The economic feasibility of lupins in a cropping system as a protein supplement for a representative dairy cattle herd in Southwest Quebec (SWQ) to substitute for soybeans was investigated using a single period linear programming (LP) model. At current prices, lupins were not a profitable base case scenario. The gross margin of the dairy farm was $147,918. This resulted in a total soil loss of 3,504 tonnes (t), 50 percent higher than soil tolerance, T (acceptable level of soil loss) of 14 t ha$ sp-1,$ and produced 541 kg of Nitrogen (N). Sale of corn grain (CG) was 442.3 t, constituting about 39 percent of the gross margin but had a negative effect on the soil. The farm purchased 59 t of straw for animal bedding. / When the environmental constraint of soil erosion, 14 t ha$ sp{-1},$ was imposed, the gross margin was estimated to be $131,148 while total N and soil loss were 2,454 kg and 1,708 t, respectively. Lupins were produced on 18.3 ha with this soil loss level constraint. The tradeoff between soil loss and gross margin and lupins was observed. The N fixation constraint reduced the gross margin while it increased soil loss. CG sales were reduced from 442 to 244.5 t but milk production increased by 163 hectolitres (HL). Sensitivity analysis on labour supply and farm size indicated that lupins became an alternative to soybeans when farm size and labour supply were larger and fewer respectively. / Lupins would substitute for soybeans if yield is increased by 5 percent; cost of production is reduced by 10 percent and home grown clean lupin seed is used for 2 or 3 more years or government provides a subsidy of $328 ha$ sp-1.$ This is approximately equal to the CG subsidy.

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