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 th e NH3 loss appears to be related to nitrogen (N) application rate. The micrometeorological mass balance method was used to measure NH3 volat ilization from rotationally grazed swards on three plots in the autumn of 1989 and throughout the 1990 growing season. The aim of the resear ch was to derive a mathematical relationship between NH3 volatilizatio n and N application rate, which would vary between soil type and weath er 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 dre ssings. 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 e quivalent 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 co mposition between the plots were relatively small. Rainfall and sward management affected the size of the NH3 volatilization rate. Volatiliz ation of NH3 was related to N excretion and N application rate. A calc ulation procedure is given to enable the estimation of NH3 volatilizat ion from N application rate. Adjustments can be made for grazing effic iency, grazing selectivity, N retention in milk and liveweight gain, c oncentrate N intake and milking duration. Losses of NH3 increase progr essively with an increase in N application rate until herbage yield re aches a maximum at an application rate of about 500 kg N ha-1 yr-1.