The effects of replacing whole cottonseed by heated whole cottonseed
at
two levels of crude protein on amino acid (AA) utilization by the bovine
mammary
gland were investigated. Four Israeli Holstein cows were used in a 4×4
Latin square
design with 2×2 factorial arrangements. Diets were formulated to
contain two levels
of crude protein (CP), normal (160 g CP/kg dry matter) and low (140
g CP/kg dry
matter), and two levels of rumen undegradable protein (UDP), high (380
g UDP/kg
CP) and low (350 g UDP/kg CP). Whole cottonseed was quantitatively
substituted
by heated whole cottonseed to formulate the high UDP diets. Intakes of
dry matter
and organic matter were similar for all treatments and averaged 15·9
and 14·4 kg/d
respectively. Yields of milk and its constituents were similar for all
treatments. Milk
yield averaged 23·1 kg/d and this contained (per kg) 32·3
g CP, 25·4 g total casein
and 47·5 g lactose. The milk fat content was lower in the cows given
the high UDP
diets, averaging 34·6 g/kg compared with 38·9 g/kg
in the cows fed on the diets with
low UDP. Plasma AA concentrations were similar for all treatments apart
from Leu
and Ile, which were higher in the cows given the high CP diets: 152 and
103 μm
compared with 183·8 and 131·5 μm for the low
CP diets. Net extraction and balance
of essential AA across the half udder suggested that essential AA were
supplied in
amounts sufficient to meet milk protein requirements for all treatments.
The surplus
AA supplied as a result of feeding the high dietary CP were probably catabolized
via
an enhanced oxidative pathway, possibly in the liver, presumably as a passive
response to their disposal.