In the basque country, in the spring, dairy ewes can spend a limited number of hours between 4 and 8 hours on pasture (Oregui et al., 1997). Besides grazing, ewes receive a supplementation of concentrates and forages in parlour. Therefore the characteristics of this supplementation are very important. In this sense, there is evidence that an increase in rumen undegradable protein could lead to an increase in milk production (Santos et al., 1998). In recent years, it has been shown the potential of tannins to decrease protein degradation in the rumen (Hervas. 2001). However, there is a lack of knowledge concerning the effect of increasing the content of rumen undegradable protein of the concentrate in a production system like the one in the basque country.