Every milk test captures thousands of data points that extend far beyond fat and protein percentages. We already use that data to make inferences about metabolism or health, but what if we could use it to reveal information about heat resilience?
New analysis suggests milk mid-infrared (MIR) spectrometry could become an unexpected tool for selecting dairy cows that are better equipped to cope with heat stress. Rather than relying on environmental measurements such as temperature-humidity index (THI), the approach estimates an individual cow’s physiological response using information collected through routine milk recording.
If validated in additional populations, the approach could offer a practical new way to select for heat resilience using data already generated through routine herd testing. More importantly, the work suggests that the biology of heat tolerance extends beyond temperature alone, with genes involved in energy balance emerging alongside those traditionally associated with heat stress.
Rethinking How Heat Tolerance is Measured
Selecting cattle that can better withstand heat stress has become an increasingly important breeding objective, but measuring heat tolerance has never been straightforward.
Most current genetic evaluations estimate heat tolerance by measuring changes in production as temperature and humidity rise. These reaction-norm approaches have advanced selection efforts, but they also have important limitations.
Traditional heat tolerance evaluations can be challenging because they:
- Rely on environmental measurements such as THI rather than an individual cow’s physiological response.
- Primarily measure production losses instead of broader biological adaptations to heat stress.
- Often show an antagonistic relationship with milk production, making simultaneous improvement more difficult.
- Require matching production records with environmental data for large-scale genetic evaluations.
Researchers from the University of Liege took a different approach. Rather than asking how much milk production declined during hot weather, they asked whether the milk itself contained enough biological information to identify cows experiencing heat stress.
Using more than one million milk MIR spectra collected from more than 82,000 Holstein cows, they applied a previously developed prediction model that classified cows as unaffected, intermediate or affected by heat stress based solely on changes detected in routine milk spectra. The subsequent genetic analysis included more than 171,000 milk records from 64,035 cows across 603 herds.
A Heritable Trait with Potential for Selection
For any new phenotype to have value in a breeding program, it must have a genetic component.
Researchers estimated the heritability of the MIR-derived heat stress phenotype at 0.10. While modest, the estimate falls within the range reported for other heat stress-related traits, suggesting the phenotype contains sufficient genetic variation to support selection.
The team also examined how the new phenotype was genetically related to other economically important traits. One of the most encouraging findings was the relatively weak antagonism with milk production compared with many traditional THI-based heat tolerance traits.
The relatively small positive correlation with milk yield suggests less genetic antagonism with production than has been reported for many conventional THI-based heat tolerance traits. More striking were the moderate negative correlations with fertility and longevity, indicating that animals genetically predisposed to lower MIR heat stress scores also tended to have more favorable genetic merit for reproductive performance and productive life.
The Genetics Behind Heat Resilience
To better understand the biology underlying the new phenotype, the researchers performed a genome-wide association study that identified 12 genomic regions associated with the predicted heat stress response. Several overlapped with genes already implicated in heat tolerance, providing additional confidence that the MIR-derived phenotype reflects meaningful biological differences rather than simply changes in milk composition.
Among the strongest candidates was HSF1, one of the best-characterized regulators of the cellular heat shock response. Heat shock factors activate protective proteins that help cells maintain normal function when temperatures rise, making HSF1 an expected and reassuring finding for a study attempting to measure heat resilience.
Researchers also identified DGAT1, a gene well known for its effects on milk yield and milk composition. Because DGAT1 sits adjacent to HSF1 on the chromosome, distinguishing whether the signal reflects production, heat response, or both remains challenging. The proximity of the two genes highlights the close relationship between production biology and heat stress adaptation.
Another notable candidate was TRIO, which has previously been linked with heat tolerance in dairy cattle. Its appearance in this analysis further supports the biological relevance of the MIR-derived phenotype.
Perhaps the most intriguing gene was ANKH, which plays a role in mineral and citrate transport. Heat-stressed cows lose minerals through increased sweating while simultaneously reducing feed intake. A gene involved in maintaining mineral availability and regulating citrate transport raises the possibility that metabolic adaptation is an important component of heat resilience rather than simply a consequence of heat exposure.
Heat Resilience May Be a Metabolic Trait
One of the most compelling aspects of the study was not a single gene, but a recurring biological pattern.
Many of the genomic regions associated with the MIR heat stress phenotype have also been linked in previous studies to negative energy balance, milk β-hydroxybutyrate concentrations, ketosis and milk citrate. Rather than appearing as isolated observations, those overlaps point toward a shared physiological foundation.
This new research suggests that a cow’s ability to cope with heat stress may depend not only on how she responds to elevated temperatures, but also on how effectively she maintains energy balance when feed intake declines and maintenance requirements increase.
That perspective aligns with what is routinely observed during periods of heat stress. Reduced dry matter intake, altered nutrient partitioning, mobilization of body reserves, and increased susceptibility to metabolic disorders often occur together. The genetics identified in this study suggest those responses may be interconnected rather than independent consequences of hot weather.
While additional validation is needed, the work offers an intriguing shift in perspective. Heat resilience may be as much about maintaining metabolic stability as it is about tolerating elevated temperatures.
Looking Beyond the Thermometer
The study does not replace existing approaches to evaluating heat tolerance, nor does it eliminate the need for environmental monitoring. The MIR-derived phenotype will require validation across additional breeds, production systems, and climates before it could be incorporated into routine breeding programs.
Even so, the work demonstrates that information already collected through routine milk testing may provide a practical way to identify genetically heat-resilient cattle. At the same time, the genetic analysis offers another reminder that the biology of heat stress extends well beyond ambient temperature, pointing instead toward the complex interaction between heat response, metabolism, and energy balance.
