Relationships between novel breeding values for heat tolerance in dairy cattle with rectal temperature during heat stress in lactating Holstein cows

Heat stress can compromise production and health in dairy cattle; themagnitude of these deleterious effects will be exacerbated by globalclimate change. The resistance of cows to heat stress is under geneticcontrol. Recently, Zoetis has developed 2 novel breeding values forheat tolerance in lactating Holsteins based upon the change in milkyield (MILKTHI) or cow conception rate (CCRTHI) with temperaturehumidity index. The goal here was to test whether variation in predictedtransmitting ability (PTA) for heat tolerance is associated with cowregulation of body temperature during heat stress. Data on afternoon(1400–1700 h) rectal temperatures during the hot months of the year(May–August) were collected from lactating cows on farms in Florida(n = 3), California (n = 1), and Turkey (n = 1) over a period spanning2007 to 2023. A total of 4,678 values of rectal temperature from 3,620cows were analyzed by the GLIMMIX procedure of SAS. Modelsincluded parity, stage of lactation, year-farm and with cow as a randomvariable. Breeding values were analyzed 2 ways: by comparing rectaltemperatures of cows above and below a PTA of 100 (94.3 ± 4.3 vs.104.4 ± 3.5 for CCRTHI and 96.3 ± 2.9 vs. 104.3 ± 3.4 for MILKTHI),and by comparing cows in each quartile of the data set. There was nosignificant relationship between MILKTHI and rectal temperature.Cows with CCRTHI &gt;100 had lower rectal temperature (P &lt; 0.0001)than cows with CCRTHI &lt;100 (38.70 ± 0.01 vs. 38.62 ± 0.01°C). Similar results were obtained when evaluating CCRTHI quartiles. Rectaltemperature was 38.67 ± 0.06°C (PTA &lt;96.49), 38.63 ± 0.06°C (PTA&gt;96.49 and &lt;100.33), 38.63 ± 0.06°C (PTA &gt;100.33 and &lt;103.92) and38.60 ± 0.06°C (PTA &gt;103.92; P = 0.044). The percentage of cows withhigh rectal temperature (&gt;39°C) was also lower (P = 0.0062) in cowsin quartile 4 (17.9%) than in quartile 1 (22.6%). Results demonstratethat CCRTHI is related to body temperature during heat stress. Thefact that CCRTHI but not MILKTHI was related to rectal temperatureis indicative to the more direct relationship between body temperatureand fertility than body temperature and milk yield.<br>