By Joel Gruver and Andy Clayton
Organic farmers know that cover crops can contribute to environmental quality and general soil improvement, but implementation of specific cover cropping strategies that cost-effectively capture benefits while minimizing challenges is easier said than done.
The Western Illinois University (WIU) Organic Research program is using field experiments, dialogue with farmers, and literature review to investigate precision cover cropping and optimize the value of cover crops. This article provides an overview of precision cover cropping concepts and summarizes recent research.
It’s clear from online ag forums and speaking with farmers that the term precision cover cropping has at least four distinct but somewhat overlapping meanings: management of cover crops to achieve specific objectives; strategic placement of cover crops in specific fields or parts of fields; strategic placement of cover crop rows in relation to other cover crop rows and/or cash crop rows (often using GPS guidance); planting of cover crops with a precision planter. We’ll explore these concepts.
Management of Cover Crops to Achieve Specific Objectives
Identification of specific cover cropping objectives or cover crop attributes (erosion control, weed suppression, N release, etc.) is a critical step in managing cover crops with precision.
Many print and electronic cover crop resources rate cover crop species with respect to a variety of attributes, including impact on soil condition, soil ecology, and aspects such as attracting beneficials. Achieving the potential implied by a particular attribute rating requires the intersection of multiple factors (suitable genetics, timely and effective field operations, favorable soil and weather), some of which are beyond a farmer’s control. Careful planning won’t trump Mother Nature, but can increase the likelihood that management objectives can be achieved at least partially regardless of what Mother Nature throws your way.
Web-based decision tools such as the Midwest Cover Crop Council’s Cover Crop Selector support integrated consideration of attribute ratings, soil properties and growth windows. Some tools, such as Green Cover Seed’s Smart mix calculator include other factors such as seed count and cost.
Historically much of the cover crop seed planted on organic and conventional farms has been VNS (variety not stated). VNS seed is generally cheapest, but is less reliable than named varieties. Most named varieties used for cover cropping (e.g., Aroostook cereal rye) were selected for forage or grain characteristics rather than cover crop potential. Expanded interest in cover crops is pushing public and private breeders to develop cover crop varieties with enhanced performance. Important characteristics to consider when comparing varieties include: winter hardiness, tolerance of wet soils/flooding, rate of maturation in the spring, shoot and root biomass production, root shoot ratios, rate of establishment and competitiveness with weeds, impact on stand and health of following crops, and seed count/lb.
As shown in this table, cereal rye varieties evaluated over 4 years in Iowa varied widely in biomass production and impact on corn stand and yield. Generally higher biomass production resulted in lower corn stand and yield. Also, cereal rye varieties of southern origin (e.g., Elbon and Maton) generally matured earlier and produced more biomass.
Annual ryegrass varieties vary widely in winter hardiness. Only varieties with proven hardi- ness, such as Bounty, Bruiser, KB Royal, King are likely to overwinter in the Midwest. These overwintered well at the WIU Allison Organic Research and Demonstration Farm in 2009-10 and have done well in other cold winter trials.
One last important consideration when shopping for cover crop seed is the difference between trademarked and certified seed. Unlike certified seed (e.g., Graza radish), a trademarked brand does not guarantee a specific genetic line.
In addition to careful selection of cover crop varieties, inoculation and fertilization/manuring can be used to enhance cover crop performance. Twenty lbs./a of N applied as Chilean nitrate (16-0-0) doubled the biomass production of spring planted Graza radish at the WIU organic research farm in 2012. In a recent on-farm trial in Southeast Illinois, inoculation of 2 hairy vetch lines increased biomass production by 67 and 80% and total biomass N by 58 and 63 lbs./a. Inoculation of legume cover crops is low cost and highly likely to improve cover crop performance.
Strategic Placement of Cover Crops in Specific Fields or Parts of Fields
Over the last decade, conservation professionals have increasingly recognized that “getting the right practices, in the right places at the right time and at the right scale is what makes conservation effective.” (Cox, 2005) While largely untested, targeted placement of cover crops (as compared to planting whole fields or whole farms) will reduce the cost of cover cropping while most likely increasing return on investment in cover crops.
Factors to consider when deciding how, where, when and at what scale to establish cover crops include: spatial variation in soil erodibility and historical degradation, opportunities for enhanc- ing the performance of conservation structures like terraces and waterways, wildlife/beneficial insect behavior and impact on the following crop. In 2013, strips of diverse cover crop cocktails featuring sources of nectar and pollen (e.g., sunflowers and buckwheat) were established along the edges of cornfields at the WIU organic research farm with the intention of enhancing biocontrol of corn pests. Based on preliminary observations of active foraging by beneficial insects and low levels of corn earworms, we plan to expand this practice.
Another example of targeted cover cropping that may be more appropriate for a highly erodible landscape is strategic early harvesting of strips within a cash crop field to facilitate early planting of cover crop strips where they will control erosion most effectively while limiting potential risk to the following crop (e.g., cereal rye often reduces the yield of subsequent corn).
Strategic Placement of Cover Crop Rows
Many large-scale organic grain farmers have improved the efficiency of their field operations through the adoption of GPS guidance, but very few have used guidance to enhance their cover cropping practices. Innovative conventional farmers have tried planting cash crops directly over or adjacent to cover crops, but strategic placement of cover crop rows is a largely untested concept within organic agriculture.
