It's a good idea to sub-divide the field anyway, if only to better utilize the grass. If you do decide to continue to poo pick, or to use a mechanical means, it is easier on shorter sward. A single electric rope or tape would allow the sheep to graze the whole area, whilst restricting the horses. Also, if you decide to top the grass, you can do this ahead of the horses so that the paddock is safe by the time they come round to it. This also applies to any spraying of weeds which may be required.
Dividing the whole into three or four paddocks would allow a few weeks' rest between grazings, during which time a harrow, and perhaps a roll, would keep the empty paddocks right. Harrowing and rolling are weather dependant, though, so things don't always go to plan. Harrow during dry conditions so that the dung is broken up and desiccated.
If you have enough sheep, you may not need to roll. Sheep are very good at compacting the surface, and their close grazing habit helps grass to tiller out and thicken the sward.
Mechanical poo pickers are a controversial subject. The brush type, which flicks the dung into a container, tend to be useless in long grass. There is also a theory that soil disturbance plays a part in grass sickness, so anything which tears at the soil surface may not be a good idea.
There are other types which 'hoover' the dung up through a big bore tube, which doesn't disturb the soil surface. Some of them cause the dung to pass through a fan-like chopping device which breaks it down and creates a mulch. However, these machines require the operator to handle the considerable weight of the tube, and to jump on and off the quad or tractor required to tow the machine. Quite hard work on a hot day.
Horses are not easy to manage at grass. There are so many factors to consider, such as the dietary needs of the individuals; their work load (if any); weather conditions and grass growth; any fertilizers used; and the time of year. No two people have the same views, either, so good luck with your project.
