What if it rains?
Leaving it lying will increase losses and potentially make it more difficult to ensile due to loss of sugars and an increase in spoilage microorganisms. So, provided it is not pouring, you should bring in what is already mown as quickly as possible. If the delay is more than 6 hours, sheet and seal the silo. If you have a lot of grass down and experience heavy and continuous rain you may find yourself in a salvage situation.