Tm. Davison et al., PHOSPHORUS-FERTILIZER FOR NITROGEN-FERTILIZED DAIRY PASTURES .2. LONG-TERM EFFECTS ON MILK-PRODUCTION AND A MODEL OF PHOSPHORUS FLOW, Journal of Agricultural Science, 129, 1997, pp. 219-231
The phosphorus fertilizer requirements and long term productivity of n
itrogen-fertilized Gatton panic (Panicum maximum cv. Gatton) pastures,
grazed by lactating dairy cows, were evaluated in a 7-year experiment
. Cows grazed at 2.6 cows/ha on pastures that received annually 100 or
300 kg N/ha and each of 0, 22.5 or 45 kg P/ha. Cows received no energ
y supplements in years 1-3 and were offered molasses at 3.5 kg/day fro
m year 4 to year 7. Cows grazed their experimental paddocks from the s
tart of the wet season until they started to lose weight in the dry se
ason. 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. This was
reflected in a reduced milk yield by cows at 300N/0P in these two yea
rs. There was no influence of rate of P fertilizer at 100N on milk yie
ld in any year. Lactation milk yields at 300N in years 6 and 7 average
d 3930, 4310 and 4610 kg/cow (P < 0.05) for OF, 22.5P and 45P, respect
ively. Nitrogen fertilizer increased milk yield in each year (P < 0.01
) except the first. Milk yields at 100N and 300N averaged 2860 and 332
0 kg/cow respectively in years 1-3 and 3720 and 4290 kg/cow in years 4
-7. The milk yield responses to P fertilizer were related to the great
er amounts of pasture and green leaf on offer, which led to a higher p
roportion of leaf in the diet, and the response to P fertilizer was de
pendent on the rate of N fertilizer applied. Phosphorus intakes were e
stimated to be below that of published requirements for cows producing
this quantity of milk. An annual model of P flow between plant, anima
l and soil pools demonstrated that at 100N/22.5P more P was returned t
o the soil as excreta (15.7 kg P/ha) than with 300N/22.5P (7.1 kg P/ha
). The major pathway of return of P to the soil at 300N was through pl
ant litter. Soil organic P was the largest, but least exploited, pool
of phosphorus. This study has illustrated how the demand for phosphoru
s by the plant in grazed pastures is modified by the input of N fertil
izer, is poorly predicted from plant analysis and published standards
for animal requirements, and indicates that a response in milk product
ion may be mediated through the effects of P on leaf growth and not on
dietary P content.