In our most recent project funded by the Pennsylvania Soybean Board we directly compared canola meal with extruded soybean meal (ESBM) in a large, long-term experiment with lactating dairy cows. The idea of the trial was to show that ESBM is equal or superior in nutritive value to canola meal, which has been heralded as a better protein supplement for lactating dairy cows, primarily as a result of recent studies funded by the Canola Council of Canada. Our study showed a statistically greater milk fat percentage and yield when the cows were fed the ESBM diet. This resulted in 3.7 lb/d greater 4% fat-corrected milk yield for the ESBM diet vs. the canola diet. Milk protein percentage and yield, however, were not different between the two diets. This may have been due to the relatively low extrusion temperature during preparation of the ESBM. In an earlier study we showed increased rumen-undegraded protein concentration of SBM extruded at 340°F vs. meal extruded at 300°F, which also resulted in increased blood plasma concentration of histidine, a key amino acid shown by our group to be limiting milk protein synthesis in lactating dairy cows. Collectively, these data led us to the hypothesis that feeding ESBM with a higher rumen-undegraded protein content (i.e., extruded at higher temperature), would result in not only greater milk fat but also greater milk protein percentage and yield in lactating dairy cows when compared on an equal protein supply basis with canola meal. Therefore, we propose to conduct a 10-week, continuous design trial with 48 high-producing dairy cows fed diets with either canola meal, which will be the control treatment, or with SBM extruded at 340°F as the main protein source. The diets will supply metabolizable protein at or slightly below the National Research Council (2001) requirements, which is expected to enhance the effect of greater rumen-undegraded protein content of the ESBM. The trial will be conducted in our free-stall facility which is equipped with Calan gates for continuous monitoring of feed intake. We will also be measuring milk production and composition (including milk fatty acids), nutrient digestibility and nitrogen losses, and blood amino acid profile. In addition, we will be continuously monitoring enteric methane production by the cows. Our hypothesis is that the greater fat content of ESBM (compared with canola meal) will result in decreased methane emission, in absolute terms and relative to feed intake and milk production of the cows. Our laboratory has extensive experience with enteric methane mitigation research, and we believe that this study will be an important contribution to the efforts to mitigate the environmental impact of dairy production. This project will demonstrate to dairy producers and their consulting nutritionists the advantages of ESBM over canola meal in terms of milk production and milk components and in decreasing the environmental footprint of milk, which will directly benefit soybean growers by expanding the marker for their products.