Eight Holstein cows were used to determine
the relationship among milk urea nitrogen
(MUN), plasma urea nitrogen (PUN),
and feeding time. We first established that
MUN concentrations were similar in concentration
among quarters by comparing milk
samples from each quarter just before milking.
In order to determine if collecting a
sample of milk from a quarter influenced the
MUN in samples taken later, samples were
obtained from the right front quarter (RF) at
2, 4, 6, and 8 hr after the AM milking and
from the left front quarter (LF), right rear
(RR), and left rear (LR) at 4, 6, and 8 h after
the AM milking, respectively. The MUN in
samples obtained from RF at 4 hr was lower
(P<0.01) than corresponding samples taken
from LF, but samples from RF at 6 and 8 hr
did not differ from corresponding samples
obtained from RR and LR. We concluded
that by 6 hr, the effect of previous milking on
MUN concentration disappeared because of
dilution. To determine the influence of
feeding time on MUN concentrations, cows
were fed half of their normal PM feeding,
injected with oxytocin at the subsequent AM
milking to reduce residual milk, and offered
surplus feed after the AM milking. Milk
samples were collected at 2, 4, 6, 8, 10, and
12 hr after feeding from RF, LF, RR, LR,
RF, and LF quarters, respectively. Blood
samples were obtained from the coccygeal
vein at hourly intervals after feeding with the
last sample collected 12 hr after feeding. The
MUN concentrations at 2, 4, 6, and 8 hr were
similar. The MUN at 10 hr was similar to
those at 2 and 8 hr, less than that at 4 and 6
hr, and greater than that for the 12 hr sample.
Concentrations of PUN peaked at 2 hr
postfeeding, then gradually declined through
12 hr postfeeding. The MUN peaked at 6 hr
postfeeding and then declined. Time after
feeding significantly influenced PUN and
MUN concentrations.
