You play as a dairy farmer at a time when aliens are invading, and officials have found that the only thing effective in killing them is the raw materials of cow fart.
Waves of enemies can sometimes be a pretty heavy barrage. It is best to have a few well-stocked grass-plots (fertilized beyond 100%) to quickly replenish meters during intense battles.
The "Trojan Cow" can sometimes be more useful then you might at first think. And the green gobs are always good to have on hand in later waves to quickly get grass plots up to full production.