We have field troughs fed from the field drains and the stock prefer the water in them than that in the mains-fed water troughs.
I take Karen's point about contamination, but if you are feeding the trough from land drains laid by yourself through your own pasture, and there's nothing higher up feeding into it, then presumeably there'd be no source of contamination.
Rainwater harvesting is a good idea too, of course.