The quantity of residue that is produced and can potentially be removed is directly related to the production yields of crops in the rotation. County-level harvested acres, yield, and total production for 1997-2001 were obtained from USDA-NASS, and 5-yr averages were determined from these data for all counties in the 10 states. These average yields were then converted to gross residue estimates using ratios of fresh grain weight to bushel factors and ratios of dry weight residues to fresh grain weight. For the three major crops considered, these factors were as follows: for corn, 25.1 kg of dry stover/bu of grain (56 lb/bu) and a 1-to-1 ratio of dry stover to fresh grain mass; for spring wheat, 35.4 kg of dry residue/bu (60 lb/bu)

Field Operations Associated with Corn Rotations for Conventional,
Reduced/Mulch, and No-till Field-Management Practices

and a 1.3-to 1-ratio of dry residue (chaff and straw) to grain; and for winter wheat, 46.3 kg/bu of grain (60 lb/bu) and a 1.7-to-1 ratio of dry residue to grain. For soybeans, these factors were 40.8 kg of dry residue/bu (60 lb/bu) and a 1.5-to-1 ratio of dry residue to beans (6).

Field Operations Associated with Wheat Rotations
for Conventional, Reduced/Mulch, and No-till Field-Management Practices

To quantify the amount of residue that can be sustainably removed, quantities of residues that must be left on the field to maintain rain and/or wind erosion at or below tolerable soil-loss levels (T) must first be esti­mated. The revised universal soil loss equation (RUSLE) and the wind erosion equation (WEQ) are used to estimate these residue quantities (7,8).

RUSLE and WEQ are designed primarily to estimate long-term, average annual soil erosion on a site-specific field characterized by a particular soil type, slope and runoff length, field length, cropping and management prac­tices used, and localized climate conditions. Residues that must be left on the field, with respect to rainfall and wind erosion, are estimated for each soil type, each crop rotation, and each tillage combination considered in this analysis, with the higher of the two estimates being the quantity needed to remain on the field.