1. Dear Graham, I regret to say that breeding horses does constitute agriculture. The breeding of bloodstock is a facet of agriculture even if it is less intensive than other forms of agriculture, such as dealing with livestock.
2. Agriculture constitutes many forms, whether it is breeding horses, sheep, piges, cattle or poultry. Once there is even intermittent use of an agricultural nature, this would constitute agricultural use. Additionally, you should realise that it is extremely difficulty in planning law, for a barn to lose the designation of agricultural use. So long as there is even intermittent use of any agricultural nature, I will continue to have that designation for planing law purposes.
3. Be aware that under Planning law, the agricultural use would have to be abandoned in order to be lost. That would require a manifest intention to abandon the agricultural use in order to lose it. Mere non-use for a year would not be sufficient to constitute abandonment.
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