I joined an online dieting website a little while back, which had "mucking out" under the exercise part. I can't remember how much it said, but it was quite impressive (approx 90 cals) per 1/2 hour
Unless it's out of the ordinary for you, it probably won't contribute much (if you, like me, are trying to lose weight!).
Try and do it really fast, to up your heartrate and get out of breath- then it'll be decent exercise.
Of course, if you are just interested, then I have no idea but bear in mind that jogging for 30 mins might burn around 300, to put it into perspective.. I know I would be near collapse, but if I mucked out 3 stables, I would be nowhere near the same level of exertion.
Disclaimer: the above is probably useless nonsense.