of each treatment. Estimated soil IN concentrations were then analyzed using repeated measures ANOVA in package lme (CAR1 correlation structure); means were calculated using the lsmeans function and contrasts among growth stages, cover crops and manure treatments were performed using glht (packages lsmeans and multcomp, R Development Core Team 2014).

 

Research results and discussion:

Soil properties and weather conditions

Soil properties were generally adequate for cover crop growth (Table 2). Among the soil nutrients analyzed, potassium was the only nutrient that was not rated as optimum for crop growth, and the sub-optimal sufficiency levels were only observed on North Farm. Inorganic N concentrations were similar in both fields, but lower in fall 2011 relative to fall 2012. This may have been a result of later soil sampling in 2011 relative to 2012, which allowed time for IN to be taken up by cover crops or leached from the plow layer. Soil pH was approximately 6 across site-years, which is within the preferred range of hairy vetch and cereal rye.

The 2011-2012 cover crop growing season accumulated more gdds than the 30-year average, especially during late fall and early spring (Table 3). Rainfall was above average in fall 2011, but below average in spring 2012. The 2012-2013 cover crop growing season experienced colder-than-average temperatures in November and early spring, but a warm December and average temperatures in April and May. Rainfall was below average for most months during the 2012-2013 cover crop growing season. The 2012 corn growing season was hotter and drier than 30-year averages for most months (Table 3). The 2013 corn growing season was cooler and wetter than 2012, with cumulative gdds and total rainfall for the season slightly above and below the total of 30-year averages, respectively. The month of June was particularly wet in 2013.

Aboveground cover crop biomass and species composition

Hairy vetch biomass in monoculture ranged from 5569 to 7211 kg/haand averaged 6384 kg/ha (mean of four site-year averages; Table 4, Figure 1). The monoculture biomass yields of cereal rye were 1.6 to 1.9x the hairy vetch biomass, ranging from 10410 to 12871 kg/ha and averaging 11113 kg/ha (mean of four site-year averages). North Farm 2013 had the highest fall soil IN concentration (IN concentration equivalent to168 kg N/ha) and produced 1.4x the total cover crop biomass averaged across sown proportions than the other three site-years (North Farm 2012: 9560, South Farm 2012: 8720, North Farm 2013: 13010, South Farm 2013: 9990 kg/ha).

Hairy vetch biomass increased as the hairy vetch seeding rate increased, and cereal rye biomass decreased as the cereal rye seeding rate decreased within each site-year (Figure 1). The effect of sown proportions on species biomass proportions in mixtures was similar in North Farm 2012 and South Farm 2013, where cereal rye was the dominant species in mixture and hairy vetch was less aggressive (equivalent biomass of both species achieved around 0.9/0.1 hairy vetch/cereal rye sown proportion). Relative to North Farm 2012 and South Farm 2013, cereal rye was an even stronger competitor against hairy vetch on North Farm in 2013, with equivalent biomass of both species achieved around 0.95/0.05 hairy vetch/cereal rye sown proportion. South Farm 2012 produced cover crop mixtures with species biomass proportions similar to the hairy vetch/cereal rye sown proportions (equivalent biomass of species achieved around 0.45/0.55 hairy vetch/cereal rye sown proportion).

In three of the four site-years (all except South Farm 2012), total cover crop biomass in mixtures was statistically equivalent to cereal rye monoculture biomass. On South Farm 2012, total cover crop biomass of mixtures was generally intermediate between the monocultures.

The cover crop biomass levels produced in our study were high relative to most other hairy vetch/cereal rye cover crop mixture studies, which have observed biomass between ~1000 and 7000 kg/ha (Clark et al. 1994; 1997, Ranells and Wagger 1996, Teasdale and Abdul-Baki 1998, Kuo and Jellum 2002, Ruffo and Bollero 2003, Sainju et al. 2005, Parr et al. 2011, Hayden et al. 2014). We attribute the high biomass productivity in this study to: 1) the late May termination dates, and 2) the large pool of available N (30-168 kg IN/hameasured in fall) provided by the soybean crop incorporated prior to cover crop planting.

