We graze ours on forage rape and always introduce them slowly - couple of hours a day to start and then build up. They’ve also got access to barley straw.
If it comes colder, don’t let them on until the frost has gone off. Heard of a few losses of cattle when it was frosty a few weeks ago
"Livestock must have access to hay, straw or grass to graze, to promote saliva production and healthy rumen function."