Translation is by Suyooo on /r/SchoolIdolFestival on Reddit – https://old.reddit.com/r/SchoolIdolFestival/comments/ees7n1/sifas_4koma_was_that_a_pun_rumbling_and_swinging/ (includes previous installment)

This refers to a Japanese proverb or superstition that reads, 食べてすぐに寝ると牛になる (Tabete sugu ni neru to ushi ni naru), which literally means that if you sleep right after the (or a) meal, you will become a cow. It has been speculated because it is considered impolite or not good for digestion.