I will weigh in with the answer that seems correct to me: there is no such word. That is, if you are referring to the period between the end of one harvest and the start of the next, which would include plowing, leaving it to lay for the winter, cultivating, planting, and growing.
Of the alternatives given, fallow, crop rotation, and intercropping are all different concepts already associated with farming. The growing season starts after planting, so does not cover the time between harvest and planting, and really refers to the period of time during which crops may be grown for a particular climate/environment. While cycle time seems to fit ok, it would properly refer to the period from the start of one harvest to the start of the other (and even then, only for a six sigma farmer :)).