I think the real issue is not whether a boarding barn is agricultural, but whether it’s annual sales total $500,000 or more or is engaged in interstate commerce. The rules regarding payment of overtime and who is qualified as an exempt or non-exempt employee are part of the FLSA, Fair Labor Standards Act, and it only applies to employers with annual sales totaling $500,000 or more or engaged in interstate commerce.**
Raising the income threshold for the automatic determination of non-exempt (as mentioned in the posted article) doesn’t change which employers are subject to the FLSA.