MODELING FREEZING-INJURY IN ALFALFA TO CALCULATE FORAGE YIELD - II - MODEL VALIDATION AND EXAMPLE SIMULATIONS

Freezing Injury can cause extensive yield loss in alfalfa (Medicago sa tiva L.) exposed to severe winters in cold climates. Alfalfa models mu st simulate freezing injury effects in a cumulative way, so that forag e dry matter (DM) yield can be accurately calculated during multiple y ears of a crop, A companion paper presents ALFACOLD, an alfalfa simula tion model developed by integrating functions of cold tolerance, fall dormancy, and freezing injury into the ALSIM 1 (Level 2) alfalfa model . The ALFACOLD model calculates forage yield on a daily basis, while s imulating freezing injury across multiple years of the same crop. The objective of this study was to test the ALFACOLD model's ability to ca lculate field-measured forage DM yield, The model was tested against m easured forage yield from 39 cultivars seeded in two years and managed under a ii-cut system far 3 yr after the seeding year at two sites in Wisconsin. Averaged across the sites, cultivars, and years, the ALFAC OLD model calculated annual forage DM yield to within 12% of the measu red yield, compared with an error of 35% with the ALSIM model, A regre ssion of field-measured yield of individual harvests (n = 874) on the corresponding ALFACOLD calculated yield indicated that 70% of the meas ured variability in forage yield was explained by the model, The avera ge difference between measured and calculated DM yield for Harvests I, L and 3 was <7%, but the model overcalculated fourth-cut yield by 38% , Averaged across all sites, cultivars, Sears, and cuts, the ALFACOLD model calculated yield with an average SD of 600 kg ha (1) cut (1).