Cow is especially recommended in the Bhagavad-gita, krsi-go-raksya vanijyam vaisya-karma svabhava-jam.  [Bg 18.44]: “Farming, cow protection and business are the natural work for the vaisyas…] That you must produce enough food grains by agriculture and give protection to the cows. That means if you have got enough food grains to eat and if you have got enough milk to get fatty substance, then your whole economic question is solved. Annad bhavanti bhutani. [From Bg. 3.14 :”All living bodies subsist on food grains, which are produced from rains. Rains are produced by performance of yajna {sacrifice}, and yajna is born of prescribed duties.] If you get sufficient food there is no question of agitation. Everyone is satisfied. Animal and man. So you must produce. That is recommendation in the Bhagavad-gita.

Srila Prabhupada, Chandigarh, October 16, 1976 – Press Interview