Horse Safe Grass Killer for Arena

My arena has become overgown with mostly grass. I would like to actually use it as an arena again but its also used as a grazing area for friends horses when they come to visit. I would like to kill most of the grass so I can more easily harrow it. I want to kill 1/2 the grass and leave the center with some grass. What would be a grass killer that would be safe for horses who may end up grazing close to where it was used? I have never used chemicals near my horses so I havent a clue what to use.

There are none that I know of that would allow immediate grazing. If you want to kill only weeds, 2-4-D is pasture safe, but it won't kill the grass. If you want to kill everything, you need to use RoundUp, but you'll have to keep the animals away for a week or more (look it up for the exact amount of time).
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Thank You! Question - keep them away as, they can't even walk on it or keep them away as in don't let them graze near it? Sorry, I know this is probably a dumb question but again, I have never used any chemicals around my horses.

You are pretty much limited to the Roundup. Just about all the other herbicides are selective broadleaf meaning they are formulated to kill only certain plant products but not grasses. Roundup kills everything it comes in contact with. The general rule is to spray the grass you want killed. Let the product dry on it. Do not allow horses out on it for 2 weeks or until you get a good rain. If you get rain before 24 hours, you won't get much of a die off.

As LHP said, off means don't let them walk on or graze on. On the other side of a fence is fine. The farmer who grows corn around our pastures sprays Round Up before planting, and it doesn't bother our pasture grass 3' away.
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What about electric fencing? You could strip graze it down in no time.

Well I guess I might either use round up and not put them in there for a while, or maybe rent a rototiller and just rototill the parts I want dirt/sand. There is or was sand under there at one time.
I do put my horses in there but I think they would have to be in there for days to graze it down. Its approx 100 x 100 and for all places for the grass to root itself so deep. I did not plant that grass in there, go figure, it comes up hearty and strong where I don't want it!!

Keep in mind that whatever you do, that grass will be back. Maybe not this year, but probably next.
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If you really want to go chemical free you can use vinegar with 10% acidity. Household has 5%. Or you can use a garden weed torch.

Horticultural Vinegar Acidifying Agent 1 Gallon | A.M. Leonard, Inc.

Or

Sievert Gardener Weed Torch | A.M. Leonard, Inc.

Or you could always just blade it up