If you are spraying, you have to keep it on your property. In Texas pesticide and herbicide use falls under the Agriculture Code, the definition of pesticide in the law includes both. There are administrative, civil, and criminal penalties for violations. This is the section where spraying on a windy day and causing drift could be a criminal offense:
(e) A person commits an offense if the person:
(1) knowingly or intentionally uses, causes to be used, handles, stores, or disposes of a pesticide in a manner that causes injury to man, vegetation, crops, livestock, wildlife, or pollinating insects;
It is a Class A misdemeanor, unless you have a prior conviction and then it would be a state jail felony.
If I let something drift off my property and are sued in civil court, I could be found liable for damages. That is why I have drift coverage as part of my ag insurance policy.