干し草 specifically means dried grass or hay, whereas まぐさ can include fresh grass or other fodder.
Standard kanji spelling; literally 'horse grass'.
Rare single-character form; may not be widely recognized.
Kana spelling is common when the kanji are not needed.
Compound of 馬 (uma, 'horse') and 草 (kusa, 'grass'), with the reading まぐさ likely from a contraction of うまくさ. The single-character form 秣 is a kokuji or specialized kanji for horse feed.