Bhashya
पशुषु पञ्चविध साम उपासीत ।सम्यग्वृत्तेष्वृतुषु पशव्य काल इत्यानन्तर्यम् ।अजा हिंकार, प्राधान्यात् , प्राथ-म्याद्वा- 'अज पशूना प्रथम ' इति श्रुते , अवय प्रस्ताव ,साहचर्यदर्शनादजावीनाम्, गाव उद्गीथ , श्रेष्ठ्यात्, अश्वाप्रतिहार , प्रतिहरणात्पुरुषाणाम्, पुरुषो निधनम्, पुरुषाश्र-यत्वात्पशूनाम् ॥
In plain words
One should meditate on the fivefold Sāman chant as manifested in domestic animals. This meditation follows immediately after the seasons, because the well-being of animals depends directly on the orderly progression of those seasons. Goats represent the Hiṅkāra, the introductory chant, because of their primary importance, or because they are the first-born—as the scripture says, 'The goat is the first of animals.' Sheep represent the Prastāva, the introductory praise, because goats and sheep are always seen together. Cows represent the Udgītha, the main chant, because they are the most excellent of all beasts. Horses represent the Pratihāra, the response, because they carry human beings. Finally, man represents the Nidhana, the concluding chorus, because all domestic animals ultimately depend on human beings for their shelter and care.
Close to the Sanskrit
One should meditate on the fivefold Sāman in animals. The sequence [here] is because the time conducive to animals follows the properly progressing seasons. Goats are the Hiṅkāra, due to prominence, or due to being first—because of the Śruti 'the goat is the first of animals'; sheep are the Prastāva, because of the observed association of goats and sheep; cows are the Udgītha, due to superiority; horses are the Pratihāra, because they carry men; man is the Nidhana, because animals depend on man.