The RSS chief was speaking in a video message to supporters.
He said the cow was a Hindu emblem and a symbol of purity.
“The cow symbolises purity and unity,” he said.
“If we can create a nation that is pure, then we can achieve the goal of achieving unity and peace.”
The BJP is yet to give any concrete date when the party will announce its first prime ministerial candidate for the 2019 elections.
Modi is expected to announce his candidate on Saturday, the day the first phase of the Uttar Pradesh assembly elections will begin.
The prime ministerial contest will be the third for Modi in his five-year term as the chief minister of the southern state.