CASE STUDY: Impact of Manure Application Timing in Dairy Pastures on the Migration of Nitrates to Ground Water

Manure application increases pasture crop production and minimizes manure storage costs. Excess N not absorbed by pasture vegetation can leach past the root zone and into ground water. A yearlong study was conducted on grazing pastures at 2 dairy farms to monitor potential nitrate (NO3) movement from the surface through the soil profile toward ground water. Precipitation, irrigation, manure application, and grazing were compared with NO3 movement. Both pastures were part of rotational grazing systems and receive periodic applications of manure by cattle and by typical manure application equipment. Sampling devices were located in the center of operational fields. A total of 9 sampling wells (lysimeters) were installed at depths of 1, 1.3, and 1.6 m below the surface of each pasture. Each depth was replicated 3 times at each site. Water samples were taken every 2 wk at each site and tested for NO3-N. Farm A applied a total of 600 kg N/ha throughout the year and farm B applied approximately 300 kg/ha. On Farm A, NO3-N concentrations averaged 15.2 ± 12.2, 6.5 ± 6.9, and 2.9 ± 3.6 ppm at 1, 1.3, and 1.6 m, respectively. 1Corresponding author: troy.downing@ oregonstate.edu On Farm B, NO3-N concentrations averaged 13.8 ± 8.5, 8.5 ± 5.7, and 1.3 ± 2.9 ppm at 1, 1.3, and 1.6 m, respectively. No relationship between farming activity and NO3 movement was noted. In addition, irrigation appeared to have no influence on NO3 movement. Substantial rainfall (>15 cm per 2-wk period) appears to be the major factor in NO3 movement.