Feed costs are a significant part of the total expenditure of the dairy business. The use of alternative ingredients in diets allows for adjustments in the food supply throughout the year, in addition to providing greater support capacity for pastures, and reducing feed costs, favoring the expansion of dairy farming. The objective of this study was to evaluate the effect of including cassava root silage (CRS) in concentrated supplementation on intake, nutrient digestibility, production, milk composition and economic viability of lactating primiparous dairy cows under grazing. Ten primiparous Girolando cows, with an average weight of 373.45 ± 63.55 kg and an initial average of 76 days of lactation, were used, distributed in two simultaneous 5 × 5 Latin squares. Five treatments were evaluated, namely: I – grazing cows without concentrated supplementation (WCS); II - grazing cows receiving 5 kg of concentrated supplement with 0% CRS; III - grazing cows receiving 5 kg of concentrated supplement with 26% CRS; IV - grazing cows receiving 5 kg of concentrated supplement with 52% CRS and V - grazing cows receiving 5 kg of concentrated supplement with 78% CRS. The inclusion of CRS was made in substitution of corn meal. Consumption, digestibility, productive performance and milk characteristics and economic viability were analyzed. The consumption of dry matter (DM), crude protein (CP) and total digestible nutrients (TDN) was lower (P<0.05) for animals without concentrate supplementation when compared to supplemented animals, whereas pasture DM consumption was higher (P>0.05) for WCS animals. Digestibility of DM and nutrients was higher (P<0.05) for animals that received concentrate supplementation, compared to WCS, except for cpNDF digestibility. Daily milk production (DMP) and daily milk production corrected for 4% fat was higher for animals receiving concentrate supplementation (P<0.05) compared to WCS. Milk fat (%) and total dry matter concentration (EST, %) were the same (P>0.05) between animals fed with and without concentrate supplementation. Regarding the inclusion of cassava root silage in the cows' supplement, the same consumption of total DM, CP and TDN was observed regardless of the level of inclusion of CRS (P>0.05), in addition to not having altered the digestibility of the DM and nutrients (P>0.05) nor the milk production of supplemented cows. The cost of supplementing a primiparous dairy cow increased linearly as CRS was included in the supplement in place of grain corn (P<0.001). The cost of the extra kg of milk produced by concentrated supplementation (Marginal Production) increased linearly with the inclusion of CRS in the supplement (P<0.05). Profitability of supplementation decreased linearly with increasing CRS inclusion (P<0.05). There was an increase in the cost of producing fat and protein (P<0.05). The profitability index of supplementation reduced by 61% as more CRS was included in the supplement. Therefore, the inclusion of CRS in the supplement for dairy cows improves the performance and milk production characteristics of dairy cows, but increases production costs by raising the cost per kilogram of the supplement, reducing viability.