I would cut it before turning the horses out. This will remove tall stems that can get seeds in their eyes. We are hearing about quite a number of seeds in eyes this year. Couldn’t mow like normal with such wet land. Tall stems left long, do not add much in reseeding value to a field. This according to MSU grass research, which is contrary to old folk wisdom. Better to buy and spread good seed if you want to improve pasture growth.
Next, cutting returns the nutrients in cut grass back to the land, acting as fertilizer. If you cut more than once a season, the nutrient value increases to the soil. Not sure if you have been fertilizing, but farmer cutting hay off, removes nutrients. Then hay or pasture has less production, less food value in the grazing without replacement nutrients. Tamara in Tenn is a production hay grower. On COTH she has shared many pieces of growing information. One that really caught my attention was their practice of soil testing and fertilizing after EVERY hay cutting on a field! For her, it keeps the field productive, hay reliably has good food vales for her many horse customers. If you have fertilized regularly, then just cutting grass short a few times over summer, leaving it lay, can add almost as much value back to the land as an application of fertilizer! And no cost but your time mowing.
Third is the cut stems break down with time, future mowing, adding organic material to your soil. Then you get the soil animals, microbial and worms, working to break down the stems more, enriching your soil. So that is 3 good reasons for mowing!
I hear you on mowing with smaller equipment my mowers are only 5ft wide. Takes a LONG TIME to mow the two 11acre parcels! But that is what makes good pastures. Glad for my tractor canopy, seems 10 degrees cooler “in the shade!” But that constant mowing is what is required here, to keep my pastures productive enough to support 9 grazing horses on 11 acres all season. Not fat horses, they get brought in during day hours, used, so they are in good condition. Horses usually only get hay in summer if we haul them someplace to play or there is drought. The other parcel we bought recently, trying to get it in good shape to do hay from. Soil test was miserable, lacking in all nutrient areas. Fertilizer man said it was greatly lacking, should fertilize twice this year to get it up to needed amounts of minerals and nutrients so soil can produce. So land has dried enough that I FINALLY got half on yesterday, will apply the other half at summer end. Plants will use it better in 2 smaller doses.
Funny thing is the tall grass LOOKED nice, but not made of nutritional value. Horse who ate that hay would be lacking needed food minerals, maybe lose weight.