A new heat load index for feedlot cattle

Area of Science:

• Animal Science
• Environmental Science
• Agricultural Engineering

Background:

• Traditional temperature-humidity index (THI) for cattle heat stress lacks key variables like solar load and wind speed.
• Management and animal factors (e.g., shade, genotype) are not accounted for in the THI.
• Predicting extreme climatic effects on livestock is crucial for welfare and performance.

Purpose of the Study:

• To develop and validate a new heat load index (HLI) for feedlot cattle.
• To incorporate environmental variables (black globe temperature, humidity, wind speed) and animal factors into a heat stress model.
• To compare the predictive accuracy of the HLI and accumulated heat load (AHL) with the THI.

Main Methods:

• Developed a new HLI using black globe temperature, relative humidity, and wind speed, validated with panting scores (PS) in Angus steers.
• Established genotype-specific HLI thresholds and adjusted them for factors like shade, coat color, and health.
• Developed an accumulated heat load (AHL) model based on HLI exposure duration.

Main Results:

• The HLI demonstrated a stronger correlation with tympanic temperature (R²=0.67) than THI (R²=0.26).
• HLI and AHL showed high predictive accuracy for panting scores (R²=0.93 and 0.92, respectively) across different cattle genotypes.
• The HLI effectively predicted panting score responses during high heat load periods, outperforming THI and THI-hours.

Conclusions:

• The developed heat load index (HLI) and accumulated heat load (AHL) model provide a more accurate assessment of heat stress in feedlot cattle.
• The HLI accounts for critical environmental and animal factors, improving predictions of heat stress impacts on cattle welfare and performance.
• This new index offers a valuable tool for managing heat stress in diverse cattle populations under varying environmental conditions.