Evaluating the potential use of winter cover crops in corn–soybean systems for sustainable co-production of food and fuel

https://doi.org/10.1016/j.agrformet.2009.05.017Get rights and content

Abstract

Climate change and economic concerns have motivated intense interest in the development of renewable energy sources, including fuels derived from plant biomass. However, the specter of massive biofuel production has raised other worries, specifically that by displacing food production it will lead to higher food prices, increased incidence of famine, and acceleration of undesirable land use change. One proposed solution is to increase the annual net primary productivity of the existing agricultural land base, so that it can sustainably produce both food and biofuel feedstocks. This might be possible in corn and soybean production regions through the use of winter cover crops, but the biophysical feasibility of this has not been systematically explored. We developed a model for this purpose that simulates the potential biomass production and water use of winter rye in continuous corn and corn–soybean rotations. The input data requirements represent an attempt to balance the demands of a physically and physiologically defensible simulation with the need for broad applicability in space and time. The necessary meteorological data are obtainable from standard agricultural weather stations, and the required management data are simply planting dates and harvest dates for corn and soybeans. Physiological parameters for rye were taken from the literature, supplemented by experimental data specifically collected for this project. The model was run for a number of growing seasons for 8 locations across the Midwestern USA. Results indicate potential rye biomass production of 1–8 Mg ha−1, with the lowest yields at the more northern sites, where both PAR and degree-days are limited in the interval between fall corn harvest and spring corn or soybean planting. At all sites rye yields are substantially greater when the following crop is soybean rather than corn, since soybean is planted later. Not surprisingly, soil moisture depletion is most likely in years and sites where rye biomass production is greatest. Consistent production of both food and biomass from corn/winter rye/soybean systems will probably require irrigation in many areas and additional N fertilizer, creating possible environmental concerns. Rye growth limitations in the northern portion of the corn belt may be partially mitigated with aerial seeding of rye into standing corn.

Introduction

The increasing recognition of the atmospheric impact of fossil fuel combustion has spurred research and development of renewable alternatives, particularly fuels derived from freshly produced biomass. Ethanol has been the most publicized biofuel to this point, primarily because it can be easily produced from plant-derived sugars and starches and can be used in internal combustion engines with minor modifications. All major ethanol plants in the USA currently use corn grain as their carbon source. However, this constrains capacity to a small fraction of total US fuel consumption, and places energy production in competition with food production for raw material.

Long-anticipated technological improvements may eventually lead to cost-competitive energy production from non-grain biomass, but this will not alleviate food versus fuel concerns if it results in displacement of grain crops with dedicated bioenergy crops such as switch grass, miscanthus, and hybrid poplar. Meeting the ambitious goals that have been set for bioenergy production without impacting food production and without massive land use change is a tremendous challenge. Some have suggested that this can be done with prairie establishment on degraded agricultural lands (Tilman et al., 2006), but others feel that this potential pool is not nearly as large as has been proposed (Russelle et al., 2007). It may be one part of a set of solutions, or one piece of the pie, to borrow the analogy that Pacala and Socolow (2004) have applied to the related issue of stabilizing atmospheric CO2 levels, but other sustainable, food-neutral sources remain to be identified. Here we explore one potential contributor – greatly increased use of winter cover crops in existing agricultural systems.

This approach represents a modification of existing farming systems so that they more efficiently use the solar radiation annually available to them to create additional biomass – to get more carbon into the system before taking more out. Under the warm, high-irradiance conditions prevalent in midsummer in the Midwestern United States, corn fields are capable of net assimilation rates as high or higher than any reported temperate zone ecosystems in the world. And yet, despite this exceptional peak photosynthetic capacity, the annual net primary productivity of a corn/soybean rotation is roughly the same as that of native perennial ecosystems in the same region that have much lower maximum photosynthetic rates (Prince et al., 2001, Schmid et al., 2000). The reason for this is evident in Fig. 1, where we have plotted half-hourly rates of NEE for corn (Zea mays L.) and a perennial species, alfalfa (Medicago sativa L.), both measured in southern Minnesota, USA. The high mid-season productivity of corn is apparent, but so is its brief lifespan. Net assimilation only lasts for about 105 days (soybean is even more ephemeral, with net carbon uptake lasting approximately 90 days). The perennial alfalfa, on the other hand, begins extracting CO2 from the atmosphere in this region on about day of year (DOY) 105 and continues until approximately DOY 315, a growing season that is twice as long as either annual crop. As Baldocchi (2008) has recently noted, the most productive ecosystems are those with long growing seasons, rather than high peak productivity.

