How do scholars know that indo-aryans raised cattle and depended upon their milk, but later turned to agriculture?

The scholars know that the Indo-Aryans was once raised cattle and depended upon their milk but later turned to agriculture for they found evidence in the Vendidad where it shows the importance of sheep and cattle-rearing and their agricultural activities was considerably developed. These have been proved from careful comparison of a number of Vedic and Avesta words relating to agriculture.

Moreover, in most Vedic hymns, one can draw a conclusion that they settled down to a peaceful agricultural life and in the book of Atharva Veda, it contains the tradition of agriculture and that Prthi-Vainya was the inventor of ploughing.

it refers to race, i am pretty sure this was a popular term in nazi germany or something.