He was named Federated Farmers' inaugural agribusiness person of the year in 2009 in recognition of his contribution to the New Zealand dairy industry.
The board was embarking on an international search to find a new chief executive and Dr Penno intended continuing in the role until a successor was found, Mr Milne said. He also intended continuing as a director.
The new chief executive would be taking on a company in strong shape and with a clear vision and growth strategy, Mr Milne said.
It had grown from a start-up to a highly profitable company listed on the NZX and ASX, with 600 staff, a "blue chip" set of customers focused on the infant formula category and committed farmer-suppliers, he said.
At Synlait's annual meeting in Christchurch yesterday, Dr Penno signalled further expansion was planned with a second milk powder manufacturing site.
The company was also looking to acquire some land in the North Island to create a second site, which would provide access to a second pool of farm-suppliers.
Synlait's revenue rose 39% to $758.9million for the year to July, earnings before interests, tax, depreciation and amortisation was up 6% at $89million and debt had been slashed by $131million.
The company was looking for a chief executive with the skills and experiences to "take Synlait up another level", Mr Milne said.
"We have already signalled to shareholders our intention to continue to develop our infant formula business, and enter new categories where there are significant opportunities," he said.
Dr Penno said it was time to move on after 17 years of "living and breathing" the company. Once the new chief executive was in place, he was looking forward to getting back to his entrepreneurial roots.
He would be looking for opportunities to get involved with start-ups and young companies, which was where his wife Maury and he wanted to continue to make a contribution, he said.
