Albert Barnes Commentary Deuteronomy 20:6

"And what man is there that hath planted a vineyard, and hath not used the fruit thereof? let him go and return unto his house, lest he die in the battle, and another man use the fruit thereof." — Deuteronomy 20:6 (ASV)

See the margin and references. The fruit of newly-planted trees was set apart from common uses for four years.