Black and white and red all over

I was curious to see sheep grazing in a field of red stalks - I cannot make out what they are.

Then I saw the magpie landing on first one then another of the sheep's backs. They do this to pick off lice and ticks (They have been accused of going deeper and causing serious harm and, although that is the exception rather than the rule, they have been persecuted for that reason - see here)