Before turning ponies out into a new field, which of the following should you check?

Checking the fencing and water supply is crucial before turning ponies out into a new field. Ensuring that the fencing is secure prevents the ponies from escaping and potentially getting injured or lost. Proper fencing maintains the safety of the ponies and helps keep them contained within a designated area. Additionally, assessing the water supply is essential because ponies need access to fresh water to stay hydrated and healthy. A lack of water can lead to serious health issues, so confirming that the water source is adequate and safe is a fundamental step in their care.

The other choices, while important in their own contexts, do not directly pertain to the immediate safety and well-being of the ponies in a new environment. Checking food and medical supplies is certainly relevant to their overall care but is less pertinent at the moment of turnout compared to ensuring their physical environment is safe. Weather conditions, while they can affect the pony's comfort and health, are secondary concerns compared to the risks posed by faulty fencing or inadequate water. Grooming equipment, although necessary for maintenance, does not play a role in the immediate safety of the ponies in a new field.