In three of the four site-years, cereal rye contributed more biomass than hairy vetch to the cover crop mixtures (North Farm 2012, North Farm 2013, South Farm 2013). Although our cover crop planting dates were not particularly late for hairy vetch, we visually observed that cereal rye usually accumulated more fall growth than hairy vetch, giving cereal rye an early competitive advantage over the slower-growing hairy vetch. Because grasses tend to outcompete legumes when ample soil N is available (Haugaard-Nielson et al. 2001), a competitive advantage of cereal rye may also be attributed to a large amount of plant-available N released by the soybean crops incorporated prior to cover crop planting each year. Among the four site-years, cereal rye was most aggressive and significantly suppressed hairy vetch growth on North Farm in 2013, which was the site-year with the most productive preceding soybean crop and greatest fall soil IN concentrations. Cold weather is expected to increase the competitiveness of cereal rye because it is more cold-hardy than hairy vetch (Teasdale et al. 2004). Therefore, the relatively cool early spring in 2013 may also explain why cereal rye represented the larger component of cover crop mixtures in 2013. We observed that the dominance of cereal rye in mixtures did not necessarily correspond to suppression of hairy vetch (e.g. North Farm 2012, South Farm 2013), likely because of capacity of hairy vetch to fix atmospheric N and to climb cereal rye plants to access light (Keating and Carberry 1993).

On South Farm 2012, the site-year in which the proportions of hairy vetch and cereal rye biomass in mixtures largely matched the sown proportions, we observed poor fall growth and geese damage to cereal rye. The poor cereal rye growth in the fall and the following warm spring probably allowed hairy vetch to compete better with cereal rye.

Cover crop nitrogen content

Cereal rye monoculture N content ranged from 48.7 to 86.5 kg/ha, and averaged 63.0 (mean of four site-year averages). Hairy vetch monoculture N content ranged from 148 to 222 kg/ha, and averaged 182 (mean of four site-year averages). In three of the four site-years (all but South Farm 2012), the dominance of cereal rye in mixtures meant that cover crop N content could not be estimated above ~0.5/0.5 hairy vetch/cereal rye biomass (Figure 2). Cereal rye N content tended to decline slightly as the proportion of cereal rye biomass in mixture decreased. Hairy vetch N content increased as the proportion of hairy vetch biomass in mixture increased. Total cover crop N content increased with increasing hairy vetch biomass in mixture but appeared to plateau around 0.5/0.5 hairy vetch/cereal rye biomass. On South Farm 2012, a maximum N content of 178 kg/ha was predicted at a hairy vetch/cereal rye biomass proportion of 0.92/0.08. On South Farm 2013, a maximum N content of 191 kg/ha was achieved at a hairy vetch/cereal rye biomass proportion of 0.54/0.46. In the remaining two site-years, the maximum total N content was achieved in the monoculture hairy vetch treatments. However, within these two site-years, the total N content of hairy vetch/cereal rye mixtures at the highest hairy vetch biomass proportion achieved were just slightly less than the N contents of monoculture hairy vetch.

Hairy vetch and cereal rye monoculture C/N ratios were 12 and 83, respectively (means of four site-year averages, back-transformed from log scale). A C/N ratio of 25-30, which represents a threshold of N immobilization/mineralization (Clark et al. 1997, Kuo et al. 1997) was predicted at approximately 0.5/0.5 hairy vetch/cereal rye biomass proportion in the three site-years that achieved this hairy vetch biomass proportion.

Nitrogen contents of cereal rye and hairy vetch in this study were similar to or greater than the N contents of the respective species observed in other studies (Clark et al. 1997, Ranells and Wagger 1996, Sainju et al. 2005, Parr et al. 2011). We observed that cover crop mixture N content was similar to or greater than monoculture hairy vetch N content at >50% hairy vetch biomass composition. Several other studies have found N content of hairy vetch/cereal rye mixtures to be equal to or greater than N content of monoculture hairy vetch (Clark et al. 1994, Ranells and Wagger 1996, Clark et al. 1997, Sainju et al. 2005, one site-year of Hayden et al. 2014). We did not measure N content of hairy vetch and cereal rye roots, which are estimated to contain an additional 10% of total hairy vetch N and 25% of total rye N (Shipley et al. 1992).

