no way will it be usefull next year it will get mited you see the powder moving
Where do you keep yours then Robert?
I seal mine up in the polythene bag it came in - having opened it carefully, not ripped it apart.
As previously mentioned, I press the bag down to expel all excess air, then seal it securely with a tight elastic band.
I frequently keep it from one year to the next and have never had a problem.
NB. If possible buy a bag with a long sell buy date - you can get them valid till the next year anyway. But if not, it is only the vitamins that are not guaranteed, and you can always give the lambs a vitamin supplement anyway.