For the sake of illustration, we introduce two hypothetical scenarios for land use conversion for cover crop seed production, with the caveat that these scenarios do not consider all variables that go into real-world upscaling of seed production for covers. In scenario one, we consider direct competition of land between maize production and cover crop seed production and assume no change in yield due to cover cropping. If based on 2019 average maize yield data we converted land used for maize production to cover crop seed production, rye seed production would result in as much as 16,459,200 MT of maize grain removed from the market, while hairy vetch seed production would result in as much as 43,525,440 MT of grain removed. This larger number is comparable to the annual amount of maize grain lost to disease in the U.S. in 2015, which amounted to 13.5% of total production12.
To avoid the tradeoffs caused by producing cover crop seed on current cash crop lands, alternatives may be proposed. This caused us to consider a second scenario, where cover crop seed might instead be grown on land held in the conservation reserve program (CRP), which pays farmers to restore marginal or ecologically sensitive land to native habitat13. Cover cropping the entire U.S. maize area would require the equivalent of as much as 18% (rye) to 49% (hairy vetch) of the 2019 CRP enrollment for cover crop seed production14. Using this much CRP land to produce cover crop seed would significantly disrupt the program’s conservation and ecosystem services benefits. While further study would be needed, it seems unlikely that CRP or other marginal lands could be used instead of cash crop land to grow cover crop seed without significant ecological tradeoffs.
Acknowledging that our simplified scenarios are subject to variation in real agricultural systems, they make clear the potentially large hidden land requirements of bringing cover crops to scale. U.S. maize seed production takes less than 0.5% of the land devoted to the crop, while from our available data, the higher yielding cover crop values would still take an average of 12 times (median 7 times) as much land. This comparison is worthwhile because it makes concrete the abstract idea of cover crop seed yield by benchmarking to a well-established, efficient seed production system. In addition, among the covers examined there was large variation (berseem clover as low seed yield; turnip and canola as high seed yield), it is important to note that ecosystem benefits of covers are not equal, and do not fit into a wide array of production systems. Hence ecologically and agronomically, it is generally preferable to plant rye or vetch over turnip, even though turnip has high seed yield15.
Planning for and mitigating projected land use needs for cover crop seed production may help pre-empt social conflicts over how to enhance agricultural sustainability16, which have included such high-profile disputes as food versus biofuels17. For example, arable lands (e.g., pasture) in other temperate regions that are not currently critical to food production could potentially be converted to cover crop seed production without major environmental cost, and in doing so may provide new market opportunities to farmers. While this could increase opportunities for participatory agronomy, it would also likely alter ecological services through changes in management intensity.
The driver behind this potential land use impact is low seed yield, though we acknowledge that yield estimates are highly uncertain. The United States Department of Agriculture does not keep statistics on cover crop seed yields, and agronomists researching these crops rarely report seed yields in the formal literature because the crops are most often terminated before maturity. This forced us to search for seed yield estimates in non-academic and private sources (Supplementary Data 1). Improving yield appraisals is readily achievable and would significantly improve assessments of land needed to produce cover crop seed. Yet, despite their uncertainty, these data highlight that most cover crops are almost certainly “underdeveloped” cultivated species in comparison to the generally much higher seed yields of cash crops of similar taxonomic backgrounds. Decreasing this breeding gap should reduce land use impacts of cover cropping.
Our results suggest that cover crop breeding research should shift to include more emphasis on increasing seed yield, in addition to environmental outcomes. Only a handful of cover crops are actively being bred for seed productivity (e.g., pennycress and camelina18). Most breeding has focused on ecosystem service values9 and forage quality11. Fortunately, advanced breeding techniques, public-private partnerships, and participatory, farmer-inclusive breeding practices could make it possible to increase the tempo of plant breeding and the subsequent adoption by farmers19. In particular, breeding might focus on classic domestication syndrome traits such as non-shattering, lack of dormancy, and flowering time20. Most of these traits have a well-known genetic basis21,22. Leveraging these known traits to improve seed yields may reduce land use impacts, provide economic benefits to seed producers, and improve farmers’ access to cover crop seed.
One potential way to speed the achievement of breeding goals could be to explore using a CRISPR/Cas9 approach to improve specific domestication traits, while still selecting for characteristics complementary to improved ecosystem service production. Rapid domestication using CRISPR/Cas9 recently has been successful in other plant species23. Specifically, the CRISPR system has been used to modify traits such as flowering, fruit size, fruit shape, plant architecture, and nutrient content in both domesticated and wild species24,25. However, a major limitation will be developing tissue culture protocols for cover crops as this has not been done and large variation exists in regeneration ability within and across species. In addition, potential regulation of these technologies in some world regions could translate into higher costs for producers.
Source: Ecology - nature.com
