Fig. 5. (a) Crop yields in till and no-till treatments between 1989 and 2002. Letters indicate the crop harvested in that year: c, corn; s, soybean; w, wheat. * Indicates statistically different responses within a year (P < 0.05) determined by slicing of the treatment by year interaction. (b) Difference between conventional till (Ct) and no-till (Nt) normalized by the overall mean crop yield for that year. Points greater than zero indicate a till response greater than a no-till response. Year means were significantly different (P < 0.05) and separated using Tukey's test. Linear regression analysis showed no trend in crop yields (P < 0.05) across all crops and years; wheat yields significantly increased in till relative to no-till over time [normalized yield difference = –5980 + 3.00 (year); r² = 0.40; n = 3 yr]. Mean ± SE.