CLIMATE-BASED ESTIMATES OF POTENTIAL FORAGE YIELDS IN CANADA USING A CROP GROWTH-MODEL

A crop growth model of the Food and Agricultural Organization of the U nited Nations (FAO) adapted for estimating yield potential of annual c rops in Canada was modified to provide climate-based estimates of pere nnial forage yields (FORYLD). Subroutines were developed to estimate a verage cutting schedules and growth periods for legumes (alfalfa) and grasses (timothy, bromegrass, wheatgrass) from climatic normals data. Model estimates of cutting schedules and potential (constraint-free) y ields without moisture stress (B(y)) were in good agreement with obser ved values. A moisture stress factor (MSF) was used to estimate dry ma tter yields (B(ya)) under rainfed conditions (i.e. B(ya) = MSF x B(y)) . The MSF was determined from the soil moisture deficit-surplus (D), w hich was based on available water content in the soil at the start of the growth period, rainfall and potential evapotranspiration over the growth period. In a test of individual cuts, using dependent data, a h igh percentage of cases (82%) had estimated values for B(ya) within 1 t ha-1 of observed values for alfalfa and timothy. In independent data tests of wheatgrasses, 95% and 58% of estimated cases were within 1.0 t ha-1 and 0.5 t ha-1 of observed values, respectively. Average const raint-free yield (B(y)) and dry matter yields under rainfed conditions (B(ya)) were estimated for all soil map units on the Soils of Canada map, using FORYLD with 30-year averages of monthly climatic data. Esti mates of B(y) for both alfalfa and grasses ranged from zero potential in northern areas to more than 18 t ha- 1 in the most climatically fav oured areas (southern Quebec, lower coastal areas in British Columbia, southern Alberta), based on a maximum of three cuts of alfalfa and tw o cuts of grasses. Estimated values of B(ya) for alfalfa in areas wher e By was greater than zero ranged from less than 2 t ha-1 in the semi- arid regions of the prairies and parts of the interior of British Colu mbia, to more than 14 t ha-1 in the most climatically favourable areas of southern Ontario and Quebec and the lower coastal region of Britis h Columbia. The results provide a reasonable basis for making comparis ons of forage production capability between regions. Additional consid eration of stand persistence and winter injury is needed if yield pote ntial data are to be used for management decisions on forages.