Development of a heat load index for grazing dairy cattle

ABSTRACT

Many indices have been developed to predict heat stress in dairy cattle, but no indices exist specifically for extensively grazed dairy cattle where wind and solar radiation modify heat stress responses. Using a large database of respiration rates and matching weather data, we developed a Grazing Heat Load Index (HLI) to predict respiration rates in extensively grazed dairy cattle. The Grazing HLI includes ambient temperature, solar radiation, and wind speed for the hour immediately preceding the respiration rate measure and was more accurate at predicting respiration rates than existing temperature humidity indices. On average, we observed an increase of 4.21 breaths per minute per 1°C increase in ambient temperature, an increase of 5.89 breaths per minute per 1 megajoule/m² increase in solar radiation and a reduction of 1.70 breaths per minute per 1 km/hr increase in wind speed. We observed marked changes in panting score and percentage of drooling animals at Grazing HLI of greater than 70. This maybe a threshold when dairy cattle welfare is seriously compromised due to heat, however, noting that respiration rate starts to increase more steeply before this threshold.

KEYWORDS:

• Heat stress
• index
• dairy cattle
• respiration rate
• temperature humidity
• heat load

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Correction

Acknowledgements

This work was conducted through the research programme ‘The New Zealand bioeconomy in the digital age’.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article was originally published with errors, which have now been corrected in the online version. Please see Correction (https://doi.org/10.1080/00288233.2024.2310802)

Additional information

Funding

This work was supported by AgResearch Strategic Science Investment Fund and funded by New Zealand dairy farmers through DairyNZ Inc.