Effect of cover crops established at time of corn planting on phosphorus runoff from soils before and after dairy manure application

Phosphorus (P) runoff from agricultural soils is a concern due to eutrophication. The simultaneous corn and cover crop system was developed by U.S. Department of Agriculture's Natural Resources Conservation Service (USDA-NRCS) for dairy farms in the northeastern United States, where short growing seasons have limited fall seeding of cover crops. The simultaneous corn and cover crop system uses post-emergence imidazolinone herbicides to allow for simultaneous seeding of cover crops with silage corn. Trials were established at two locations in the Cannonsville Reservoir watershed, New York, part of New York City's drinking water supply system, to assess the effects of this cover cropping system on water quality. Rain simulations (60 mm hr-1) were conducted to evaluate the initial 30 minutes of runoff from small (1 x 2 m; 3 ft x 7 ft) plots before and after surface application of dairy manure (50 or 100 kg total P ha-1; 45 or 89 lb P ac-1). Corn yields from plots interseeded with red clover compared most favorably with the conventionally cropped controls, with no significant differences in yields noted between the two treatments at either location. Prior to dairy manure application, losses of P in runoff were primarily a function of erosion. Because all cover crops increased ground cover (up to 81 percent greater than the control), total P loads in runoff were significantly lower from cover cropped plots (averaging 10 mg per plot) than from conventionally cropped controls (averaging 39 mg per plot). At the same time, suspended solids loads averaged 25.3 g (1 oz) from the control plots and 5.9 g (0.2 oz) from the cover crop plots. Despite concern that release of soluble P from the cover crops could enrich dissolved reactive P in runoff, dissolved reactive P losses from the simultaneous corn and cover crop system were generally not different from conventionally-cropped silage corn losses. Application of manure obscured cover crop/conventional silage corn treatment differences with regard to P runoff, with dissolved reactive P becoming the dominant form of P in runoff due to contributions of readily soluble P in manure. Because runoff P losses were already high from unmanured conventional silage corn plots, application of manure did not significantly increase P losses from some of the conventional silage corn treatments. Results highlight the agronomic and water quality benefits of the simultaneous corn and cover crop system, particularly when implemented with red clover.