The National Agricultural Library has an excellent calendar at http://www.agnic.org/events.
It covers major international and U.S. national events (1,500+) with
emphasis on those of scientific significance. It includes all subject
areas and has special-focus sections for Animal Sciences, Food and
Nutrition, Forestry, Natural Resources, Plant Sciences, and Soil
Sciences. It also links to 200 calendars maintained by others organized
by broad subject category.