Perhaps the author has personal experience with this? I suspect a British author, as an American one wouldn't rhyme grain/again. Though I do wonder what the title has to do with it. Nothing in the poem speaks to something unique to farming, I suppose unless the specific grass is wheat or oats or some such. I do like the way there's almost an additional rhyme: stain/again works (inasmuch as grain/again does, depending on nationality), and nascent/grain is kind of close to one.