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Horses (Equus caballus) were introduced with European settlement both in Australia and New Zealand. Over time, animals escaped and were released and were first recognised as pests in Australia in the 1860’s. In contrast to Australia, the New Zealand population is protected.
In 1992 Australia was estimated to have 300,000 feral horses, mainly in the cattle raising districts of Queensland, Northern Territory and Western Australia. Smaller populations are located in eastern Australia, mainly in the alpine and sub-alpine areas. The number of horses varies considerably depending on the effectiveness of management programs and the impact of drought and bush fires. For example, it is estimated that the population of feral horses in eastern Australia declined by 70% in 2002/2003 due to the effect of management, drought and bushfires. New Zealand has approximately 1,000 feral horses, mainly at Kaimanawa on the North Island.
Although not well quantified, there is good evidence that horses cause significant environmental damage including fouling waterholes, damage to native vegetation and through soil compaction. Areas used by horses during drought are believed to be important refuge areas for many native plants and animals. The major impact of feral horses is on cattle production. The diet of both is similar and although there is likely to be more pasture than either can consume in normal seasons, competition is great during drought. The presence of feral horses can prevent effective management of pasture and water, especially during drought. While cattle can readily be de-stocked to preserve pasture and breeding stock, feral horses can’t.
Horses are well adapted to the sparsely distributed and unpredictable resources of arid Australia but also do well in the sub-alpine and alpine districts of Australia and New Zealand. They can move up to 50 km a day to food and water and have few predators and diseases. Mortality in arid Australia is mainly associated with drought which causes starvation, lack of water and consumption of usually avoided toxic plants.
Mares breed in spring to summer and on average produce one foal every two years. Under good conditions the population can increase by 20% a year.
The aim of management is to reduce the damage due to horses to an acceptable level. The most common practice is to muster and harvest horses around key points such as feeding areas and water points. Harvested animals can then be sold. Further reduction in density in arid Australia is usually obtained through helicopter-based shooting using highly skilled, trained shooters. On a smaller scale, brumby running (culling the population using horses) and ground shooting may have a role. Fertility control has also been suggested but has limited application for widespread populations because of the difficulty in delivering the fertility agent, which usually has to be administered on a regular basis to ensure ongoing control of the population.