Not sure what you mean about grazing or hay testing.
Those horses were all stalled and yes we did test any new hay crops.
I could adjust my mixed grain feed accordingly.
Now, our horses are out most of the time (decent weather conditions.)
We do fertilize and plant every year, so we know what they are getting from the pasture. Plus I keep a rack of dry grass out for them to free choice.
If the pasture gets to low then I start throwing alf. to them. once a day.
If a horse is laid off in a stall or needs to be kept quiet, I will put them on straight grass hay, with supplements and or grain depending on the horse and situation.
Hope I answered you