It going through an aurtum { heck i can not think of how to spell it lol} flush at the moment i opened up some of my track to my horses and within 24 hour they all had pulses.i would leave it a bit longer. Also if your grass is long and seeded its not as bad as shorter 6-8 inches long grass.
she's probally been on it no more than 10 hours, it's really long, i'm strip grazing it, but worry is my middle name, she seem to be ok at moment, i'll just keep an eye on her, I've got too much blooming grass.
Long grass is better, as regards sugars, then short stressed grass, And in fact frosted grass is among the worst for stress sugary grass - the photosynthesis continues in the sunlight but the growth has stopped so the sugars are more concentrated.
The old theory about winter grass having less food value related to protein and vitamins I suspect, before it was known about the sugars. Check out Safergrass.org, she has done the research http://www.safergrass.org/