Animals offered CS-based diets had higher urea levels
in blood plasma (PUN; P < 0.01) when compared with
animals offered GS-based diets. Similar effects of CS
were noted for MUN (P = 0.03). No effect was observed
(P > 0.05) of HMBi supplementation on urea N levels in
plasma, milk, or urine. Animals offered G+ had a lower
levels of PUN than those offered C− (P < 0.01) and
C+ (P < 0.01) but not G− (P = 0.15). Plasma urea N
concentrations were higher in animals offered C+ than
in animals offered G− (P < 0.01) but not C− (P =
0.21). An interaction existed (P = 0.06) between forage
and HMBi for PUN concentration. Animals offered C+
had higher concentrations of MUN than those offered
G− (P = 0.06) and G+ (P = 0.06). Concentrations of
MUN from animals offered C− were not different from
those from animals offered other treatments (P > 0.05).
Concentrations of urinary urea nitrogen (UUN) were
greater from animals offered CS-based diets (P < 0.01)
than from animals offered GS-based diets. Daily excretion
of urea (g/d) was greater from animals offered
CS-based diets (P = 0.01) when compared with those
offered GS-based diets. Urea output was lower from
animals offered G+ than from animals offered C− (P
= 0.06) and C+ (P = 0.06). Urea output from animals
offered G− was not different from those offered other
treatments (P > 0.05). Urea output was lower from
animals offered G+ than from animals offered C− (P <
0.01) and C+ (P = 0.06). Urea output from animals offered
G− was not different from that of animals offered
other treatments (P > 0.05). A positive linear relationship
was observed (P = 0.01; R2 = 0.30) between urea
N excretion and feed N intake (Equation 1).