One possible way to contribute to prospective bioenergy needs and continue to meet the demand for food and feed in a sustainable manner is to take advantage of both the peak summer efficiency of C4 corn, and the broader, spring-fall capabilities evident in perennial C3 systems through the inclusion of cover crops. There are a number of cover crops that are well adapted to the cooler conditions that prevail in the spring and fall. Some do not overwinter, and hence would be useful as either spring or fall crops, but not both. Others, such as winter wheat (Triticum aestivum L.) and rye (Secale cereale L.) can survive even the harsh winters of the northern Corn Belt, and thus could assimilate C in both the fall and the following spring. Winter rye has been extensively tested as a cover crop in corn/soybean rotations (Ruffo et al., 2004, Coelho et al., 2005, Feyereisen et al., 2006a, Feyereisen et al., 2006b, Kaspar et al., 2007). It is sometimes planted following corn harvest, then killed just before planting of soybean or corn the succeeding spring. We have measured NEE continuously in such a farming system for several years at Rosemount, MN, USA (Baker and Griffis, 2005). Half-hourly data from an entire year are shown in the lower panel of Fig. 2. Comparison with a nearby conventional soybean field (upper panel) reveals substantial additional C assimilation in the cover cropped system. The accumulated rye biomass in this system was nearly 5 Mg ha−1, but to accomplish this it was necessary to allow the rye to continue growing for several weeks after the succeeding soybean crop was planted, and this ultimately had a negative effect on soybean yield, decreasing it by 15% relative to a control field. It may be possible to maintain the rye production and still remove it early enough to avoid impacts on the following crop if the rye can somehow be planted earlier in the fall. Conventional planting with a grain drill must wait until the preceding corn crop is harvested, but some producers have experimented with helicopter seeding into the standing corn canopy, allowing the rye to germinate and begin vegetative growth prior to corn harvest. We hypothesized that this would increase the amount of rye biomass available for harvest at an earlier point in the spring to minimize effects on the following crop. Furthermore, since an established cover crop also provides erosion protection in the fall and spring, some fraction of the stover from the previous corn crop could probably be sustainably harvested without negative soil erosion or quality effects, a point noted in a recent life-cycle analysis of stover ethanol production (Kim and Dale, 2005). However, it is difficult to draw any conclusions regarding the potential regional or national biomass contributions from winter rye cover crops in corn/soybean systems on the basis of a few isolated field experiments. Also, producers are hesitant to adopt cover cropping without a broader assessment of the potential risks and benefits. In particular, there are concerns that the water used by a winter cover crop will increase the likelihood that the subsequent summer crop will be affected by drought stress. Ideally these questions might be addressed with multi-year field experiments at multiple sites, but such experiments are difficult to conduct and even more difficult to fund. As a preliminary step, we have developed a model to tentatively address these questions, and to use heuristically to guide further field research. Our specific modeling goals were:

  • 1.

    To estimate the potential additional biomass that can be produced at locations across the corn belt through the use of winter rye cover cropping.

  • 2.

    To estimate the additional water use associated with winter rye cover cropping.

  • 3.

    To estimate how planting date (particularly through aerial seeding into standing corn) and kill date affect the answers to goals 1 and 2.

  • 4.

    To delineate uncertainties and areas needing further research.

  • 1.

    The model must be simple and robust, but should be based to the greatest extent possible on sound physical and physiological principles.

  • 2.

    The model must operate with the meteorological data available from standard agricultural weather network stations, i.e. – temperature, relative humidity, solar radiation, shallow soil temperature, and precipitation.

Section snippets

Approach

The model estimates potential biomass production and water use of winter rye, i.e. – it assumes no limitations due to water stress or nutrient deficiencies. It scales leaf photosynthesis and transpiration to the canopy with a sun/shade approach (Norman, 1993, dePury and Farquhar, 1997, dePury and Farquhar, 1999). A number of physiological parameters must be known or estimated, and management parameters are also required, including harvest date for the previous crop and seeding date for the

Model results

Fig. 6 summarize the model estimates of rye biomass if it is produced conventionally, i.e. – planted with a grain drill after fall corn harvest and removed prior to spring planting of either corn (top panel) or soybeans (bottom panel). For each site the vertical solid bar represents the mean yield over 5 years with moderate N fertility (Vm = 80 μmol m−2 s−1) and the shaded bar represents the high fertility case (Vm = 95 μmol m−2 s−1). The latter would likely require significant additional N, while the

Conclusions

We have used a simple, robust model to estimate potential biomass production from a winter rye cover crop in continuous corn and corn–soybean systems at eight locations across the Midwestern US. The results indicate that this could provide 1–8 Mg ha−1 of additional biomass annually. The potential productivity is proportional to total available PAR and GDD during the period between fall corn harvest and spring planting, thus the lower amounts correspond to the northernmost locations in MN and WI.

Acknowledgements

Support for this project was provided by the USDA-ARS Renewable Energy Assessment Project (REAP) and the Office of Science (B.E.R.), U.S. Department of Energy, grant DE-FG02-03ER63684.

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