Nutrient digestibility, nitrogen excretion, and milk production of mid-lactation Jersey × Friesian cows fed diets containing different proportions of rumen-undegradable protein oleh ABDUR REHMAN, MUHAMMAD ARIF, MUHAMMAD SAEED, ABDUL MANAN, ADHAM AL-SAGHEER, MOHAMED E. ABD EL-HACK, AYMAN A. SWELUM, ABDULLAH N. ALOWAIMER