Two old Shetland ewes I picked up a few years ago as part of my first bunch of lambs/hogs had been fed a slice of dry toast every day as a treat by their previous owner... I gave it to them occasionally, but the toast had to come out of a rustling plastic bag, otherwise would be ignored... they used to dash across the field when I rattled the bag - good hearing when they wanted to hear! They were over 10 when I got them, both produced a couple of lovely lamb crops (and I still have their daughters) and then had to be dispatched because of old age (not many sheep have a stroke I guess....)
But in general toast with Marmite or honey is reserved for any one sick and not eating as a last resort. Has been known to work!