For WTO notifications related to the Agriculture agreement, you can comb through the WTO’s “DocsOnLine” database of Members’ notifications to find the data, or use the ‘shortcuts’ provided on the WTO’s ‘Gateway’ page for agriculture. But—as in the case of the tariff schedules—you’ll have to extract the data from the original documents (mostly MS Word files) and make your own summaries. This is what I did for the database of SPS measures I built a year or so back.
Or, if you’re feeling lucky, you could check the most recent Annual Reports of WTO, which often contain summary tables of some of the data from the past year.
Fortunately, the United States Department of Agriculture has filled the gap for several of the most important data series in the WTO notifications (duties, export subsidies and production subsidies). The USDA site is not pretty, but it is much more functional than anything the WTO supplies. You can find both summary spreadsheets and, if you’re comfortable with the jargon, small on-line databases that allow you to drill down to the detailed level of the data buried in the heap of Member notifications.