You pose a very good question, Lori. While some experts theorize that dogs eat lawn grass to help resolve gastrointestinal issues, others believe that dogs munch because they are satisfying a need for green leafy roughage. Still others think that some dogs nibble on lawn grass because, well, they’ll nibble on anything.
Whatever the reason, lawn grass—like any plant material—can potentially cause gastrointestinal tract irritation, and vomiting and/or diarrhea may occur as a result of ingestion. However, lawn grass that has not been treated with pesticides or other potentially harmful agents is not considered to be toxic, and systemic or serious clinical effects would not be expected should your Labrador/pit mix decide to sample your lawn.