In May 2010, we planted popcorn directly over radishes that had been planted on 30” rows with a four-row Buffalo planter in August 2009, and then ridged with a four-row Buffalo cultivator about a month after planting. The popcorn planted beautifully into the radish ridge, which had winter-killed in December and left little residue visible at the time of planting. However, rainfall that was three times higher than normal during the month after planting necessitated full field tillage and replanting.
In 2011, we evaluated corn following volunteer oats, radish on 30” rows planted with a four-row Buffalo planter with volunteer oats, and radish drill-planted on 7.5” rows with volunteer oats. Both treatments with radish produced significantly higher yields than the no-radish treatment (see chart on page 13). Corn yield following radish on 30” rows was not significantly different than corn yield following radish on 7.5” rows, but the 30” radish seeding rate was ~ 70% lower. In this experiment, neither the corn nor radish was planted with GPS guidance, but careful flagging enabled us to plant the corn rows very close to the preceding 30” radish rows.
In 2012, we compared corn planted with RTK guidance (Real Time Kinematics, the most reliable form of GPS guidance allowing sub-inch accuracy) over 30” rows of Graza and Tillage Radish that had been planted carefully with flags but without GPS guidance. The radishes winter-killed, and the whole field was shallowly rotavated and then field cultivated prior to planting corn. There was no difference in corn yield with respect to radish variety, but corn following radishes that had been cultivated about a month after planting yielded ~20 bu/a more than corn following radishes that had not been cultivated. The strong beneficial effect of one cultivation the previous fall is somewhat of a mystery, but may have been related to reduced growth of winter annual weeds in those plots.
In 2013, we evaluated corn planted following 30” radish rows, 30” radish rows with twin rows of peas in the inter-row and no-cover. The Tillage Radishes were planted with RTK guidance in late August 2012 and twin rows of Admiral yellow peas were drilled the next day between the radish rows. The radishes without peas were cultivated once about a month after planting. Radish root biomass in late November 2012 was ~ 20% higher in the plots with the twin rows of peas.
We intended to plant directly over the radish rows without any pre-plant tillage but had to modify this plan due to record rainfall in spring 2013. We ended up using a high residue cultivator to terminate many of the weeds in the radish and radish+pea plots prior to planting. Weeds in the fallow plots were terminated using a rotavator. Planting was difficult but our ridge-till planter did an admirable job of removing in-row weeds, and the soil tilth was much better than in other fields on the farm where we did full-field spring tillage. The highest yielding plot was a radish+pea plot (~200 bu/a) but there were no statistical differences between the treatments.
In addition to these large-scale plots, we have planted many additional cover crop species using push planters such as the Earthway seeder and Planet Jr. seeder. The #22 plate for the Earthway works well for oats, rye, barley, wheat, vetches, lentils and buckwheat. The #14 plate works well for field peas, Austrian winter peas and faba/bell beans. The Planet Jr. seeder provides more control of seeding rate, but does not singulate seed. We found the following cover crop species/Planet Jr. seed plate hole combinations work well: turnips (1), radish (11), winter peas (36). Using small plots, we have observed that in-row soil test P and K levels are significantly higher following winter-kill of radishes, mustard, oats and phacelia than in the inter-row zone.
Precision planting of cover crop seeds
Cover crop seeding methods range widely in cost and risk. Precision planters that singulate seed and place it at uniform depth (generally into moisture) represent the optimal planting technology on most Midwest farms, but historically have rarely been used for establishing cover crops. According to Iowa custom rates, the average cost for operating a no-till planter is $18.45/acre, which is significantly higher than traditional cover crop seeding methods. Savings may be realized if seeding rates can be reduced. The principal advantage of establishing cover crops using a precision planter is a very uniform stand using the lowest possible seeding rate. Other potential advantages include opportunities for bio-strip-till (i.e., planting cash crops directly over or in close proximity to cover crop rows), opportunities for intercropping using a split planter, and more rapid germination and growth of cover crops during the limited fall window for growth.
We have had good success using the small 60-cell milo plates in Kinze meters to plant radish. The lowest possible seeding rate using our planter transmission is ~ 3 lbs./acre or a little over four radish seeds per foot. Precision planter plates suitable for planting radish are listed in the following table. Insecticide boxes and air delivery systems (e.g.Valmar and Gandy) also can be used to precisely meter cover crop seeds.
Table 1. Recommended planter plates for cover crop radish.
|White||60-cell sugar beet|
|Deere||small sugar beet 4/64 inch|
|Kinze 2000 and 3000 series||small 60-cell milo|
|Kinze Edge Vac w/ e-sets||60-cell small sugar beet 1/16 inch|
|Monosem||6020 plate; vacuum set to 15|
Conclusions and future options
Greater precision in cover cropping practices should increase the positive effects while reducing negative effects. We believe that we have just scratched the surface with respect to precision cover cropping. Our larger goal is to develop more efficient and effective organic farming systems through the integration of GPS guidance, controlled traffic, minimum tillage, precision cultivation, precision placement of inputs such as seed treatments and organic fertilizers and precision cover cropping.
Joel Gruver is an assistant professor of Soil Science and Sustainable Agriculture at Western Illinois University and the Director of the WIU Organic Research Program. He invites readers interested in precision cover cropping to contact him at firstname.lastname@example.org.
March | April 2014