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Animal and pasture maintenance, a partnership

A.L. Vizard and J.Z. Foot

McKinnon Group, Veterinary Clinical Centre, Werribee, VIC 3030
Department of Agriculture, Pastoral Research Institute, Hamilton, VIC 3300


The sustainability of a grazing system requires that the partnership between animal and plant is both biologically and economically sound. Many sheep farms in Australia fail to generate enough income to be able to maintain their pasture resource. Australian wool is produced from a wide range of climatically and ecologically diverse regions. Each has differing constraints that hinder the long term success of wool enterprises. In the majority of cases, a mismatch between the energy and protein available from pasture and the nutritional demands of the sheep flock is the most important biological limitation. Changes to time of lambing, time of shearing and flock structure, in conjunction with manipulation of the pasture base, are the most powerful tools for improving the grazing system. For example, across southern Australia, the slow winter growth rate of pastures restricts the number of sheep that can be carried on a property year round. Pasture cultivars that improve winter growth will have a great impact on annual carrying capacity in these regions. Many producers in southern Australia amplify the mismatch between winter feed and animal requirements by lambing in autumn, thus maximising the flock's demands for nutrients at a time when pasture availability is at a minimum. Lambing in late winter/spring is a cheap and effective method of increasing annual carrying capacity. However, a combined approach, in which time of lambing is changed and pasture growth rate in winter is increased, has the most potential for long term improvement.

Failure to meet the nutritional requirements of some classes of stock will cause severe loss of production and can result in deaths. This is unacceptable both from an animal welfare and economic viewpoint. Ewes in late pregnancy and lactation, and sheep during their first year of life are particularly vulnerable. Any grazing system must accommodate their particular requirements. Lactating ewes have more than double the energy requirement of a dry ewe and need to be eating close to their maximum voluntary intake in order to maintain adequate milk production. Pastures with less than about 1.3 tonnes of dry green matter per hectare will not allow them to achieve this. Wethers increase the flexibility of grazing management. They are able to remain productive on pastures as low as 0.5 tonnes of dry green matter per hectare, and for short periods can be stocked heavily when it is advantageous to the pastures or when extra pasture is required by other stock.

Lamb survival is particularly sensitive to perinatal ewe nutrition. Lamb growth rate to weaning is important, since the weaned lamb cannot afford to lose bodyweight until it has adequate fat reserves. Weaners will not be able to maintain live-weight on pastures with less than about 12% green material without supplementation. Infection with internal parasites, and other factors such as trace element deficiencies, can interfere with efficient utilisation of pastures by sheep. Grazing strategies should provide parasitologically safe pastures for vulnerable stock, some strict rotational patterns of grazing do not allow this.

All too often, the choice of pasture species, and the requirements for renovation of pastures are made on agronomic grounds alone. Tailoring pastures to meet specific grazing systems requires a knowledge of the annual cycle of pasture growth and flock nutrient requirements, an assessment of the short and medium term economic implications of any change, as well as a long-term overview of sustainability of the proposed sheep-pasture grazing system. This integrated approach to the sheep-pasture partnership is necessary to ensure the long term economic stability of the pasture dependant wool industry.

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