Addressing the national council at the organisation's annual meeting yesterday, Dr Rolleston said Britain's membership of the European Union over the past four decades had given New Zealand increased access to a continent that could be difficult to penetrate.
It had also provided the opportunity to build relationships and the strength of those relationships would now be put to the test, he said.
"We have lost an Anglo-centric, rational, free-market voice in the European Parliament just as we are starting our trade talks.
"We know that to trade with Europe, Norway has adopted 5000 EU laws and 70% of the EU directives, rules which they, as a non-member, have no democratic ability to influence.
"Is this what the new Europeans will be pushing for in our negotiations?'' he said.
Dr Rolleston questioned whether the European Parliament would learn from Brexit that "good decisions must have their base in science, not in the opinion of the lobby group''.
He had spoken with Tony Blair when he visited New Zealand as British prime minister and Mr Blair had revealed his frustration at the "stranglehold'' NGOs, particularly those environmental groups who "ignored science when it suited them'', had on the European Parliament.
"Britain voted Brexit in part because of rules they saw as interfering and irrational; single-issue lobby groups should take some responsibility,'' he said.
Many voters chose Brexit because they felt disenfranchised and powerless against big business and a globalised world. "Do we, as farmers, have lessons to learn here? I think we do,'' he said.
The world was a less certain place and New Zealand must ensure it found its way in it. He was looking forward to the challenge amid that uncertainty because out of challenge came opportunity, he said.
Farmers were up for the challenge to address water quantity and quality issues but they needed to work with the rest of society, who, in turn, needed to understand that the answers were ``not just linear, nor are there quick fixes''.
"We must be informed by the science, not slogans and unsubstantiated rhetoric,'' he said.
"Farmers have come a long way since the conversation on water quality began. Farmers across the country have spent hundreds of millions of dollars to improve the environment.
"They have fenced 20,000km of rivers. They are changing their farming practices to reduce runoff, reduce sediment and reduce nitrogen leaching.
"Farmers are engaging with councils, with zone committees, and in water groups right down to catchment level. For it is through working together as local communities, focusing on the problem areas and taking ownership of the outcomes, that we can reach our economic and environmental aspirations.''
Increasingly, agriculture relied on its social licence to operate. To work effectively, that licence needed to have its base in evidence.
That was why science was a priority for Federated Farmers and why the organisation was disappointed to see environmental groups distorting or using selective evidence in the water debate to undermine what farmers were doing, he said.
Dr Rolleston proposed establishing the Land and Water Stewardship Initiative, a "forward-thinking think tank'' of primary industry players with iwi and environmental representation who were willing to work together to propose solutions that took both the economy and the environment forward.
