That has been shifted to Scott Simpson, who is also National's environment spokesman.
Simpson leads National's "Bluegreens team".
Simpson's Workplace Relations and Safety portfolio has been handed off to Todd McClay, who picked up economic development from Paul Goldsmith after he moved into the finance role.
Bridges said the reshuffle shows that the party is "brimming with talent".
He said Muller was a hard-working and highly performing MP who was deserving of a promotion.
"I have no doubt that Todd will hold this Government to account on behalf of rural New Zealand."
Bridges also praised Guy for his contribution to National, and the rural sector, during his time in Parliament.
"Nathan has been a champion for rural New Zealand. As a farmer and a businessman, he understood more than most what the sector needed and he delivered for them."
Farming lobby group Federated Farmers said Guy was a pragmatic and knowledgeable Minister for Primary Industries.
"His door was always open, and he was always level-headed and considered in his dealings with people," Federated Farmers president Katie Milne said.
Speaking to media this afternoon, Guys said his National Party colleagues were a bit "shell shocked" when he made his announcement in caucus today.
But he said he's sure that National will win the next election.
