The “grain” and “rain” lines are related. In order to grow grain, a regular rainfall is essential.
The Buddha lived in a period of a significant cultural shift. It was the development of agriculture, and, related to that, the establishment of bigger settlements and the development of an urban culture. The “old” way is represented by the cattle, which would be linked to a more nomadic lifestyle, or small settlements. The “new” way is represented by the grain.
And we have to imagine that this transition didn’t come without conflicts. In the Bible, we find this very same topic in the conflict between Cain and Abel. Here we see it in the form of one verse pronounced by a deva, and a reply given by the Buddha.
There has already been more discussion on this topic, and I just saw that there is a talk on this Sutta by Bhante Sujato (the very first in the list).