納屋には干し草が積んである。
Hay is piled up in the barn.
牧草 is grass grown in a pasture (fresh or as a crop), while 干し草 specifically refers to that grass after it has been cut and dried into hay.
Standard kanji spelling; the most common written form.
Abbreviated kanji spelling; less common but still encountered.
Irregular okurigana usage; very rare and non-standard.
Rarely used kanji form; 乾 is not the standard character for this word.
Compound of 干し (hoshi, 'drying', the continuative form of 干す) + 草 (kusa, 'grass'). The reading ほしぐさ is a rendaku variant.