My girls are on pasture grass during the summer months. I think it all comes down to what type of grass it is and what time of year. I was told by an old horse/cattleman that your pasture should be like a good salad. Not just one type of grass but many different ones with a few legumes thrown in (alfalfa clover bean types)
I'm sure you have a department of agriculture or NRCS office somewhere in your area. You should contact them and have them take a look at your pasture and do an assesment. They are a wealth of information and should be able to help you out.
Here is a link for florida NRCS find an office http://offices.sc.egov.usda.gov/loca...a&stateCode=12