Heat Tolerance as a Breeding Goal: Preparing for a Warmer Climate Without Losing Production

Every summer, milk shipments dip, conception rates fall and cows pant in the shade. Cooling systems are the first answer, and our guide on heat stress signs, causes and cooling covers them. But if you are making breeding decisions today, the cows born from them will be milking in a warmer world. That raises a fair question: should heat tolerance be a selection goal, and what does it cost?

Is heat tolerance genetic?

Yes, partly. A study of 353,376 test-day records from 38,383 first-lactation US Holsteins, published in the Journal of Dairy Science and indexed in the FAO AGRIS record, found that individual cows differ in the temperature-humidity index (THI) at which their milk yield begins to fall. About 56 percent of the variation in that threshold was additive genetic. In plain terms, some cows start losing milk earlier in the heat than others, and a good share of the difference is inherited.

Other work looks at body temperature. A genomic study in Frontiers in Genetics, available through PubMed Central, notes heritability for rectal temperature as a heat-stress indicator in the range of 0.13 to 0.17. That is modest, comparable to many health and fertility traits, and enough to make progress with consistent selection, just not fast progress.

The catch: heat tolerance and yield pull in opposite directions

The same AGRIS-indexed study reported that the THI threshold is negatively correlated with milk yield, at about minus 0.53, and the rate of decline beyond the threshold was even more strongly tied to yield at minus 0.62. The Frontiers paper likewise reports negative genetic correlations between milk production and heat tolerance, ranging from minus 0.18 to minus 0.68 depending on the measure. Put simply: cows bred for more milk tend, on average, to cope worse with heat. That is not surprising, since a high-producing cow generates more metabolic heat to dump.

This is the core tension. Breeding programs that rewarded yield alone for decades may have quietly pushed the herd toward heat sensitivity. A trade press report on Misztal, Brito and Lourenco's 2024 analysis in JDS Communications, summarized by The Bullvine, says the THI at which Holsteins begin losing production dropped from 72 to 69 over roughly two decades. We could not read the underlying paper, so treat that as a reported finding, not settled fact. It is, however, consistent with the negative correlation above.

What evaluations exist?

Australia's DataGene has published a Heat Tolerance Australian Breeding Value since December 2017, according to the same trade report, which also says the Holstein version had a reliability of 48 percent after a reference-population expansion. Reliability is not heritability; it tells you how much to trust a bull's number, and 48 percent is moderate. The report adds that, at the time of writing, heat tolerance was not among the official CDCB traits in the US, though a commercial heat index had been launched by a genetics company outside the official indexes. If you are in the US, ask your supplier what they use and how it was validated before you buy semen on that basis.

A sensible breeding approach

1. Do not overcorrect. Giving up yield for heat tolerance wholesale would cost you at the milk check every month of the year, while heat stress bites for a few. Aim to stop the slide, not to reverse the breed.
2. Use heat tolerance as a secondary trait. Where bulls are close on overall merit, favor the one with a better heat evaluation from a source you trust. Our piece on genomic testing and net merit explains how to rank on the main economic index first.
3. Watch your own herd. Look at how your cows' production and conception rates respond in the hottest weeks. Records from your herd tell you more about your barn than any national average.
4. Keep investing in cooling. Genetics is slow. Shade, soakers, fans and ventilation work this summer. Water matters too: Ohio State Extension figures show a dry cow's water needs rising from about 9.7 gallons at 40 degrees F to about 16.2 gallons at 80 degrees F, so tank capacity and flow are part of heat preparation (see the OSU water usage article).

For a more practical breeding walkthrough, see selecting for heat tolerance without sacrificing milk yield or fertility.

The slick gene question

Some programs have looked at the slick hair-length gene found in certain tropically adapted cattle. The paper identifying it as a candidate gene influencing hair length and thermotolerance in Bos taurus is Olson and colleagues in the Journal of Animal Science, cited in the PMC article above. Moving such a gene into a Holstein population is a separate, specialized project, and it needs to be done without losing the breed's production traits. We have not verified field results, so view it as an area to watch rather than a tool to order next week.

The bottom line

Heat tolerance is heritable, moderately so, and it is unfavorably tied to yield. That makes it a goal to weigh carefully, not to chase blindly. Keep breeding for the traits that pay, add heat tolerance where good data exist, and keep your cooling system in good repair, because no sire will do that for you. For related reading, farm.fm's dairy section and agriculture.directory are useful starting points.