Bare ground might be fine - but you'll be the best judge. A good rule of thumb is to think about how you would fair running on the surface you're considering: if you would get hurt if you slipped or fell, it's probably not good for your pup to run and chase on.
Excessive dust, loose gravel, standing water, mud, debris, etc. might not be the most ideal surfaces to play on. If you have less than ideal ground take care to set up the best and safest course you can. If the ground is wet, be sure to protect the main drive unit from water by placing it on a piece of cardboard or something similar. The main drive unit is not waterproof and can be damaged if it gets wet.
Jul 10, 2024