Tropical Grasslands (2006) Volume 40, 113
Pasture production, pasture quality and their relationships with
steer gains on irrigated, N-fertilised pangola grass at a range of
stocking rates in the Ord Valley, Western Australia
R.J. JONES1 AND R.P. LeFEUVRE2
1CSIRO Sustainable Ecosystems, Davies
2Formerly CSIRO Division of Tropical
Agriculture, Townsville, Queensland, Australia
Pasture presentation yield, composition and
quality attributes were measured on grazed, irrigated
pangola grass pastures fertilised at a range
of nitrogen rates (NR; 90–350 kg/ha/yr) and continuously
stocked at a range of stocking rates
(SR; 5.2–12.6 steers/ha) over 2 years on the Ord
Irrigation Area at Kununurra, Western Australia.
A central composite design was used to model the
responses obtained. Pastures remained pangoladominant
throughout under all treatments. Mean
presentation yields of dry matter (DM), digestible
dry matter (DDM), nitrogen (N), phosphorus (P)
and sulphur (S) all declined with increasing SR
at each NR. NR increased yields with responses
inversely related to SR.
The %N of the pastures increased linearly
with SR from a mean of 0.7%N at 5.2 steers/ha
to 1.5%N at 12.6 steers/ha. By contrast, the effect
of NR on pasture N% was small at any given
SR. The %P of the pastures also increased with
increasing SR but declined with increasing NR,
especially at high SR. The predicted highest %P
of 0.26% occurred at 50 kg N/ha/yr at a SR of
12.6 steers/ha. Sulphur concentration followed
a similar trend to that of P, with highest %S of
0.14% also at 50 kg N/ha/yr with 12.6 steers/ha,
and the lowest S concentration of 0.07% with
350 kg N/ha/yr and 5.2 steers/ha.
In vitro dry matter digestibility (DMD) of total
pasture was generally low (< 55%) and varied
seasonally, with lower values in the cool dry
season and higher values in spring and summer.
Mean values tended to increase with SR, especially
at low NRs.
Annual steer gains/head were positively correlated
with yields of DM, DDM, N, P and S
(r ≥ 0.92***) but negatively related to the quality
attributes of %N, %P, %S and %DMD. Both DM
yield and LWG/head were closely related to pasture
height (r ≥ 0.95***).
Moderate stocking rates of around 5 steers/ha enabled pastures to produce well and remain
stable, giving high steer gains/head and also a
satisfactory economic outcome (Izac et al. 1990).