The unfortunate arrival of some particularly hairy dairy cows has led to the discovery of a gene that could make NZ's national herd better able to tolerate hot summer days.

Richard Spelman - chief scientist for Livestock Improvement Corporation (LIC) - says the farmer-owned co-operative has successfully bred cows that have an internal temperature one degree lower than similar-aged cows raised on the same farm.

As the climate changes, these heat-tolerant cows will be able to remain comfortable at higher temperatures, Spelman says.

About 10 years ago, farmers began complaining when a bull in the LIC line-up produced very hairy offspring.

"They couldn't tolerate heat. You know, we had examples of these animals spending time in troughs trying to cool down and they also didn't lactate very well. The farmers weren't happy but as a science team, we were a little bit excited by this.

"You know, there are three billion base pairs (in genomes). Which one of those base pairs is causing that effect? And the team went and discovered the gene and the one variation that actually caused the effect."

Spelman says the team became aware of the Senepol cow - a heat-tolerant Caribbean beef breed with a slick coat.

"Given that we knew the biology about the gene that causes hairy animals, we could look at related biology and say 'actually we think that is the gene that maybe confers the slick (coats) and heat tolerance."

It did.

A Senepol bull was used to successfully inseminate dairy cows, however, its progeny milked poorly. That trait is now being bred out.

"The key thing is as we dilute that Senepol background, we want to continue to ensure we have the variation; that piece of DNA that gives us the slick coats," Spelman says.

When research into the gene began, it was thought heat-tolerant animals could be targeted at the international market.

"We called it 'The Tropical Cow' and as we have gone through this process, and we are now 10 years into this breeding programme, we have become very aware of climate change... so we are very much about trying to breed an animal that has a variation that will allow them to graze in the heat of the day and actually still be comfortable and not impact on feed intake, not impact on reproductive performance and milk production.

"So it's important from an animal welfare point of view, it's becoming more important. Our consumers want to know that the welfare of the animals is top-notch."