Modeling Nitrogen Use Efficiency of Forage Crops Fertilized with Liquid Manure Applied with Irrigation Water.

Animal manure is a valuable source of nutrients and organic matter. A large proportion of the nitrogen (N) in manure is in organic form, which makes the amount of N available to a crop difficult to determine. However, optimal manure management that ensures adequate crop nutrition while avoiding pollution problems requires accurate and reliable estimates of N availability.

The objective of the project were (1) to calibrate the Root Zone Water Quality Model (RZWQM) for three fields on dairy farms located in California's Central Valley to simulate crop growth and the fate of N, and (2) to use the simulations to develop improved management practices.

A unique characteristics of the dairy manure management in the region is that liquid manure is applied multiple times to each crop, including silage corn and winter forage, together with the irrigation water by flood irrigation.

The RZWQM simulated crop yield and N uptake well. In general, the model was also able to simulate the long term-trends in soil moisture and mineral N content in the top 90 cm of the profile. However, the simulation of short-term changes and differences with soil depth was less accurate.

On average, 520 kg N ha^-1 were applied to corn, 37% as in the form of organic N. Corn took up 290 kg N ha^-1. Based on the simulation, 120 kg N ha^-1 leached below a depth of 180 cm during the corn growing season, and losses due to denitrification were insignificant in these sandy soils.

The amount of N leached was correlated with the amount of water leached (R² = 0.75) and therefore with the amount of water applied in excess of crop demand. In a next step, different management practices will be evaluated for their potential to improve water and N use efficiency.