Tillage system by planting date interaction effects on corn and soybean yield

Spring soil temperature and soil water content can be influenced by tillage system. If a tillage system and planting date interaction exists, planting on a single date, as is done in most tillage trials, could bias yield results. We tested for this interaction by comparing corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields using strip tillage, no tillage, and disk-chisel tillage systems with planting dates determined by soil temperature and water content conditions within each tillage system. A 3-yr study (2002-2004) was conducted on a site near Newton, IA that had three soil types: Cumulic Hapludolls, Aquic Hapludolls, and Cumulic Haplaquolls. A split-plot design was used with tillage as whole plots arranged in four randomized complete blocks. Crops in all tillage system treatments were planted on three dates that comprised the split-plots. The criteria to determine the planting dates were soil temperature (>10 degrees C for corn and >13 degrees C for soybean for 12 consecutive hours) and soil water content (less than or equal to the lower plastic limit for any of the tillage treatments) at the 0.05-m depth. A planting date occurred for each of the tillage systems as these criteria were met. For both crops, the earliest date having these soil conditions occurred simultaneously for disk-chisel and strip tillage. The no-tillage plots exhibited these conditions between 4 and 28 d and between 6 and 15 d later for corn and soybean, respectively, than for the other tillage treatments. Corn planted with disk-chisel tillage yielded 0.8 Mg ha(-1) more than the mean of the other two tillage treatments across years. Planting date affected corn yield only in 2003. For soybean, planting date affected yield. Soybean planted at the early planting date yielded 0.16 Mg ha(-1) more than the mean of the other planting dates across years. There was no interaction of tillage x planting date for yield of either crop. This research indicates that recommendations derived from existing tillage research using a common planting date are valid.