These devices are used to wean a calf from its mother. It attaches to the calf nose so that when the calf tries to nurse, the mother is poked and refuses to allow the calf to nurse.
It looks like a rather cruel device, but really is not at all. It is part of a two-step process that dramatically cuts the calf stress while still allowing it to graze and drink water.
The device is put on the calf nose for a period of four-seven days, after which he/she is separated from its mother.