What DM should I aim for?

Aim for at least 28%, but only if you can achieve this within 24 hours, preferably 12 hours. DMs above this may well lead to higher DM intakes but not necessarily to higher animal production; field DM losses will also be higher and the risk of aerobic spoilage will be significantly increased.

It is worth finishing the clamp off with a few loads of direct cut grass as this will help seal the surface and reduce aerobic spoilage.

How can I tell what DM the grass is?

Take a few representative grab samples of the swath and squeeze them.