Canola and Wheat Double-Cropping Systems in Central Iowa.

A field experiment was initiated near Boone, IA to study two double-cropping systems that each includes spring- and winter-sown crops, as well as an underseeded green manure crop, for their productivity and effect on successive corn (Zea mays L.). A comparison with a traditional corn-soybean [Glysine max (L.) Merr.] is also provided. In one double-cropping system, spring canola (Brassica napus) is succeeded by winter wheat (Triticum aestivium L.) and in the other system; spring wheat is succeeded by winter canola. The winter crop in either system is underseeded with a red clover (Trifolium pratense L.) green manure crop in the following spring just before winter crop green up. Red clover acts as cover after winter crop harvest, is terminated the next spring, and along with liquid swine (Sus scrofa domestica L.) manure, acts as a source of biological N for the succeeding corn. The yields of winter wheat and winter canola are nearly double those of the respective spring varieties. Combined wheat+canola yield, however, is similar between the two systems (4.1 Mg ha^-1). Fall red clover biomass just prior to a killing frost combined with spring regrowth just prior to termination did not differ between the systems across the two years (4.4 Mg ha^-1). Total N content of combined fall+spring red clover biomass also did not differ between the systems (124.3 kg N ha^-1). Pre-sidedress soil nitrate (7.6 g kg^-1) and corn stalk nitrate concentration (1105 g kg^-1) did not differ between the systems. Based on pre-sidedress soil nitrate tests, corn following both double-cropping systems did require less sidedress N than corn following soybean in 2013 (68 kg N ha^-1 vs. 102 kg N ha^-1). On average, corn yield was similar between the two double-cropping systems at (9.3 Mg ha^-1) but less than corn following soybean (11.0 Mg ha^-1). However, corn stalk nitrate levels at physiological maturity were greater in the double-cropping systems than the traditional system (1,106 g kg^-1 vs. 415 g kg^-1). These results show the potential of alternative cropping systems in maintaining overall productivity and properly managing N.