Hello there. Hope you are doing good. I would like to tell you that no it is not possible to switch from one field to another field in M.Sc. If you want to pursue M.Sc in Agriculture, then you must have gained the basic knowledge about the subjects of Agriculture in your bachelors, hence B.Sc Agriculture is required for a candidate who want to pursue Masters in Agriculture.

I hope you found this answer helpful. Good luck for your future.