The phosphorus fertilizer requirements and long term productivity
panic (Panicum maximum cv. Gatton) pastures, grazed by lactating
dairy cows, were evaluated over
7 years. Cows grazed at 2·6 cows/ha on pastures that received
annually 100 or 300 kg N/ha at each
of 0, 22·5 or 45 kg P/ha. Phosphorus treatments were applied
as single superphosphate, balanced for calcium by applications of gypsum.
The soil had an initial available soil phosphorus content of
40 mg/kg (bicarbonate extraction). At
zero P fertilizer (0P), extractable soil P declined at the rate of
1·9 mg/kg each year; at 22·5P it was
maintained close to the original level while at 45P it increased at
6·6 mg/kg each year. Increased P
fertilizer caused significant (P<0·01) increases
in plant P concentration from year 2 onwards. In
years 6 and 7 there was significantly less green pasture and leaf on
offer in 300N pastures at 0P than
with 22·5P and 45P. There was no influence of rate of P fertilizer
at 100N on pasture quantity on offer
in any year. There were clear trends at 100N of decreasing total pasture
and green dry matter (DM) on offer over the 7 years, but not at 300N.
Cows at 300N consumed more leaf in the diet in autumn and winter
than at 100N. Leaf was
55–60% of the diet in summer and autumn, but decreased to 21%
(100N) and 37% (300N) in winter.
Dead material in the diet was always higher at 100N. Pasture leaf percentage
and leaf yield were the
best individual predictors of leaf percentage in the diet. Diet P selected
from pasture was reduced by
the higher rate of N fertilizer in each season. Estimated P concentrations
of the diet selected from
pasture for summer, autumn and winter averaged 0·30, 0·38
0·28% DM for 100N and 0·19, 0·24
and 0·18% DM for 300N treatments, respectively.
The response to P fertilizer was dependent on the rate of N fertilizer
applied. The critical
bicarbonate extractable soil P level for this soil type, below which
pasture responses occurred, was
30 mg/kg at 300N. The critical level at 100N was not reached, but
was <23 mg/kg P.