Hello!  Thank you for reaching out to OSU Extension Service. I recommend that you mow the grass with the brush mower during the season, specifically when the seedhead is emerging beyond the top leaf (flag leaf) if you can.  This will allow the basal buds at the base of the plant to grow, helping each plant grow outward from the crown to fill in the area.  Haying would remove all of the nutrients that are in the grass from the field, so I would wait to hay until next year when the perennial plants are more robust.  You want leave adequate leaves on these baby plants for photosynthesis this year, so set the mower so that it doesn't cut shorter than 4" from the ground.  You'll be able to run the cattle on the area when the ground is dry enough that there will not be any soil compaction, excess trampling of plants and there is enough growth to not over-graze, again allowing at least 4" of stubble.  This first season of growth is important and sets the field up for productivity. If over-grazed or taking nutrients from the plant without giving them back, you may end up with weedy annual grasses taking the place of the forage perennial species you've just planted.

For more information about pasture & grazing management we have a free online course you can take and a condensed version of the course in a downloadable, printable format available at the links below. The "How Grasses Grow" section is very informative for the stage you're at right now.  There is also information on soil fertility, rotational grazing and more!

https://workspace.oregonstate.edu/course/pasture-and-grazing-management

https://catalog.extension.oregonstate.edu/em9302