Crop yields over the first nine years of LTRAS, a long-term comparison of field crop systems in a Mediterranean climate

The Long-Term Research on Agricultural Systems (LTRAS) experiment has been monitoring the long-term effects of conventional and alternative practices on crop yields and soil properties since 1993, with a planned duration of 100 years. Data for the first 9 years are presented here. Ten, 2-year rotations, each with three 0.4ha plots per phase of the rotation, range from low-input rainfed unfertilized wheat to high-input irrigated conventional and organic maize-tomato systems. For years 3-9, average yields of alternative (organic and green manure) systems were significantly lower than those of comparable conventional systems, except that there were no significant differences in average yields of tomato. Delayed seeding after a winter legume cover crop (LCC) limited maize yields. Yield trends over years were somewhat obscured by variability due to weather. Negative yield trends over years in unfertilized wheat controls were statistically significant, but only when data for the 9th year were included. Yield trends for organic maize were also significantly negative, but tomato yields in the same system showed a significant positive trend, as did yields of conventional maize. As this experiment continues, long-term trends in soil properties and processes could alter the magnitude or direction of trends seen to date.