Which of the following is NOT a method to provide water to horses in the field?

The mobile watering can is not a standard method for providing water to horses in the field due to its impracticality for regular use. While a watering can can be useful in specific contexts, such as for smaller animals or in certain situations, it is not efficient for supplying water to horses over larger areas or for multiple horses at once. Streams, automatic water troughs, and buckets are all widely accepted and effective methods used in equine care. Streams can provide a natural water source, automatic water troughs can maintain a constant supply without needing human intervention, and buckets are versatile and can be filled as necessary. Each of these methods is more suited to the needs of horses in terms of accessibility and volume of water than a mobile watering can.