Since pesticides are federally regulated, you must adhere to uses and application rates specifically outlined on the container's pesticide label. If the specific mention of forestry applications/use is not mentioned, then you are using the herbicide "off label", i.e., in violation of federal law.
Seeing that there are numerous manufacturers and different formulations of 2,4-D, you must read and follow the label. Please contact your local Iowa District Forester for forestry herbicide recommendations on your property as the answer to your question requires additional input including which species you intend to control.
Best of luck!