The profitability of nitrogen fertiliser usage on Canterbury seasonal supply dairy farms : A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science (Honours), Lincoln University

A simple model was constructed to investigate the effects of nitrogen fertiliser on milkfat production and liveweight changes in early lactation. The model incorporates pasture responses to nitrogen, allowance/intake, intake/milkfat production, and intake/liveweight change responses. These were developed from reviewed trials. The model was used to test different situations of stocking rate, initial pasture mass at the beginning of the lactation, and varying application rates. The expected milkfat responses varied from 0.099 to 0.376 kgMF/kgN depending on the conditions of stocking rate and initial pasture mass. These results are comparable to those obtained in a number of seasonal trials. The highest milkfat responses occurred when there was a high stocking rate relative to the total annual pasture production. The marginal increase in milkfat production was greater at lower intakes. As the initial pasture mass increased, the degree of underfeeding reduced thus a lower milkfat response was experienced. Liveweight differences followed a similar trend to that of milkfat response. As the rate of nitrogen application increased, the expected milkfat response declined. The extra pasture produced was increasingly able to satisfy the cows' requirements. High stocking rates and low initial pasture masses were again identified as leading to high milkfat responses at all application rates. At an application rate of 33 kgN /ha, 11 of the 24 situations exceeded the economic break even response of 0.20 kgMF/kgN. The probability of obtaining an economic milkfat response increased with stocking rate and initial pasture mass. Under Canterbury conditions, a high stocking rate relative to the pasture production of 11,000 kgDM/ha/yr obtained an economic response. An economic response occurred at average to high stocking rates relative to the pasture production of 13,000 kgDM/ha/yr. A number of additional benefits and possible negative side-effects were also identified