Land Equivalent Ratios

Across site-years, biomass LERs of cereal rye tended to increase as hairy vetch seeding rates increased, suggesting that cereal rye competes better against hairy vetch than against itself (Figure 3). Cereal rye biomass LERs were always significantly greater than one at the highest hairy vetch sown mixture proportion, and greater than one in the three highest hairy vetch sown mixture proportions in all site-years except South Farm 2012. Hairy vetch biomass LERs were generally less than or equal to one, except on South Farm in 2012 where they were greater than one at the two lowest hairy vetch sown mixture proportions. High LER values for one species normally corresponded with low LER values for the other species. None of the mean biomass mixture LERs fell below one, and there were six cases of mixture LERs significantly greater than one out of all site-years and sown mixture proportions. Land-Equivalent Ratios significantly greater than one were observed on all four site-years for at least one sown mixture proportion, and thus were able to detect overyielding on site-years that the RCC products did not detect significant overyielding.

Nitrogen LERs showed similar trends as the biomass LERs, except that cereal rye N LER values often exceeded cereal rye biomass LER values for the same treatments (Figure 4). As a result, there were seven more cases of mixture N LER values significantly exceeding one.

Overall, South Farm 2013 had the most sown proportions with biomass or N LER values significantly greater than one, and South Farm 2012 had the fewest. North Farm 2012 and North Farm 2013 had the same number of LER values significantly greater than one, which contradicts the relatively low RCC product calculated for North Farm 2013.

In summary, we found that hairy vetch/cereal rye mixtures consistently performed at least as well as the weighted average of monocultures, and produced more biomass and N content than the highest producing monoculture species at some mixture proportions. Other studies of annual grass/legume mixtures have found that LERs, measured by both biomass and N, generally exceed 1.0, ranging from 0.95 – 1.75 (Karpentstein-Machan and Suelpnagel 2000, Szumigalski and Van Acker 2008, Bedoussac and Justes 2010). Overyielding observed in this study may be attributed to reduced competition for soil N due to the capacity of hairy vetch to fix atmospheric N, as well as reduced light competition due to the tall architecture of cereal rye, which provides a scaffold for hairy vetch to climb.  

Cover crop mixtures: conclusions and recommendations

The high biomass levels achieved by cover crop mixtures provided excellent weed suppression during cover crop growth, and would be expected to provide excellent weed suppression in a subsequent no-till crop. Other authors have found that approximately 8000-9000 kg/ha of biomass is required to achieve reliable weed suppression in a no-till system (Mohler and Teasdale 1993, Teasdale and Mohler 2000, Smith et al. 2011), a level that was met at even the 0.8/0.2 hairy vetch/cereal rye sown proportion in three out of four site-years. Achieving maximum cover crop N and fixed N required at least 50% hairy vetch biomass component, which was usually produced at the 0.8/0.2 hairy vetch/cereal rye sown proportion. Thus, the 0.8/0.2 hairy vetch/cereal rye sown proportion provided near-optimal N content and sufficient biomass for reliable weed suppression. Considering the cost of seed used in this study ($6.61 and $0.75/kgfor hairy vetch and cereal rye, respectively), this mixture can accumulate N levels similar to a hairy vetch cover crop and enhanced weed control at a lower cost than the hairy vetch monoculture. Important to note is that substantial variability in proportional compositions among site-years suggests that the proportions of each species in mixture cannot be consistently predicted by cover crop seeding rate. Thus, we recommend that, at least for now, producers growing hairy vetch/cereal rye mixtures obtain an estimate of cover crop biomass and species biomass proportions in the spring prior to planting to inform management decisions.

