Many crop producers will be attempting to leave a specific amount of residue cover on fields this spring following tillage by a chisel plow, disk, or other full-width tillage equipment. Amounts to be left are specified in their soil conservation compliance plans.

A set of typical Iowa tillage operations might include: a disk- chisel with twisted shovels and a field cultivator, followed by a row-crop planter. Using suggested numbers (Soil Conservation Service and Equipment Manufacturers Institute) for percentage residue remaining, the amount of cornstalk residue cover may range from 14 to 34 percent. More fragile soybean residue may range from 5 to 11 percent. In soybean residue, a crop producer will need to decide what tillage equipment to leave parked inside the machine shed if a significant amount of residue cover is required.

Crop producers should be aware that although many factors affect the amount of residue remaining, the equipment operator also influences cover by selection, adjustment, and operation of the tillage implements. In general, a less aggressive tillage implement configuration, shallower tillage depths, and slower speeds tend to leave more residue cover.

Field tests in undisturbed standing cornstalks at Ames have indicated that adjusting gang angle on a tandem disk to a less severe position left 4 to 12 percent more residue remaining of that which was present before tillage as compared to a more aggressive disk gang angle. Replacing 3-inch twisted shovels on a chisel plow with 2-inch straight chisel spikes or 16-inch wide sweeps left 8 to 12 percent more residue. Reducing tillage depth from 6 inches to 3 inches left 4 to 11 percent more residue. Decreasing disk speeds from 4.5 to 3 mph left 3 to 6 percent more residue. This and other research indicates that changing tillage implement configuration and depth can leave 5 to 10 percent more surface corn residue. Reduced speed has a somewhat lesser effect.

Although spring equipment operations influence residue cover, other factors prior to tillage also affect cover and are probably responsible for the wide range in residue amounts from identical tillage systems. Such factors include: the type of crop (e.g. corn or soybeans) and yield level from the previous growing season, uniformity of spreading by the combine straw and chaff spreaders, residue harvest by livestock or machine, and overwinter climatic conditions for residue decomposition.

Because of the inherent uncertainties, tillage equipment operators are advised to make a residue cover check after a couple of tillage passes before tilling the entire field. A single check with a 50- foot camline takes about five minutes. An average of three checks produces a better measure of the actual percentage cover. Thus far, the weather has not been cooperative with regard to early preplant (EPP) herbicide applications.