Modeling freezing injury in alfalfa to calculate forage yield: I. Model development and sensitivity analysis

Freezing injury can cause extensive yield loss in alfalfa (Medicago sativa L.) exposed to severe winters in cold climates. Existing simulation models for alfalfa either do not account for freezing injury effects or do not differentiate cultivars for their varying response to freezing stress and subsequent effects on crop productivity. The objective of this project was to incorporate cultivar-specific effects of freezing injury on forage dry matter (DM) yield into an existing alfalfa growth model to calculate annual forage yield during the life of an alfalfa crop. Numerical functions of cold tolerance, fall dormancy, and freezing injury were developed and integrated with the growth processes in the ALSIM 1 (Level 2) alfalfa model. The new model, ALFACOLD, requires input of: daily solar irradiance, maximum and minimum air temperature, and precipitation; latitude; dates of harvest; cultivar's fall growth score (FGS); initial plant density; and maximum plantavailable water in the root zone. The plant components modeled were leaf, stem, buds, and total nonstructural carbohydrates accumulated in the crown and root tissue. The model integrates the effects of soil moisture deficit, temperature, solar irradiance, and plant density on alfalfa growth, and the effects of subzero (°C) temperature on freezing injury and associated yield loss. Sensitivity of calculated forage DM yield to a ±25% change in selected model parameters was measured in cultivars of FGS 2, 3, and 4 during three production years of a 4-yr alfalfa crop. The model did not simulate growth during a seeding year. A cultivar's rate of cold hardening and lowest temperature tolerance during autumn and winter influenced forage yield more than did the rate of dehardening in the spring. Field validation of ALFACOLD calculations of forage yield is presented in a companion paper.