Technical Abstract: Two experiments with growing beef steers fed hays of warm season (gamagrass and switchgrass) or cool season (tall fescue) grasses showed a strong linear relationship between urea production by the animal and urinary urea excretion as functions of nitrogen (crude protein) intake. The nature of the response for total nitrogen or readily degradable nitrogen was similar for warm and cool season grasses. However, less degradable, true protein fractions of warm and cool season grasses have distinct relationships with urea production and excretion.