I've found that although horses and donkeys love clover it makes them slobber badly. A little clover will encourage growth of a main grass so it won't hurt to put it in a mix of grass. The best thing to do is check with a Farmer's Co-Op or Ag Ext. Office in your area to see what types of grass work best for your area. Orchard grass, clover and some types of Fescue work well where I'm at but may not do well there. Be careful with fescues however if they do work well there. As with anything they have pros and cons. They are very durable and cold hardy, handle heavy clay soils and do well with high traffic. But I'm in the process of changing most of my fields over to an Orchard grass mix because Fescues are creating more and more problems with lactating animals. I have people come to me all the time looking for goats milk to give to their foals because the mares never produce milk due to fescue only grazing. But the best bet is to check with people locally to see what works best in your area.
Mike

clover also gives donkleys ,horses the runs....

Fescue also causes abortions in some animals; not sure about donkeys.

We've been planting oats for 2 years now, horses love it, no ill effect noted. We get it from the co-op very inexpensively by the ton. We feed it to them, too, and they plant as they graze.

Brendasue

I don't know how many donkeys you have but a third of an acre is not much for even one equine with out careful management.

Only red clover has adverse effects on donkeys and horses (drooling) and only at certain times of year. There is a type of mold that forms on it. It's harmless. Never had any problem with the runs. My seed includes lots of white clover.

I agree that you should contact your county co-op extension. They will no doubt want you to provide a soil sample to see about amending the soil. It's inexpensive and in the end saves you money because you can buy seed and fertilizer all day long but if it's not what your soil needs it won't help.

Donkeys like horses ca easily overgraze a field in no time. Careful rotation and limiting the time on the pasture helps.