Cover crop decomposition

Cover crop residue mass declined rapidly immediately after termination, but the rate of mass loss decreased as the corn growing season progressed (following “exponential decay”). The proportion of mass remaining in cover crop residues at any given time decreased with increasing hairy vetch biomass proportion in the residues. For example, in 2012, decay models predicted that 50% of pure hairy vetch residue remained in the no PPL treatment after ~40 days, while 68% and 87% of the 0.50/0.50 mixture and cereal rye residue remained, respectively (Figure 5). In 2013, a similar pattern was observed, but decomposition proceeded more rapidly overall, probably due to greater rainfall (Figure 6).

The decomposition patterns of the cover crop residues were similar in the subsurface band treatment as in the no PPL treatments in both years. However, for cover crops with high cereal rye composition, broadcast PPL application decreased the proportion of mass remaining at any given time relative to the no PPL and subsurface band treatments, causing decomposition patterns to be more similar among cover crop proportions in the broadcast treatments. Incorporation of residues and PPL through tillage also tended to decrease the mass remaining of cover crops relative to the no PPL and subsurface band treatments for all cover crops, except for cereal rye in the first ~85 days of the 2013 season. By the end of the corn growing season, approximately 60% of residue mass remained for pure cereal rye and 25% of residue mass remained for pure hairy vetch with no PPL or subsurface banded PPL. Broadcast PPL application and incorporation decreased the mass remaining of pure cereal rye residues from 60% to ~40%.

Residue N followed a similar exponential decay pattern as residue mass for most treatments. One important exception was the pure cereal rye in the no PPL, subsurface band and incorporated treatments, in which the proportion of N remaining over time tended to oscillate around 1.0 rather than declining (Figures 7 and 8). Consistent with the trends in residue decomposition, increasing hairy vetch proportion resulted in a lower proportion of N remaining at any given time during the corn growing season. However, N loss proceeded more rapidly than mass loss in both years. The rapid rate of N release early in the 2012 and 2013 seasons meant that the proportion of N remaining in cover crop residues had reached a stable level by 65 days in 2012 and 43 days in all PPL treatments in 2013.

Subsurface band PPL application did not alter N release patterns of the cover crops relative to no PPL, but broadcast application decreased the N remaining at any given time in residues with high cereal rye composition (>0.50) throughout both seasons relative to the no PPL, subsurface band and incorporated treatments. Incorporating the residues and PPL reduced the N remaining in residues with greater than 50% hairy vetch composition, but increased N remaining in residues with mostly cereal rye relative to the other PPL treatments. By the end of the corn growing season, 80% of residue N remained in pure cereal rye residues with no PPL or subsurface banded PPL, but only 60% of residue N remained from pure cereal rye residues + broadcast PPL. Increasing hairy vetch biomass in the residue decreased the percentage of N remaining at corn maturity to a minimum of 20% for monoculture hairy vetch. Important to note is that N lost from the cover crop residue was not necessarily immediately available for plant uptake due to potential N losses and microbial N cycling prior to corn uptake.

Overall, adding cereal rye to hairy vetch residues decreased decomposition and reduced the proportion N released from residues during a corn growing season. However, adding cereal rye to hairy vetch residues did not slow down the rapid rate of N release from the hairy vetch residue component. Subsurface application of PPL did not affect decomposition or N release relative to no PPL application, suggesting that subsurface band application may best conserve surface residues while providing additional N to corn. Broadcast PPL application increased residue decomposition and N release of residues containing mostly cereal rye. Incorporation of residues and PPL with tillage increased decomposition for all cover crop residues relative to no PPL and subsurface band PPL. Incorporation also increased the rate of N release for all cover crop residues except pure cereal rye relative to the no-till treatments (no PPL, subsurface band, broadcast).

Soil inorganic N spatial distribution

Random walk models and inverse distance-weighted interpolation were used to generate a surface of IN concentrations for the soil profile of each treatment at each growth stage in 2012 and 2013. Spatial IN distribution will be discussed by growth stage.

