Feed your trainees in something they're used to.  The Vandenberg heifers were used to being fed in troughs, so I used them for the training process.
You don't have to give animals a lot of the new food.  It's just a "snack."
Here's an example of weeds and familiar food mixed for the first day.

Just like people, animals may not try new foods even if they're good for them. Fortunately, an animal who has had lots of good experiences with trying new foods is less afraid to try another new things.

Give trainees good experiences by giving them new, foods like ground alfalfa and rolled grains twice a day for four days. Create a routine by feeding them at the same time each day, using a cue, like tubs or a familiar trough or cake feeder. By day five they should be so used to you giving them something new and a little strange, that they'll be ready to try weeds.

Expect your cows to try each new food slowly, perhaps leaving after a small taste and returning later in the day to finish it off.

Don't worry if your cows don't clean their plates the first day. Since they have no food safety labels, they try things a little at a time.