Legacy effects of liquid dairy manure in grain production systems

Transferring liquid manure from dairy farms to nearby cash-grain farms has the potential to support high grain yields at reduced cost, while making use of animal wastes that might otherwise cause environmental hazards if not utilized appropriately. Yet, empirical evidence of the legacy of long-term liquid manure application, and its interaction with crop rotation management on grain yield and soil properties is scarce. Here we evaluate the benefits and legacy of repeated liquid dairy manure (115 Mg ha⁻¹ yr⁻¹) over 12 years (2003–2014) on corn yield, soil organic matter (SOM) content, and soil nutrient availability in three annual cash grain cropping systems under different crop rotation management in southern Wisconsin. The three systems include: continuous corn (CC); minimum-till corn-soybean (CS); and an organically managed corn-soybean-wheat/oat-clover rotation (CSW). Results showed that, liquid manure application increased corn yield, soil test phosphorus (STP), and soil test potassium (STK) content across the three systems, and soil pH and nitrate (0–90 cm) in CC over the 12 years. The lack of significant impact of manure on SOM content suggested that the liquid manure application was insufficient to build SOM on these fertile Mollisols. The legacy benefit of liquid manure application on corn yield was significant in 2015, but no longer detectable in 2017, three years after the last application, while STP and STK content remained higher in manure treated plots than no manure plots. In 2017, cation exchange capacity (CEC), soil pH, and soil nitrate content were also greater in manured plots, but the effect was crop rotation specific. We show that manure is an equivalent or superior substitute for synthetic fertilizers providing a potential win-win scenario for both grain and dairy farms in the North Central US, where synthetic fertilizer is costly (cash-grain farms) and land for manure spreading is often limited (dairy farms). Our study suggest that liquid dairy manure application is beneficial for agroecosystem productivity but continual applications are needed to sustain its benefit in the crop production systems in the region.