At corn emergence in both years, most no-till treatments exhibited a trend of decreasing IN from the soil surface to depth, with greatest IN concentrations at a depth of approximately 0-10 cm (Figures 9 and 14). The IN concentrations at the soil surface, and/or the depth to which the pulse of elevated IN concentrations extended, increased with greater hairy vetch composition in the residue, and with broadcast PPL application. Without broadcast PPL application, a depth trend was not present for cereal rye and very little IN was present throughout the soil profile. Incorporating the cover crop residues and PPL caused a region of elevated IN concentrations to a depth of 20 cm (plow layer) relative to 20-30 cm for all three cover crops in both years. The IN concentrations within the plow layer were greater for hairy vetch and the mixture relative to cereal rye in 2012, and greater for hairy vetch relative to the mixture and cereal rye in 2013. There was a slight trend of decreasing IN with depth, even within the plow layer. In 2013, IN tended to be more concentrated at the soil surface near the corn row than away from the corn row. The effect of the PPL starter may have affected sampling points around its delivery location in 2013, causing elevated IN in this region.

At the fifth-leaf stage in 2012, soil IN concentrations were greater overall relative to emergence, and the spatial trends observed at emergence remained evident (Figure 10). At fifth-leaf in 2013, spatial predictions revealed a trend of greater IN at the soil surface than at depth for the cereal rye and cover crop mixture in all PPL treatments, while this depth trend was not observed for the hairy vetch treatments (Figure 15). Within the subsurface band treatment, a region of high soil IN concentrations was present at the location of subsurface band PPL delivery. Inorganic N concentrations at the center of the band were 483 mg/kg (standard error = 79) in 2012 and 387 mg/kg (standard error =21) in 2013. Inorganic N concentrations decreased with distance from the center of the band, and the band’s zone of influence extended to a radius of 5 cm. Elongated regions of elevated IN concentrations were predicted below the subsurface bands, although concentrations in this region did not exceed 20 mg/kg.

At silking in both years, other than the mixture and hairy vetch incorporated treatments in 2012, IN was generally below 10 mg/kgand only slightly elevated at the soil surface (0-10 cm) vs. at depth (Figures 11 and 16). Within the subsurface band treatments, soil IN remained elevated up to 160 mg/kg in the center of the PPL bands, and 40-80 mg/kg in the 5 cm radius surrounding the bands. In 2012, elevated IN concentrations of 5-10 mg/kg extending halfway to the corn row in the hairy vetch treatment, and 10 cm laterally in the mixture, provided evidence of IN movement away from the band. In 2013, there was no evidence of IN movement away from the bands. In the 2012 mixture and hairy vetch incorporated treatments, IN remained elevated (up to 40 mg/kg in mixture and 80 mg/kg in hairy vetch) at 0-20 cm relative to 20-30 cm. Within the mixture and hairy vetch incorporated treatments in 2012, depletion of IN was observed in the corn row, particularly at 20-30 cm depth relative to the rest of the interrow profile.

At the milk stage in both years, IN concentrations remained less than 10 mg/kg throughout the soil profiles for all cover crop residues in the no PPL treatment (Figures 12 and 17). In the subsurface band treatments, IN was elevated to between 40 and 80 mg/kg in 2012 and between 20 and 40 mg/kg in 2013. In both years, there was some evidence for IN movement away from the band in the hairy vetch residue, although concentrations were less than 10 mg/kg even in elevated regions away from the band.

At maturity in both years, IN was concentrated at the soil surface for nearly all cover crop and PPL treatments, except for the cereal rye residue with no PPL and the cereal rye incorporated treatment in 2012 (Figures 13 and 18). Inorganic N concentrations of 40-80 mg/kg were observed in the PPL bands in 2012, and 10-40 mg/kg in 2013. Inorganic N concentrations greater than 20 mg/kg were isolated to within 5 cm surrounding the application site. In 2012, regions of IN concentrations between 5-20 mg/kg in the mixture incorporated treatment, and between 10 and 80 mg/kg in the hairy vetch incorporated treatment were observed extending from approximately 5 cm to 30 cm depth across the profile except in the corn row.

