How would you safely put your horse to pasture for the summer without fear of founder?

i had her on pasture last summer. This winter she was on 1/2 alph alpha and 1/2 gr bales. So when the snow melts.. how do I safely get her back on pasture? Apparently she foundered before I got her... but she was good on pasture last summer when I got her, so I plan to put her on the same field again this year as she did good on it before