School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemVegetative and sheep production on a semi-arid Danthonia caespitosa (Gaudich) -Stipa variabilis (Hughes) grassland in response to fertilizers and deferred grazing( 1977)The initial and residual effects of nitrogen and phosphorus fertilizers applied over four consecutive years and gypsum applied once only, were assessed on a Danthonia caespitosa - Stipa variabilis grassland, growing on the semi-arid Riverine Plain of south-eastern Australia. The treatments were combined factorially. The residual effects were assessed over a five-year period from when the fertilizers were last applied. Totals of 155 and 310 kg ha-1 of N, 100 and 200 kg ha-1 of P, and 3.34 and 6.68 tonne ha-1 of CaSO4.2H20 had been added. Seasonal production of total dry matter and individual species, and nitrogen and phosphorus contents of the plant tops, were measured. A simultaneous, but separate, study was made over five years of the effects of nitrogen and phosphorus fertilizers and deferred spring grazing on the bodyweight and wool production of Merino wether weaners grazing a D. caespitosa - S. variabilis grassland at three stocking rates. Seasonal forage availability, botanical composition of forage, bodyweight and wool growth of sheep, were measured. Two years after grazing treatments ceased, and three years after the final addition of fertilizer, further measurements were made on the vegetation. In the first study, during the four years of fertilizer application, forage production increased in response to the additives in years in which the annual rainfall varied from well below to near average. They accentuated the normal pattern of a spring peak, but also gave lesser increases in production in winter and autumn. Summer production was not measured because of the absence of effective summer rainfall. Nitrogen and phosphorus fertilizers and gypsum all contributed to the increased production, and the combination of the three additives produced the greatest yield. Over all seasons the yield of grassland treated with nitrogen plus phosphorus plus gypsum averaged 250 per cent of the control, or approximately 1,500 kg ha-1. The main contributors to increases in yield were D. caespitosa, S. variabilis, annual herbs, and legumes. The perennial grasses were particularly favoured by nitrogen fertilizer whereas the yield of legumes was suppressed. The yield of legumes increased greatly in response to phosphorus fertilizer. With the exception of two species, all plants which increased in yield are acceptable to sheep. Nitrogen and phosphorus contents were above the minimum requirements for domestic herbivores. The study of the residual effects revealed that the increase in dry matter production in response to nitrogen ceased one year after the last application of nitrogen fertilizer. After five years, grassland which had received 200 kg ha-1 of phosphorus and 6.68 tonne ha-1 of gypsum yielded 6,500 kg ha-4 of dry matter, compared with 1,300 kg ha-1 in the absence of fertilizers and gypsum. Legume growth was still suppressed five years after nitrogen fertilizer had been applied, whilst legumes increased production greatly in response to the previous phosphorus and phosphorus-gypsum treatments. Nitrogen content of the herbage averaged 2.1 per cent four years after the previous high level phosphorus treatment, compared to 1.6 per cent with the control after four years. It was unaffected by previous nitrogen fertilizer. Phosphorus content was reduced for the first three years after nitrogen fertilizer was last added, but was increased by phosphorus fertilizer to 0.28 per cent, compared with 0.18 per cent on the control, four years after phosphorus fertilizer was last added. Soil moisture characteristics, and total soil nitrogen and carbon showed no differences between treatments. The Truog phosphorus in the soil under the high phosphorus level decreased from 103 ppm to 53 ppm, compared to the unaltered control level of 7 ppm over the five year period. In the grazing experiment there were very few differences in plant and animal response between the continuously and deferred grazing systems. Forage availability increased on fertilized grassland at all stocking rates by up to 50 per cent. D. caespitosa, S. variabilis, perennial chenopods, and annual herbs contributed to the increase in forage present. Medicago polymorpha became prominent on all fertilized grassland. At 2.0 and 2.7 sheep ha-1 without fertilizer, and 2.7 sheep ha- 1 with fertilizer, the relative contribution by the more palatable species decreased and EriochIamys behrii increased greatly. After grazing treatments ceased, this trend was reversed. Sheep bodyweight and wool growth rates were increased on fertilized grassland. In September 1971, at 2.0 sheep ha-1, the bodyweights of sheep on unfertilized and fertilized grassland were 35.0 and 45.2 kg, respectively, and the clean fleece weights were 3.2 and 4.3 kg per sheep, respectively. Animal production from this grassland can be increased with the addition of fertilizer to the grassland and by increasing the stocking rate by about 50 per cent above the district average of 1.2 sheep per hectare.