TILLAGE EFFECTS ON SOIL-TEMPERATURE, SHOOT DRY-MATTER ACCUMULATION AND CORN GRAIN-YIELD

Shoot dry matter data from a multi-year experiment with four tillage treatments (conventional (MB), chisel (CH), ridge till (R) and no-till (NT)) conducted on two sites at Ottawa, Canada (45 degrees 23'N, 75 degrees 43'W) were analyzed to determine to what extent tillage differences influenced shoot growth over the vegetative period. Over three years the ranking of corn shoot dry matter at 6-leaf, 12-leaf and silking stages was MB > CH > R > NT. Tillage effects were only statistically significant (P less than or equal to 0.05) at early vegetative stages. Phenological data indicated a greater delay in silking than in reaching the 6-leaf stage under conservation tillage. Thermal units (growing degree days, base temperature 10 degrees C) in the seed bed (0.05 m) were also ranked MB > CH > R > NT for development between planting and the 6-leaf stage. By the 12-leaf and silking stages, tillage treatment had little effect on soil temperature. Soil temperatures at the 0.20 m depth were unaffected by tillage treatment. Shoot growth gradually became a function of air temperature after the growing tip emerged from the soil at the 6-leaf stage. Differences in grain yield and moisture content at harvest among tillage treatments were not attributable to differences in vegetative dry matter accumulation, but to developmental delays caused by fewer soil thermal units available to the 6-leaf stage under conservation tillage. These findings suggest that reducing the residue covering at planting, and thereby raising seed bed soil temperature, would improve grain yields on conservation tillage.