Um Oeung says 20-month-old Tha Sophat started suckling the cow in July after he saw a calf do the same.
Um Oeung told The Associated Press he pulled the boy away at first. He relented after his grandson protested loudly and the boy has suckled the cow's milk once or twice a day since then.
Tha Sophat has lived with his grandparents in Siem Reap province in northwest Cambodia since his parents moved to Thailand looking for work.
Um Oeung said Friday the cow doesn't mind the boy suckling but he is worried about his grandson's health if he continues.
