How often should I irrigate my pasture?

I am looking to put my 2 horses on 3 arces. If I suppliment feed once a day how often should I irrigate to keep my pasture with enough feed to keep them healthy? I live in the central valley of CA.
Answers:
if the ground is hard and dry maybe one or two times a year. Once in the spring and once in the fall. if you choose to do it once a year do it when it's the rainy season (when ever that is in your part of the world) so you can saturate the ground better.

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In my area irrigation is not needed. As long as the horses stay in good condition and there is grass to nibble they will be fine. 3 acres of good pasture should be nice as long as you seed every few years. The length of grass should dictate water needs. If it stays at a nice height they are well fed and avoid overgrazing. (parasites are an issue with over grazing)
A little grain never hurts. always offer mineral/salt block for vitamins. Not all grasses or hay provide all needs.

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The other person gave a good answer.
But there is one more thing you should do. Watch that the weeds do not take over your pasture. With not small of pasture. I would not you poisons. Do not like that much. Pull them or cut them. Weeds need to be longer to take over the pasture. So if you cut them cut the grass long. Had a friend with a weed pasture. I read to much. So my friend try what I read. Her pasture came back in one year. They had grass to eat and not so much weeds. Brake up the horse leftovers. Its the nice way to put it.