University of Tasmania

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Superphosphate : effects on pasture and sheep production

posted on 2023-05-27, 16:54 authored by Reid, Robert Neil Douglas
A grazing experiment with Polwarth ewes was carried out over seven years on a perennial ryegrass/subterranean clover pasture near Bothwell in central Tasmania. Treatments consisted of all combinations of five stocking rates (6, 8, 10, 12, and 14 ewes/ha) and four levels of superphosphate application (nil, 62. 5,125, and 250kg/ha/annum). Changes in soil pH, bicarbonate phosphate and bicarbonate potassium were studied and measurements were made of pasture availability, growth, and botanical composition. Sheep production records included wool weight, fibre-diameter and value, as well as ewe liveweight and lamb production and value. Gross margins were calculated for each treatment in each year. The results showed a small but non-significant decrease in soil pH with the use of superphosphate, but with annual fluctuations associated with season. Superphosphate treatments produced significant differences in soil phosphate levels, and the nil treated area showed seasonal effects. Pasture growth responses to superphosphate application were recorded in autumn and spring. Pasture availability differences were greatest between extremes of stocking rate, rather than between levels of superphosphate. Changes in botanical composition resulted in a reduction in ryegrass over time, along with seasonal fluctuations in the clover component of the pasture.At the end of the experiment, the highest rye grass and clover content occurred under the heaviest stocking rate. Sheep liveweights showed greatest treatment effects in late winter, with spring being a period of compensatory gain. Wool values were influenced by treatment to some extent, with clean fleece weight and fibre diameter affecting returns. Lambing performance was associated with stocking rate in some years, and fertiliser influenced lamb growth rates in one of the seven years. Wool and lamb production per hectare, and monetary returns, increased sharply with increasing stocking rate. On the other hand the gross margin analyses showed that only at medium to high stocking rates was there any money to be made by using superphosphate, and then only at a level between 62. 5 and 125kg/ha in most years. The future of soil tests, test strips, models and remote sensing in assisting on-farm fertiliser decisions is briefly discussed.


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Thesis (M.Ag.Sc.)--University of Tasmania, 1990. Bibliography: p. 170-193

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