Soil inorganic N summary

At corn emergence, IN levels under the hairy vetch residue with no PPL amendment were 35 kg/ha in 2012 and 50 kg/ha in 2013 (Tables 5 and 6). At the same growth stage under the cereal rye residue with no PPL, IN levels were 12 kg/ha in 2012 and 14 kg/ha in 2013. The 2012 cover crop mixture provided IN concentrations similar to 2012 hairy vetch, while the 2013 mixture provided IN concentrations similar to 2013 cereal rye at emergence and fifth-leaf stages in both years. Broadcast and incorporated PPL amendment increased soil IN by ~50% and by 100-200% at emergence, respectively relative to no PPL for all cover crop residues. At emergence and most subsequent growth stages in the no-till treatments, IN was generally concentrated near the soil surface under the cover crop residues and decreased with depth, except in the cereal rye residue, which did not usually show a depth unless broadcast PPL was applied. Sidedress subsurface PPL application provided similar or greater IN concentrations as the broadcast and incorporated treatments at fifth-leaf, and soil IN remained localized to a 5-10 cm-radius zone around the delivery point throughout the growing seasons. In the incorporated treatment, soil IN was generally concentrated to a depth of 20 cm (plow layer).

In 2012, the drier year, soil IN increased between emergence and fifth-leaf, decreased between fifth-leaf and silking and increased slightly between milk and maturity for most treatments. A similar pattern was demonstrated in 2013, the wetter year, except that soil IN remained constant or decreased between emergence and fifth-leaf, probably due to early-season leaching or denitrification losses. In 2012, substantial IN (100 kg N/ha) remained in the hairy vetch incorporated treatment profile after corn maturity. The subsurface band PPL application at sidedress avoided early-season N losses in 2013, providing 1.3-2.6x the soil IN as the broadcast and incorporated treatments at the fifth-leaf stage. However, high concentrations of IN, C and increased soil moisture in the PPL band may have led to denitrification losses.

Research conclusions:

Data collected on cover crop N content, residue decomposition and corn N use efficiency are being used to validate the cover crop module of an adaptive-N management tool (Adapt-N, Cornell University). This module will provide users with estimates of N availability from cover crops. Spatial predictions of soil IN will be used to develop soil sampling protocols that accurately represent nutrient concentrations in fields that have received subsurface banded manure.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary

Education/outreach description:

The results of this study have been presented at the annual meetings of the American Society of Agronomy in 2012 (October 21-24, Cincinnati, OH) and 2013 (November 3-6, Tampa, FL). This research was also featured at the 2012 Sustainable Agricultural Systems Laboratory Field Day (August 2012), and as part of the 2013 Penn State “Cover Crop Innovations Webinar Series” (March 18, 2013). Three publications are now being prepared for submission into peer-reviewed journals.

Project Outcomes

Project outcomes:

Nitrogen costs 

Hairy vetch and cereal rye seed used in this study cost $6.61 and $0.75/kg, respectively. Considering the cost of seed used in this study, the average N content of monoculture hairy vetch across four site-years, and assuming a N fixation rate of 80% for hairy vetch, the cost of fixed N is estimated to be $1.54/kg N for the pure hairy vetch treatment in this study. A second calculation was performed for the 0.8/0.2 hairy vetch/cereal rye sown proportion, which yielded a mixture biomass composition of approximately 0.5/0.5 hairy vetch/cereal rye in North Farm 2012 and South Farm 2013. The average cost of fixed N, including cost of both hairy vetch and cereal rye seed, was calculated to be $2.07 for this seeding rate on North Farm 2012 and South Farm 2013. Hayden et al. (2014) calculated the cost of N fixation by hairy vetch in mixture with cereal rye to be $0.79 to $1.90/kg N across mixture proportions. The costs of fixed N calculated for the present study and by Hayden et al. (2014) do not account for fixed N in root tissue, and are thus likely to overestimate the cost of total fixed N in hairy vetch biomass. Of the total aboveground N accumulated in hairy vetch monoculture, approximately 80% is released during the first corn growing season (Figure 9), although not all N released may be plant-available. Therefore, the cost of plant-available fixed N is slightly higher than the cost of fixed N.
Pelletized poultry litter ($0.24/kg material; ~$12.13/kg plant-available N, assuming 2% plant-available N) was used in this experiment to optimize uniformity in manure application rates, but non-pelletized poultry litter is also available at a cost of $0.03/kg material; ~$1.65/kg plant-available N. The average cost of urea fertilizer over the past five years ranged from $1.10 to $1.35/kg (USDA-ERS 2013).  

