Some things, like weather and major political issues are easily obtained bits of information and are high-impact events. Data like satellite imaging of fields are available at a price and will tell an awful lot about what best-case production figures will be like. Lastly, insider information is around too - asking companies what they planted, sales data on seeds/fertilizer, etc. If a disease is hitting one company, it's probably hitting a lot of them, especially true with live stock.
People do pay for this information, and they use it to trade futures. Which is how it eventually gets "priced in."
Forecasting is a huge factor in a successful agriculture business. Which is part of why a lot of niche industries tend towards vertical integration - it's much easier to forecast demand when you produce a lot of derivative products. Buy your own stuff; if you planted a surplus then create new products / markets, if there's a shortage, cut production on lower profit items.