Short answer: No, humans can’t temporarily survive by eating lawn grass because our bodies don’t break down the cellulose in grass properly.
Imagine a disaster hitting the area where you live, causing the food supply to halt temporarily. All grocery stores, mini-marts, and restaurants close. You’ve eaten everything in your pantry and are now out of food. What are you going to eat to survive? Well, the answer isn’t grass (from your lawn).
There are two main problems with humans eating grass.
First, humans don’t get any energy from grass. Unlike many other animals, our digestive system is unable to break down the cellulose in grass properly. In fact, grass has a negative calorie count in humans. This means you will actually lose energy as your digestive system tries to break down and digest the grass.
The second problem is the damage grass will do to your teeth. Grass has a lot of silica in it. Silica is a strong abrasive that will quickly wear down your teeth. Grazing animals, such as cows, have adapted over time and continually grow and replace worn down tooth surfaces.