What do meteorological indices tell us about a long-term tillage study?

The pattern of hydrological events is expected to become more variable as the climate changes at specific geographic locations; rainfed agricultural systems will be potentially more vulnerable. Understanding how to adjust management practices will be critical for successful adaptation, therefore, long-term agriculture experiments, conducted over a range of years provides an opportunity to gain insight into future crop responses. The objective of this study is to analyze the results of a long-term tillage-rotation-nitrogen rate study (1986–2016) in northeast Nebraska using two meteorological drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), to determine if either helps differentiate the tillage, nitrogen and rotation treatments across the indices ranges and in making management decisions. The experimental design is a split-split plot with four replications. Whole plots are no-till (NT), spring/fall plow (PL) and disk (DK); subplots are corn-soybean [Glycine max (L.) Merrill] (C–S) rotation and continuous corn [Zea mays L.] (C-C); and sub-subplots are five (0, 40, 80, 120 and 160 kg ha−1) nitrogen (N) rates (NR). The SPEI accounted for about 20% more variation in corn yields compared to SPI and was used to segregate years into dry, normal, and wet years. Under normal conditions (-0.99 ≤ SPEI≤0.99) and dry conditions (SPEI≤-1.00), optimum NR was 120 kg N ha−1. Under dry conditions for both rotations, NT had 10% and 24% greater yield than DK and PL. However, under wet conditions (SPEI ≥ 1.00), corn yield in PL and DK was 3% greater than NT and yield continued to increase with NR from 0 to 160 kg N ha−1 without reaching a maximum. Rotation (C–S) under normal and wet conditions increased corn yields 28% and 17% compared to C-C, respectively, but for extreme drought conditions (SPEI≤-2.00) and prolonged drought conditions (more than a year) corn yield was 17% greater in C-C compared to C–S. The results of this study indicate that if the future seasonal climatic conditions will be wetter, NR may need to be increased, and reduced tillage will reduce yields. Similar analysis of other long-term research will provide insight into future management practices.