Approximately 277 millions chickens are grown under the GAP standards on 600 farms, and these standards are used by retailer Whole Foods Market.

"Supporting this additional commitment to improve chicken welfare is a step forward in continuing to provide our customers with products of the highest quality and flavour,” said Theo Weening, global meat buyer for Whole Foods Market.

The new rules will require genetic potential growth rate equal to or less than 50g per day averaged over the growth cycle (roughly 23 per cent slower growth than conventional chickens).

Lower stocking densities will also be implemented, and producers will have to provide enrichment such as perches and pecking substrates to meet the GAP standards.

“At GAP, our goal is to improve the welfare of farm animals,” said Anne Malleau, executive director for GAP. “By addressing fast growth, we will be getting to the root of the welfare problem facing chickens today. Implementing this transition will require significant work, but we are confident we can get there."

Production of alternative, slower-growing broiler breeds has been gaining attention in various countries, particularly in Europe.

Many believe that using such breeds is better for animal welfare - GAP said that fast-growing breeds were less able to engage in natural behaviours due to detrimental effects of fast growth on their immune and musculoskeletal systems.

“GAP will continue to be a multi-stakeholder process,” said Ms Malleau. “GAP chicken farmers have pioneered animal welfare in North America, and we’re excited to continue that journey with them on this new ground-breaking project.“

She continued: “GAP has reviewed scientific literature and existing genetics across the world in making this decision, and is confident that this new direction will bring benefit to birds, producers, consumers, and food purveyors alike."