U REA is the main source of nitrogen in liquid
supplements for ruminants. Good reviews
on utilization of urea by ruminants have
been published recently (Waldo, 1968; Conrad
and Hibbs, 1968; Chalupa, 1968, 1970;
Loosli and McDonald, 1968) and further mention
of the basic aspects of non protein nitrogen
(NPN) metabolism by ruminants would
be redundant. However, several aspects which
apply specifically to lactating dairy cows will
be discussed.