This question is partially answered in the Idaho Statutes, but my questions goes a little deeper. Idaho Code Section 36-1603 (part of the Fish and Game code) prohibits entry of another person's land to hunt, fish, trap or retrieve game if the land is cultivated or posted (by sign or orange paint), but my question goes a little beyond that. If the property is "cultivated" in the spring and summer, but left fallow in fall and winter (since it is under several feet of snow), does this section still apply? I have never seen posting signs and the property is not fenced.
Fish and Game encourages all hunters to “Ask First” before hunting on private property to respect the property rights of others. Our actions as hunters influence landowners' support for wildlife and public access. Idaho Code Section 36-1603 defines "cultivated" as "soil that is being or has been prepared by loosening or breaking up for the raising of crops, or used for the raising of crops, or artificially irrigated pasturage." State agencies and the courts are supposed to interpret laws to give them a sensible construction that reflects legislative intent. In the situation you describe, the soil beneath the snow has been loosened and broken up for the raising of crops, and it makes sense to consider the land "cultivated" under the definition of the recreational trespass law. The definition of "cultivated" does not require that crops be grown on the land year-round, especially when weather conditions make that unreasonable.