Duplessis, M., Girard, C. L., Santschi, D. E. and Pellerin, D. 2014. An economic model evaluating the supplementation of folic acid and vitamin B₁₂ given around parturition and in early lactation on dairy farms in Québec, Canada. Can. J. Anim. Sci. 94: 737-747. The aim of this study was to estimate the potential profitability of a combined supplement of folic acid and vitamin B₁₂ given around parturition and in early lactation in commercial dairy herds in Québec. A total of 791 dairy cows from 14 herds were enrolled. Cows were assigned to weekly intramuscular injections of saline or 320 mg of folic acid and 10 mg of vitamin B₁₂. Treatments began 3 wk before the expected calving date and lasted until 8 wk of lactation. Within each herd, data on production, reproduction, and incidence of metabolic disorders and other diseases were recorded. With regard to the Canadian dairy industry, which operates under a supply management system, two scenarios were studied: (1) quota kept constant and (2) number of cows kept constant. For scenarios 1 and 2, eight and seven herds out of 14, respectively, obtained a positive annual net margin per cow following the vitamin supplement. The average net margins were Can$31.18 and Can$-4.86 N cow^-1 yr^-1 for scenarios 1 and 2, respectively. The variability of the response highlights that supplies of these vitamins by ruminal synthesis were probably different among herds and actual knowledge does not allow predicting supplies according to the diet.