Abstract
Nitrogen flow through a typical dairy farm (40 heads in milk; 25 heifers and 6 ha for forage production) in southern Japan was investigated during a year from October 1994, in relation to nitrate pollution of groundwater. Nitrate-N concentration of the well-water (approx. 70 m deep) on the farm exceed 10 ppm in May 1994 and gradually rose to 11.5 ppm after two years. Annual total nitrogen supply for cattle was 7,326 kg, of which only 26% was derived from on-farm forage production, with the remaining 74% obtained from purchased feed. Of the total N supply, 22% was utilized in animal production (milk, calves, body weight gain and stock replacement), and 71% was estimated to be excreted (all of dung and urine was stored as slurry, and applied to the field twice a year to boost forage production). The amount of nitrogen applied as slurry to the field for forage production was 41% of excreted N (2,110 kg) with the fate of the remaining 59% (3,066kg) being unknown - probably part as leachate to well-water and part as gaseous loss to the and atmosphere. Annual nitrogen uptake by forage plants (Italian ryegrass from autumn to spring; with a corn and sorghum mixtures from spring to autumn) was 2,143 kg, equivalent to 79% of total nitrogen application (450kg N/ha) by slurry and chemical fertilizers. The above results suggest that the nitrogen cycle on this farm acts as an open system, with a substantial amount of nitrogen being lost (either as leachate or a gaseous product) from the system. This is due to the substantial nitrogen input from purchased feed mostly imported from overseas and seems to be partly responsible for the high nitrate-N concentration of the well-water in this farm.
