In the fast-moving farm bill deliberations, contacting Senators who are on the Agriculture Committee may be especially meaningful on April 24 and 25. That Committee is expected to determine language for their version of the farm bill on the 25th. This committee process is called "mark-up". The National Sustainable Agriculture Coalition (NSAC) is helping coordinate input on the a number of farm bill issues before mark-up.
The NSAC policy blog has the most recent analysis and updates on amendments that are being offered. http://sustainableagriculture.net/