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
How much straw do they eat
"Livestock must have access to hay, straw or grass to graze, to promote saliva production and healthy rumen function."
http://beefandlamb.ahdb.org.uk/wp/w...ing-brassicas-for-better-returns-manual-6.pdf