Potential for a Rye Cover Crop to Reduce Nitrate Loss in Southwestern Minnesota G. W. Feyereisen,* B. N. Wilson, G. R. Sands, J. S. Strock, and P. M. Porter ABSTRACT Cover cropping practices are being researched to reduce artificial subsurface drainage NO 3 –N losses from agricultural lands in the Upper Mississippi watershed. This study was designed to investigate the in- fluences of fall planting date and climate on cereal rye (Secale cereale L.) biomass and N uptake in the spring, and to assess subsurface drainage NO 3 –N loss reductions. A soil–plant–atmosphere simulation model, RyeGro, was developed and used to predict rye cover crop establishment and growth, soil water balance, N cycling, and drainage NO 3 –N losses from mid-September through May in southwestern Minnesota. An imbedded stochastic weather generator provided model climate inputs. Inclusion of a rye cover crop sown on 15 September reduced N losses by 11.1 kg N ha 21 or 45% for a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] crop rotation. Fall sowing dates of 1, 15, and 30 October resulted in reductions of 7.8, 5.8, and 4.6 kg N ha 21 , respectively, by the end of May. Desiccation of the rye on 1 May resulted in reductions of 4.5, 2.2, 1.2, and 0.7 kg N ha 21 , for the 15 September and 1, 15, and 30 October sowing dates, respectively. Cover cropping practice provides promising opportunities for reductions in N losses for cropping rotations wherein the primary crops are harvested before mid-September and planted after mid-May. We predict that a winter rye crop can reduce drainage NO 3 –N losses on average 7.4 kg N ha 21 for southwestern Minnesota if planted on 15 September and desiccated on 15 May. H YPOXIC ZONES occur in several coastal estuaries around the world, and one of the largest zones can be seen in the northern Gulf of Mexico at the mouths of the Mississippi and Atchafalaya rivers (Rabalais et al., 2001). The low levels of O 2 in the Gulf waters can be traced to a cycle that is exacerbated by high levels of N entering the Gulf from these rivers (Rabalais et al., 1996). Nutrient loading in the Mississippi River has been increasing in quantity since the 1950s (Antweiler et al., 1995). Analysis of the sources of the N in the Mississippi River indicates that the Upper Mississippi watershed, including Minnesota and Iowa, is a significant contrib- utor. Agricultural subsurface drainage systems can ex- acerbate N losses from agricultural lands to surface waters (Zucker and Brown, 1998). These systems are used to increase crop productivity and reduce the risk of lowered crop yields from root zone excess water stress during wet years (Fausey et al., 1995); however, agri- cultural drainage systems have created a pathway by which nutrients can escape from the fields they are in- tended to enhance (Skaggs et al., 1994). One general strategy to mitigate the loss of NO 3 –N through subsurface drainage systems is to minimize the amount of nutrients reaching the drains (Mitsch et al., 2001; Randall and Mulla, 2001; Dinnes et al., 2002). Examples of methods proposed to implement this strat- egy include managing nutrient application more effec- tively, changing cropping systems, and using appropriate tillage practices. One of the methods related to cropping system modification is the use of fall-planted cover crops to assimilate residual soil NO 3 2 before establishment of the succeeding summer crop. Cover crops can affect the water balance, reduce the soil NO 3 –N level, and provide residue cover on agricultural fields that are normally fallow between summer crops. A cover crop growing in fall and spring takes up soil NO 3 –N, which is a leachable mineral form of N, and produces a nonleachable pool of organic N (ON) in the biomass of the plant (Hoyt and Mikkelsen, 1991). The ON in the plant residue is left on the surface of the ground, where it will be broken down and recycled during a period of months and years. Be- cause of their ability to reduce NO 3 –N leaching, cereal cover crops have become a major part of the proposed strategies to reduce nutrient loadings to the Chesapeake Bay (Boesch et al., 2001). In addition to N scavenging benefits, cover cropping can provide the advantages of surface cover, erosion protection, snow trapping, and weed suppression on fields from which silage corn or shorter season canning crops are harvested. The majority of research to date on the use of winter cover crops to reduce NO 3 –N leaching to groundwater or drainage effluent has been performed in warm, humid climates where the majority of nutrient loss occurs during the winter. In colder climates where the soil profile freezes during the winter, the majority of nutrient loss through subsurface drainage occurs during the spring, before sig- nificant biomass accumulation of the summer row crop. Moreover, the precipitation regimes are considerably dif- ferent between the warm, humid climates and the drier, colder northern climates. For example, the percentages of average annual precipitation falling during the period of October through March for a Washington state ex- perimental site (Kuo et al., 1997), a Maryland site (Ranells and Wagger, 1997), and Lamberton, MN (Strock et al., 2004), are 75, 45, and 26%, respectively. The challenge of obtaining the benefits of winter cover crop use in the northern Corn Belt is the short and cold growing season between summer row crops (Dinnes et al., 2002). There is a lack of research quantifying how effective the technique of growing cover crops between G.W. Feyereisen, USDA-ARS Southeast Watershed Research Lab., Tifton, GA 31793-5737; B.N. Wilson and G.R. Sands, Dep. of Bio- systems and Agricultural Engineering, Univ. of Minnesota, St. Paul, MN 55108-6005; J.S. Strock, Southwestern Research and Outreach Center and Dep. of Soil, Water, and Climate, Univ. of Minnesota, Lamberton, MN 56152-1326; and P.M. Porter, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108-6026. Re- ceived 5 May 2005. *Corresponding author (gfeyereisen@tifton. usda.gov). Published in Agron. J. 98:1416–1426 (2006). Modeling doi:10.2134/agronj2005.0134 ª American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ON, organic nitrogen; RSN, residual soil nitrate; SMC, soil moisture content; SWROC, Southwest Research and Out- reach Center. Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. 1416 Published online October 3, 2006