Why should you avoid using a body brush on a grass-kept horse?

Using a body brush on a grass-kept horse is not advisable primarily because it can remove their natural oils. Horses that are kept on pasture have a natural protective layer of oils on their skin and coat. These oils play a crucial role in maintaining the health of the coat, preventing excessive drying out, and providing a barrier against dirt, parasites, and harsh weather conditions.

By using a body brush, which is designed to remove dirt and debris, you also risk stripping away these essential oils. This can lead to a variety of skin issues, including dryness, irritation, and an unhealthy coat appearance. The natural oils are vital for maintaining the proper sheen and luster of the coat, and their removal can contribute to skin problems over time.

The other choices, while they touch on valid concerns regarding grooming, do not capture the primary reason effectively. Irritation caused by brushing, potential coat damage, or ineffectiveness in cleaning are secondary concerns when compared to the detrimental effects of losing the horse's natural oils.