I'm assuming this question is pertaining to animal farms? Because I try to eliminate grass from growing in my flower beds! :-)

My best answer is that unhealthy grass makes for unhealthy animals who depend on it for nutrition.

Everything on a farm depends on soil. Soil needs to have the right amount of elements (nitrogen, for example) to support the plants growing in it. If the plants don't have the nutrition they need, they can't grow as well, and the animals eating the plants won't grow well either. Plants get their energy from the sun, but still require essential elements to build the parts they need for photosynthesis.