Tropical Grasslands (2003) Volume 37, 6583
Effect of stocking rates on animal gain, pasture yield and composition, and soil properties from setaria-nitrogen and setaria-legume pastures in coastal south-east Queensland
R.M. JONES1 and R.J. JONES2
1 CSIRO Livestock Industries, Brisbane
2 CSIRO Sustainable Ecosystems, Townsville,Queensland, Australia
production, pasture production and pasture composition were measured
from 3 fertilised sown pasture systems in the subtropics of south-east
Queensland. The pastures were Setaria sphacelata cv. Nandi (setaria)
+ Desmodium intortum cv. Greenleaf (desmodium), setaria + Macroptilium
atropurpureum cv. Siratro (siratro) and setaria + 333 kg N/ha/yr
as urea (N-fert). The legume treatments were initially stocked continuously
at 4 stocking rates (SR) of 1.1, 1.7, 2.3 and 3.0 head/ha, and the N-fert
at 5 rates of 3.0, 3.6, 4.2, 4.8 and 5.4 head/ha in a completely randomised
design with no replications.
For the first
4 years, all treatments were stocked as planned. For the next 8 years,
the highest legume SR was reduced to 2 head/ha and only the lowest and
highest SRs for desmodium were maintained. For the remaining 12 years,
measurements were made on only 3 SRs on siratro and 2 SRs on the N-fert
treatment. Each November, new yearling heifers (in later years steers
also) replaced those on the trial. The 2-yr-old heifers were then mated
and breeding success measured.
in the experiment where 3 or more SRs were operating, annual animal
gain from all 3 systems declined linearly with increasing SR. Rates
of decline/unit of SR with the legume treatments were 2–3 times those
for the N-fert treatment. Annual animal gain (Y) was more closely related
to pasture presentation yield than to mineral composition of the pasture
or faeces. The best fit for all pastures at all SRs over a 12-year period
was with the log of DM presentation yield/head in autumn (X): Y = 89.8
+ 72.7X (r2 = 0.87; P<0.0001).
Pattern of gain
through the year was similar for all pastures from November–May, with
only a slightly reduced gain with increasing stocking rate. Thereafter,
the effects of SR became more apparent and gains from May–October declined
at a rate related to SR. Annual animal gains also declined over time,
especially on the legume systems. This resulted in a decrease in the
estimated SR for maximum gain/ha from 2.2 and 5.2 head/ha in the siratro
and N-fert treatments in 1969–1973 to 1.3 and 3.7 head/ha for the same
pastures in 1977–1981. Breeding performance was good for heifers from
all treatments except for animals grazing at the highest stocking rate
on siratro-based pastures for 1 year when heifer weights were below
250 kg at mating.
to pasture yield of both legumes declined with increasing SR and over
time. Siratro was far more persistent than desmodium but declined due
to inadequate recruitment from seeds or stolons. A subsidiary experiment
showed that spelling in autumn enabled partial recovery of siratro.
Associated with the yield decline at the higher SRs was an increase
in grazing-tolerant grasses and native legumes. However, these did not
support good liveweight gains as pasture yields were low.
the legume-based siratro pasture, nitrogen application reduced topsoil
pH, cation exchange capacity and exchangeable Ca, Mg and K. In contrast,
topsoil of the N-fertilised pasture had higher levels of organic carbon,
NO3–N, exchangeable acidity, exchangeable aluminium and extractable
In the light
of these results, management options for Siratro to persist better and
be productive under grazing in this environment are discussed.