Nitrate Leaching Occurring in Tifton-85 Hay Field Receiving Nitrogen Fertilization.

Nitrate leaching from livestock operations, and subsequent contamination of water resources is a major environmental issue that could undermine long-term sustainability of cattle production in many watersheds. A field study was conducted in an established Tifton-85 bermudagrass pasture within the Suwannee River Basin in Florida to evaluate rates of nitrogen (N) fertilizer application on nitrate leaching in rapidly permeable upland sandy soils. Treatments included four N application rates (0, 33, 67, and 100 kg N ha^-1 harvest^-1) corresponding to 0, 0.4, 0.7, and 1.1 times, respectively, of recommended N rate for hay production in Florida. Soil water was extracted from the soil after each rain using suction lysimeters installed below the rooting zone of the grass (~1 m deep), and analyzed for total N and nitrate concentrations. Tifton-85 bermudagrass was harvested at ~28 days intervals and analyzed for yield, N uptake, and herbage nutritive quality. The greatest total DM yield (~18 Mg ha^-1 yr^-1) was obtained from plots receiving the highest N rate (100 kg N ha^-1 harvest^-1) followed by the treatments of 67 kg N ha^-1 harvest^-1 (~16 Mg ha^-1 yr^-1), 33 kg N ha^-1 harvest^-1 (~12 Mg ha^-1 yr^-1) in that order, with the least DM yield (~3 Mg ha^-1 yr^-1) coming from the control plots. Across harvests, herbage nutritive value was similar among the treatments. The greatest N uptake efficiency of ~0.89 g g^-1 occurred in plots receiving the 33 kg N ha^-1 harvest^-1 treatment, with the least N uptake efficiency of ~0.31 g g^-1 occurring in the 100 kg N ha^-1 harvest^-1 treatment. Across the sampling period, the treatment with 33 kg N ha^-1 harvest^-1 rate, had the lowest amount of nitrate-N (<10 mg L^-1) leached below the rooting zone. Nitrate-N leaching patterns of plots receiving N application rate ≥67 kg N ha^-1 harvest^-1 exceeded the Florida's groundwater NO₃-N standard of 10 mg L^-1, and suggest high risk of impacting groundwater quality, if such rates are applied to soils with coarse (sand) textures. Further studies should evaluate N application rates between 33 and 67 kg N ha^-1 harvest^-1 that will optimize DM yield, without increasing N losses to the environment.