We found previously that the current recommendations for Na+,
K+, and Cl−
contents in the diet do not meet the needs of lactating cows. The responses
of
cows receiving a ration with increased amounts of Na+,
K+, and Cl− (E cows) were
compared with those of cows consuming the same ration with a fixed concentration
of these ions (C cows) between weeks 2 and 8 post partum (PP). Milk, protein,
fat and
lactose yields, and dry matter intake between weeks 2 and 4 PP were higher
in E
than in C cows. These differences did not occur between weeks 4 and 8 PP,
mainly
because of a higher incidence of PP complications in E cows. A greater
increase in
plasma insulin-like growth factor-1 concentration in E than in C animals
during
weeks 2 and 3 PP was consistent with the milk responses. A reduction in
aldosterone
concentration in E cows in weeks 2 and 3 PP was a consequence of their
Na+
requirements being satisfied as a result of their enhanced Na+
intake. A subsequent
elevation in aldosterone concentration in E animals was probably related
to a
moderate excess in K+
intake. This increase in aldosterone explains the urinary
potassium loss that was detected at week 6 PP. The absence of differences
between
E and C cows in plasma renin activity was consistent with an absence of
differences
in urine volume and with the apparent utilization of the enhanced ion intake
for body
functions.