Mr McIvor said it had been an honour to work nearly half of his working career leading the two organisations over two stints.
His last day at both organisations will be July 24.
"I’m fully committed to
B+LNZ and the NZMB and have plenty to still achieve in the next three months."
He will start as the Ospri chief executive in August.
Ospri board chairman Paul Reynolds said Ospri was "thrilled to welcome Sam".
"His extensive experience, coupled with his profound understanding of the agricultural sector, uniquely positions him to drive Ospri forward and deliver value to our shareholders, stakeholders and the farming community."
B+LNZ chairwoman Kate Acland said the board thanked Mr McIvor for the significant contribution he made to the company and the sector in the 17 years he worked for the organisation, including the past eight as chief executive.
"We are delighted that Sam has chosen to continue with a role in the agriculture sector and are confident he will continue to make a positive impact in this position."
The board would begin the search for a new chief executive in the coming weeks, she said.
- Entries are open for the Otago-Southland Hoof & Hook Beef Competition.
Competition convener Barry Gray, of Owaka, said beef steer and heifers would be judged on the hoof at Charlton Saleyards and on the hook at Alliance in Mataura.
Entries close on May 23.
