There are other non-slip finishes that may be more appropriate for large expanses. If still thought necessary, several farmers have commented to me that diamond shaped patterns wash down better than square cut grooves. A single long hose can often be very heavy and difficult to drag around the whole yard. Several smaller hoses may be more operator-friendly and save time. The idea is to get plenty of volume onto the concrete.
Pressure helps but volume is required to move the muck.
I saw a good example of a wash down system that works at a local dairy recently. The system was incorporated in the backing gate on a circular yard. Chains dragged along under the gate and the water came from 3 or four outlets on the backing gate. The backing gate was not needed for cow flow and was activated at the end of milking. As the gate swung round the chain dragged through the cow pats and the water flushed it away.