ECONOMIC OPTIMAL AMOUNT OF NITROGEN-FERTILIZATION FOR SUGAR-BEET - CRITICAL OBSERVATION OF DIFFERENT METHODS EXEMPLIFIED ON 39 FIELD TRIALS IN LOWER SAXONY

Data were taken from 39 field trials with stepwise nitrogen fertilization in Lower Saxony (FRG) from 1980-1992. The compared economic parameters are the corrected revenue, the corrected revenue plus quality payment and the latter plus varying shares of C-sugar beets. Farmers cannot influence effects of the year on sugar beet yield. It is assumed that farmers organize the sugar beet acreage in a way that they can fulfill the contract with the sugar factory in years with low sugar beet yield. In years with high sugar beet yield, above the A-/B-quota some C-sugar beets are produced. To define the amount of C-sugar beets, two models with different assumptions were developed to calculate the influence of the year on sugar beet yield. Model A defines the line between B- and C-sugar beets at levels of 14% or 20% below the highest yield of each field trial. In model B this line is at the yield without nitrogen fertilization. It is shown that 20% lower revenue, quality payment and 43% higher fertilizer price had no important effect on the optimal amount of nitrogen fertilizer. Whereas including of C-sugar beets in model A and B resulted in 34-50 kg N/ha lower fertilizing optimum, the level is at 80-90 kg N/ha. This makes clear that the corrected revenue is not a satisfactory parameter to estimate the optimal amount of nitrogen fertilizer if C-sugar beets are not involved. The models A and B differ hardly in their results.