The USDA is not part of the political structure, so there is no such thing as a “political contribution” to anybody in that agency.
Of course, but it’s very naive to think that the USDA is not influenced by Congressmen and Senators that take contributions as their main business. There is no part of government that is apolitical.
OK, let's just say that ADM wanted to "influence" the corn crop statistics that are gathered annually. Since the crop statistics are gathered by the local county agents office (you can look up crop statistics BY INDIVIDUAL COUNTY on the USDA site), this means that ADM would have to bribe EVERY COUNTY AGENT (not "Congressmen and Senators") in every corn-producing county in the US. And they would ALL have to be sufficiently dishonest to accept the bribe and not blow the whistle. Frankly, I don't think that scenario is of a very high probability of liklihood.