Rising temperatures are doing more than reducing milk production on dairy farms. New research from Cornell University suggests heat stress is also lowering the quality of milk, creating economic losses that may be twice as large as previously estimated.
The study, published in the journal Environmental Research Letters, found that warming temperatures reduce the fat and protein content of milk in addition to lowering overall production. Because dairy farmers are paid based in part on those components, researchers say the financial impact is substantial.
Using data from approximately 6.5 million dairy cows across 43 states between 2007 and 2016, researchers compared milk production records with localized weather data. The analysis found that while milk output declines sharply once temperatures and humidity reach certain thresholds, milk quality begins to deteriorate at lower temperatures and continues to worsen as heat increases.
Researchers estimated that a 10-point increase in the temperature-humidity index leads to a 1.2% reduction in milk production but a 2.8% drop in dairy farm revenue over the course of a year. Across the U.S. dairy industry, that translates to an estimated $1.65 billion annual loss.
Cornell economist Ariel Ortiz-Bobea, the study’s senior author, said the decline in milk fat and protein content has largely gone unnoticed compared with more visible reductions in milk output.
The research also found little evidence that dairy cows are becoming more resilient to heat. Scientists observed few differences in heat tolerance based on a cow’s age, farm size or geographic region. Instead, adaptation appears to occur primarily through where dairy farms are located, with cooler states such as New York and Wisconsin remaining important centers of milk production.
Researchers said the findings could influence future breeding and research efforts aimed at developing cows that are more resistant to heat stress while maintaining strong milk production and quality.
The study was supported by the U.S. Department of Agriculture’s National Institute of Food and Agriculture and the National Science Foundation.
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