I have to site an online article in APA format, but the only information it gives me is a association (United States Department of Argiculture) the d
It is very common when dealing with government information to have a "corporate author," meaning an organization, as opposed to an individual, is given credit for authorship.
I think I would treat your source as a "corporate author, government report." Here is the citation example given from p. 205 of the Publication Manual of the American Psychological Association, 6th edition.
U.S. Department of Health and Human Services, National Institutes of Health, National Heart, Lung and Blood Institute. (2003). Managing asthma: A guide. for schools (NIH Publication No. 02-2650). Retrieved from http://www.nhlbi.nih.gov/health/prof/lung/asthma/asth_sch.pdf
Because government agencies are hierarchical, the largest agency is listed first, and then the descend in order, as you can see in the example. Department, agencies, & committees are separated by a comma in the author statement.
APA is a rather inelegant citation style (it is designed for publishing journal articles in the psychological literature) but for some reason has become the gold standard. MLA is much more flexible. Their example is for a government document with an identifying number, which yours does not have. This goes back to (I believe) the days when all information was published in print. The number made it unique, so much more easy to trace. Now the URL makes the document unique. So, what do we do about the missing number?
For lack of a better plan I would omit that section in your citation. If you professor frowns upon that, then just ask innocently, "Well, what would you do about the missing number?" :)