Does our FPO need GST registration and what compliance must we keep up?

Yes, in most trading cases your FPO will need GST registration. You must register once your turnover crosses the threshold, and you need it anyway to sell taxable goods like fertiliser, pesticide and many processed/packaged items, and to claim input tax credit. Many agricultural commodities sold unprocessed are exempt or nil rated, but the moment you deal in branded/packaged or taxable inputs, GST applies, so most active FPOs register early. Beyond GST, a Producer Company FPO must keep up: annual statutory audit of accounts, filing of annual returns and financial statements with the Registrar of Companies (ROC) on the MCA portal, holding board meetings and an annual general meeting with proper minutes, maintaining the members and shares register, deducting and depositing TDS where applicable, and income tax return filing. The simplest way to stay safe is to engage a CA/accountant from the start and keep monthly books clean. Missed ROC and GST filings bring penalties that have sunk otherwise good FPOs. Confirm the current GST turnover threshold and rates with a tax professional as these change.