ततः परं मासोत्तरमर्धवृद्धिर्यवप्रस्थ आत्रिवर्षात्द्रोण आचतुर्वर्षात् ॥ कZ_०२.३०.१२ ॥
tataḥ paraṃ māsottaram ardha-vṛddhir yava-prastha ā-trivarṣāt droṇa ā-caturvarṣāt
Thereafter (as the horse grows), the allowance increases by a half-unit each additional month: barley up to one prastha until the third year, and up to one droṇa until the fourth year.
To enforce uniform procurement and prevent both underfeeding (loss of readiness) and over-issue (embezzlement/wastage) in state stables.