Ian White (2011). GRAZING MANAGEMENT SYSTEMS FOR CATTLE IN THE TROPICAL SAVANNAS OF NORTHERN AUSTRALIA PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

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Author Ian White
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2011
Thesis type PhD Thesis
Supervisor Prof Dennis Poppi
Dr Leigh Hunt
Dr Steve Petty
Total pages 226
Total colour pages 30
Total black and white pages 196
Language eng
Subjects 0702 Animal Production
0703 Crop and Pasture Production
0502 Environmental Science and Management
Abstract/Summary Four investigations were undertaken to examine the response of tropical savanna pastures of northern Australia and cattle to different grazing systems. These were: (1) Plant and animal response to the four grazing management systems (a) set stocking, (b) set utilisation (20% annual pasture utilisation), (c) wet season spelling (WSS; early and late wet season), and (d) cell grazing (through 26 small paddocks). Each management system was established under large landscape scale areas (0.70 to 21 km2); (2) Sampling requirements for predicting diet quality using faecal near infrared reflectance spectroscopy (F.NIRS); (3) Wet season spelling regimes and defoliation rates on plant dynamics; and (4) Response of cattle and pastures to progressive defoliation over 14 days. In (1) the large landscape grazing systems experiment, pasture response (yield, ground cover, perennial grass basal area and species composition) and animal production (mortality, liveweight, body condition score, and reproduction) were measured biannually, diet quality monthly, and costs and returns calculated over four years. In all systems, pasture utilisation rate was approximately 20%, pasture attribute responses were similar and no degradation occurred. Hence, 20% utilisation appears sustainable irrespective of grazing system. Cattle in WSS and cell grazing had markedly better dry season weight gains, but the trial duration and lack of treatment paddock duplication meant paddock differences and grazing treatments effects were unclear. Cell grazing was a difficult system to implement due to poor economic, livestock production, management and biophysical outcomes. Set stocking was unable to control patch overgrazing, and in variable seasons pasture underutilisation or overgrazing can occur. Set utilisation was most economically viable, but with variable rainfall, issues would arise involving buying and selling cattle to meet the desired utilisation level and costs would increase. WSS underperformed set utilisation economically, but it can produce a more resilient system with better biological responses such as increased seed banks and root reserves. A single WSS system was investigated in (1), while in (3) Chapter 3, alternatives for timing and duration were investigated over three years in an intensive small plot clipping experiment. Early or late WSS for eight weeks, similar to (1c), were compared with 16 weeks over a full wet season, and wet season clippings which removed from 20 to 60% of the leaf height of Dichanthium fecundum, Astrebla elymoides, and Aristida latifolia plants. In the dry season all plants, including controls, were clipped to 10 cm. Results indicated pasture benefits from full WSS including greater yields and flowering rates and fewer mortalities than early or late spelling, while removing less than 30% of leaf height restricted the negative effects of clipping. Annual yields and basal area responses improved further if spelling was extended until perennial grass dormancy (as with the controls). In (4) the response of cattle and pasture to progressive defoliation over 14 days was investigated in wet seasons and dry seasons in two small trial paddocks with different pastures. Cattle removed 48 to 60% of the herbage dry matter. Defoliation patterns and species selection changed as defoliation progressed. Intake was measured using F.NIRS and n-alkanes. Pasture intake declined after three days in the wet season (6.5 to 5.2 kg/day), but was lower and showed no change in the dry season (4.5 kg/day). The decline in intake appeared related to removal of around 14% of the volume of preferred species and the first re-grazing of plants. Crude protein (CP) declined around 2% units in all trials, dietary non-grass 20% units and dry matter digestibility 6% units, all greater in the wet season. In the grazing systems experiment (1), F.NIRS revealed seasonal differences in diet composition, but no differences in the grazing systems. The reliability of the recommended sampling method (bulk faecal samples from15 animals) was tested (Chapter 2.2) by analysing up to 70 samples from cattle in paddocks varying in size from 2.9 to 111 km2. Using the variability in dietary CP, non-grass content (NG), and dry matter digestibility, two standard statistical methods determined required sample size. For reliable experimental sample means the recommended 15 samples were often inadequate and required sample numbers varied with dietary attribute. More samples are needed with increasing paddock size for CP and NG. In the variable tropical savanna systems of northern Australia, grazing management needs to become more dynamic incorporating time as a management tool. Grazing management should vary between wet and dry seasons and in years when spelling is to be used, full wet season spelling should be adopted as it benefits perennial grasses, producing advantages for the plant to better cope with adverse events or variable seasons. It is recommended that defoliation of 30% of annual pasture height or 20% of pasture dry matter is sustainable in these landscape systems.
Keyword grazing systems, pasture spelling, Australia, tropical savanna, native pasture, animal production, NIRS
Additional Notes colour pages: 47, 66, 94, 95, 97, 106, 111-120, 162-170, 193, 194, 196, 199, 200. landscape pages: 34

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Created: Sat, 21 Apr 2012, 12:48:59 EST by Mr Ian White on behalf of Library - Information Access Service