Hi Andrew,
Alfalfa is a long lived
perennial that is greatly improving to the soil. Generally, once an alfalfa crop is established, it is left in place for a minimum of five years, and often as long as fifteen years. It just gets going in its second and third years, so ripping it out now really makes no sense from a farming standpoint.
It sounds like the tenant farmer wants to grow corn, because there is a heck of a lot more money there than in alfalfa. Corn, in terms of soil improvement, is about the polar opposite of alfalfa. Corn is about the heaviest feeder there is. Plus, as you suggest, corn gets a lot of chemical use, both herbicide and fertilizer.
From the standpoint of your land, you definitely
should keep the alfalfa crop in place. It will provide any animals you have with excellent forage and
hay. It will continue to improve your soils year after year. Corn would do nothing beneficial for your land now.
From the standpoing of your neighborhood, well, that's anybody's guess. Shooting from the hip here, as someone who once was new to the country, it sounds to me like the tenant farmer is trying to take you for a ride. Get the most money out of your land that he can, while he can. His response to your suggestion that the alfalfa planting is still very young, with years of productivity ahead, will likely tell you a lot about his motives. Also will be interesting to see what he says when you talk about alfalfa being a soil improving crop, whereas corn is a depleting crop. You'll likely be able to ascertain quite a bit about his motivations and morals in the ensuing discussion.
Country
politics are no joke. You want to get along with your neighbors. But, IMHO, being seen as a sucker is the only thing worse than being seen as uncooperative. Taking a stand, for what's in your best interest, might be tense at first with the tenant, but will gain you respect from all the other neighbors in the long run.
good luck!