RELATIONSHIPS BETWEEN AMMONIA VOLATILIZATION AND NITROGEN-FERTILIZER APPLICATION RATE, INTAKE AND EXCRETION OF HERBAGE NITROGEN BY CATTLE ON GRAZED SWARDS

Grazed pastures emit ammonia (NH3) into the atmosphere; the size of the NH3 loss appears to be related to nitrogen (N) application rate. The micrometeorological mass balance method was used to measure NH3 volatilization from rotationally grazed swards on three plots in the autumn of 1989 and throughout the 1990 growing season. The aim of the research was to derive a mathematical relationship between NH3 volatilization and N application rate, which would vary between soil type and weather conditions. In both years the plots received a total of 250, 400 or 550 kg N ha-1 as calcium ammonium nitrate (CAN) split over 6 to 8 dressings. The number of grazing cycles ranged from 7 to 9 for the three N plots. In the last two grazing cycles of 1989, NH3 losses were 3.8, 12.0 and 14.7 kg N ha-1 for the 250N, 400N and 550N plots, which was equivalent to 5.3%, 13.9% and 14.4% of the amount of N excreted on the sward, respectively. In 1990, NH3 losses were 9.1, 27.0 and 32.8 kg N ha-1 for the 250N, 400N and 550N plots, which was equivalent to 3.3%, 6.9% and 6.9% of the N excreted, respectively. Differences in urine composition between the plots were relatively small. Rainfall and sward management affected the size of the NH3 volatilization rate. Volatilization of NH3 was related to N excretion and N application rate. A calculation procedure is given to enable the estimation of NH3 volatilization from N application rate. Adjustments can be made for grazing efficiency, grazing selectivity, N retention in milk and liveweight gain, concentrate N intake and milking duration. Losses of NH3 increase progressively with an increase in N application rate until herbage yield reaches a maximum at an application rate of about 500 kg N ha-1 yr-1.