Genomic Predictions for Heat Tolerance in U.S. Holsteins: What the New Research Means for Your Breeding Program

What the research actually did

Most breeders have watched the same cows fade every July. Same ration, same barn, same fans, and yet a few cows hold their milk while their pen mates slide. The obvious question is whether those cows are different on paper, in their genes, or only in where they happen to stand. A large U.S. study published in the Journal of Dairy Science in 2023 set out to answer exactly that, and its numbers are worth knowing before you start asking your semen rep for "heat tolerant" bulls.

The researchers pulled single-step genomic predictions for heat tolerance of production yields from a very large data set: 12.8 million test-day records on 923,026 Holstein cows in 27 states, with matching records on Jerseys, covering 2015 through 2021. They measured heat load with a five-day hourly average temperature-humidity index and looked for the point where average adjusted yields started to fall. For Holsteins that threshold came out at 69, and for Jerseys 72. About 15 percent of Holstein test-day records fell on or above the Holstein threshold.

The key finding for breeders is about variation. Using a reaction-norm model, the team separated each cow's "general production" genetics from her "heat tolerance production" genetics. In Holsteins, the heritability of milk, fat and protein yield under heat stress increased, with a small standard error, which is the signature of a real additive genetic component for heat tolerance. The pattern was less clear in Jerseys. In plain terms: some Holstein families really do lose less milk as the index climbs, and a genomic model can start to rank them.

What a reaction norm tells you, and what it does not

A reaction-norm model treats a cow's yield as a line that runs across an environmental gradient, here heat load. Two cows can start at the same yield on a cool day and slope away from each other as it gets hot. The slope is the heat-tolerance signal. That is a very different thing from a cow's raw summer yield, which mixes in stage of lactation, parity, pen position and whoever got the better fan that day.

Two cautions follow. First, this is a research evaluation. CDCB's published Net Merit history, which runs through its April 2025 update, lists yield, fertility, health, feed intake and composite traits but no heat tolerance trait, so you will not find a heat figure on a standard U.S. sire summary the way you find Daughter Pregnancy Rate. Check current sire summaries and your A.I. company's materials before assuming otherwise. Second, a ranking learned from records collected at one range of heat loads and humidity says less about a herd in a very different climate.

The trade-off breeders should expect

Do not expect heat tolerance to arrive free. A 2019 Frontiers in Genetics study of U.S. Holsteins used about 254,000 test-day records from 17,522 cows and found the genetic correlation between general production level and thermotolerance was negative, ranging from -0.18 to -0.68. The authors read that as evidence that high-producing cows are more susceptible to heat stress. Cows that make the most milk make the most metabolic heat, so this is biologically sensible.

Australia is the best-documented example of what a national heat trait looks like in practice. When DataGene introduced its Heat Tolerance Australian Breeding Value in December 2017, it reported a reliability of 38 percent, and described heat tolerance as favorably linked with fertility but unfavorably linked with production. Its guidance was to look for bulls with a strong overall index and a heat tolerance value above 100, and to use a team of bulls to offset the low reliability. That advice is more useful than it first sounds. It says "balance," not "chase."

Question	What the evidence supports
Is there a genetic component?	Yes. Heritability of yield traits under heat stress rose in U.S. Holsteins.
Is it free of trade-offs?	No. Published correlations with production level are negative.
Is it on a standard U.S. sire summary?	Not in CDCB's published Net Merit trait history through April 2025.
Is accuracy high?	Early national heat evaluations launched at modest reliability.

The single-gene shortcut: slick hair

There is one heat-tolerance variant that is well understood. The SLICK mutation sits in the prolactin receptor gene and produces a short, shiny coat. A recent review in the Journal of Animal Science describes SLICK as conferring heat resilience and offering a realistic route to mitigating heat stress by introgression into cattle without the mutation, while noting that definitive cause-and-effect evidence for prolactin's many roles is still being worked out. For a Holstein breeder, that is a long-horizon option, because bringing a variant in from other lines means managing everything else that comes with it. It is not a quick fix for next summer.

How to use this in a real breeding program

1. Quantify your own heat problem first. Pull test-day records and compare June-to-August yield and conception against your spring baseline. If the dip is small, genetics is not your biggest lever.
2. Fix the environment before the genome. Reduced feed intake explains only part of the milk loss in heat-stressed cows, according to one recent review of pair-feeding research, but cooling, shade and ventilation still pay first. Our guide to heat stress signs, causes and cooling covers the barn side.
3. Keep the main index in charge. Rank on Net Merit first, then use heat information as a tie-breaker among bulls with similar overall merit and sound fertility. Our guide to genomic testing and Net Merit explains the base index.
4. Spread risk. Use several sires rather than one heat-tolerant sire, for the same reason DataGene suggested a team of bulls.
5. Do not trade away fertility or health. Check Daughter Pregnancy Rate, livability and the recessive status of any sire you add.

Where this is likely heading

The direction of travel is clear. More hot-season data, wearable sensors and automated milking records will give researchers better phenotypes, and better phenotypes mean higher-reliability predictions. Until a trait appears in the sire summaries you actually use, treat heat tolerance as a promising secondary signal, not a selection goal on its own. The herds that gain most will be the ones that measure their own heat losses, cool their cows well and breed for a balanced cow. For more on herd-level economics, browse the Agriculture Directory and farm.fm's dairy coverage. This site is independent and is not affiliated with any breed association or evaluation agency.