[Foobar]

How long should you have kept livestock off a field before you can cut it for hay?
Can I get away with 4 weeks? Or does it need to be more like 6?

[Marches Farmer]

Depends on the weather and what type of grass you have.  Damp soil, warm weather and Italian rye grass and you might get away with 4.  Old pasture and 28 degrees as we have here and you won't get a cut this summer if you haven't got it in already.

[Foobar]

My question is more about contamination rather than growth.
As it happens I'm not after quantity as I'm doing it by hand, for the first time, so really it's more of an experiment .  We have some odd corners of fields that always grow quicker than the rest, so I would just cut those bits rather than the entire field.

[Backinwellies]

presuming you not going to incude dung piles in your cut there is nothing to contaminate with.

[Foobar]

My sheep don't really do dung piles, they do "raisin sprinkles" so they are a bit harder to see .  And of course they wee

[Fleecewife]

Raisin sprinkles will shake out as you make the hay and if one has had the runs then you would have to remove the contaminated bit by hand.   The sheep, if left in the field, may well sleep on the hay you are making, eat it and as you say, pee on it - the finished product won't be tasty for them.   We shut up our field destined for hay as soon as the grass starts growing in spring, then we cut it when it's ready and we have a haymaking weather window.  Takes about 10 weeks.   We wouldn't leave the sheep on it at all during that time, for the reasons above.
However, if you are just cutting grassy corners, then I would think that as long as the sheep are out once you are ready to cut that will be ok.   If they are contaminating it that means they have been eating it and sleeping on it, so there won't be anything left worth scything.   In fact I'm wondering why the corners are not being grazed if there are normally sheep on it - not worth the effort of making hay if it's stuff they won't eat.

[Foobar]

Okay thanks.  The sheep have been out for 4-5 weeks, and there are just corners that grow better as we have a mixture of soil types and I suspect those areas retain more or less moisture or something like that, maybe with the shade of the trees, or fertilizer run off from the neighbours. Who knows .  When the sheep are in there they eat it all, so it's not like they have turned their noses up at it.