fodder
秣 centers on dried plant material used as animal feed: fodder, specifically hay or grain stalks for livestock.
秣 combines 禾 (grain plant) with 末 (tip, end), likely suggesting the cut ends of grain stalks used as fodder. The exact historical development is uncertain, but the component combination fits the meaning.
The grain plant 禾 beside the tip/end 末: picture the cut ends of grain stalks being gathered as fodder for animals.
For マツ, imagine a farmer spreading a mat of dried grain stalks as fodder: mat -> マツ, and the animals eat from the mat.