Water and Nitrogen Dynamics On Grazing Dairy Farms in Georgia.

Animal confinement systems are the primary type of dairy farm in Georgia. Because of increasing grain and energy costs, these farms are becoming costly to operate. New Zealand-style rotational grazing dairies are now being viewed as an alternative to confinement systems.  Grass-based dairies have lower operating costs, better animal health, less environmental impact, and are more energy efficient. The first objective of this project is to monitor the soil water content on two grazing farms with center-pivot irrigation systems to provide efficient irrigation scheduling. The second objective is to develop a farm-gate N balance that includes N losses through ammonia volatilization and nitrate leaching. Soil water content was continuously recorded with reflectometets connected to dataloggers that were installed on each of the main soil types present under each center pivot. Cup lysimeters were installed on each soil type to a depth of 1m to monitor nitrate concentrations, which together with 2-D Hydrus modeling are used to estimate amounts of leached nitrate.  Ammonia volatilization losses are being measured with a micrometeorological technique that uses passive ammonia samplers that are replaced on a monthly or bimonthly basis. During warmer months, with high evapotranspiration demand, plant-available water averaged between 40 to 100%.  Also, during that period, nitrate concentrations in leachate ranged from 0 to 4.3 mg/L, but 2-D Hydrus modeling did not indicate drainage below 1 m.  Monthly ammonia volatilization losses ranged from 0.2 to 13 kg N/ha depending on environmental conditions and fertilizer applications. Our current results seem to indicate that grazing dairy farms have a relatively low environmental impact.