Value of weed reduction        
    
Weed biomass results have not yet been summarized for this study, but observations made during the corn growing seasons and initial review of the data suggest that hairy vetch/cereal rye mixtures greatly reduce weed pressure relative to no cover crop and hairy vetch monoculture. Thus, planting cover crop mixtures is expected to reduce the cost of other weed management tactics in a no-till system such as high-residue cultivation and chemical control. 

Equipment costs  

The cost of a subsurface litter applicator is approximately $40,000, which is more than twice the cost of a broadcast spreader. The implement is currently cost-prohibitive to most individual farmers, although not to contract applicators and local conservation districts.  The cost is expected to decrease as the technology moves into commercial production (Kleinman 2009). The roller/crimper (10.5 ft model) is available for purchase at a cost of $3,360 (I and J Manufacturing, GAP, PA). Plans are also available on the Rodale Institute website (http://rodaleinstitute.org/our-work/organic-no-till/no-till-rollercrimper-plans/).

Farmer Adoption

A recent survey of 759 farmers who use cover crops throughout the north-central U.S. found that the number of cover crop acres planted by conservation-minded farmers has been on the rise since 2008 (SARE-CTIC 2013). The majority of survey respondents reported that they used cover crops within continuous or rotational no-till systems. Approximately one-third of respondents have used two-species or multi-species mixtures. Among the desired cover crop benefits, weed control and   N source were rated fourth (40% of respondents) and sixth (36% of respondents), respectively. However, few farmers mentioned chemical reduction as a desired benefit. In a recent survey of 1700 farmers who grow cover crops, 45% reported that their primary termination method is herbicides, 20% use tillage, 19% grow cover crops that winter kill and 10% mow. Only 1% of the respondents use the roller/crimper as the primary termination method (SARE-CTIC 2014). 
The sidedress poultry litter subsurface applicator used in our research is a prototype and not practical for farmer adoption at this time, largely because it stores and delivers a relatively small volume of poultry litter, requiring frequent refilling and slow tractor speed. A similar subsurface band applicator, the “Subsurfer”, is considered commercially-viable and has attracted much attention among researchers and conservation organizations within the Cheseapeake Bay watershed. The relatively high cost of the Subsurfer has limited adoption among individual farmers, but cost is expected to decrease over time.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

In order to develop cover crop mixture recommendations for specific management goals (e.g. N provisioning vs. weed control), additional research is needed on how weather conditions, field nutrient properties, timing of management and other factors interact with seeding rates to influence mixture composition. For example, within the four site-years of this study, which were conducted on similar soils in the same geographical location, we observed three different competition outcomes from identical seeding rates. Cover crop mixture composition affects total cover crop N and residue decomposition, so determining how to manage species proportions in mixtures will be important to achieving desired N supply and weed control benefits. While the present study characterized the effects of cover crop composition and manure application method on residue decomposition, it did not test the effect of cover crop residue initial mass on decomposition. Understanding the effects of both cover crop composition and total initial biomass would improve estimates of cover crop N release. Finally, in order to address corn stand establishment challenges in high-biomass residues, research is needed to optimize no-till planter modifications and timing of roller/crimper passes relative to planting.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.