Our horses have good grass hay virtually 24/7, and are fed alfalfa pellets/beet pulp every day. However, it's been spectacularly hot this summer, and they seem to have lost weight. Not a lot, but a noticeable amount - they all look a bit ribby. However, they spend more time standing in the shade than eating - even when we put the hay in the shade, they tend to ignore it for the most part and just doze.
Hopefully, their appetites (and weight) will pick back up when the weather moderates?