Science and policy to manage nitrogen with cover crops—The Maryland experience

During the 1980s, research showed nitrogen-stimulated eutrophication was contributing to the Chesapeake Bay's deterioration, and cover crop research shifted from legumes to replace fertilizer, to winter cereals to reduce nitrogen (N) leaching. The Maryland Department of Agriculture (MDA) adopted nutrient regulations and added technical and financial resources promoting N-capturing cover crops and N-use efficiency. Today, Maryland's "Clean the Bay" effort includes regulations and generous cost-sharing for planting and terminating cover crops. The MDA supports cover cropping on ∼180,000 ha annually, out of ∼350,000 ha of land in row crops not planted to winter cereals, achieving by far the nation's highest percentage (∼50%) of cropland in cover crops. Brassica species gained popularity because they effectively capture N when planted early, and also provide immediate farmer benefits, such as alleviating soil compaction, suppressing certain weeds, and avoiding N immobilization. The MDA tweaked its program to include brassicas and, later on, legumes in mixtures with grasses. Still, research showed large profile N pools remaining in fall, much of it deeper than 90 cm. Further work indicated this residual N could be cleaned up by early-planted, deep-rooting cover crops. Additional progress on enhanced cover cropping included inter-seeding to start growth early, multiple species for stand reliability and immobilization avoidance, and planting "green" into living cover crops for maximizing soil health and water conservation benefits. Maryland's Cover Crop Program is modified annually, with recent changes incentivizing more time for cover crops to function at each end of their growing season.