A new heat load index for feedlot cattle

¹ The University of Queensland, Gatton, Queensland, Australia, 4343
² Haskell Agricultural Laboratory, University of Nebraska-Lincoln, 57905 866 Rd. Concord, Nebraska, USA 68728
³ Hubbard Feeds Inc. PO Box 8500, Mankato, Minnesota, USA 56002-8500

^* To whom correspondence should be addressed. E-mail: j.gaughan{at}uq.edu.au.

	Abstract

The ability to predict effects of extreme climatic variables on livestock is important in terms of welfare and performance. An index combining temperature and humidity (THI) has been used for over 4 decades to assess heat stress in cattle. However, the THI does not include important climatic variables such as solar load and wind speed (WS; m/s). Likewise, it does not include management factors (the effect of shade) or animal factors (genotype differences). Over 8 summers a total of 11,669 Bos taurus steers, 2,344 Bos taurus crossbred steers, 2,142 Bos taurus x Bos indicus steers, and 1,595 Bos indicus steers were used to develop and test a heat load index for feedlot cattle. A new heat load index (HLI) incorporating black globe temperature (BG; °C), relative humidity (RH; decimal form) and WS has been initially developed using panting score (PS) of 2,490 Angus steers. The HLI consists of 2 parts based on a BG temperature threshold of 25 °C: HLI_BG>25 = 8.62 + (0.38 x RH) + (1.55 x BG) - (0.5 x WS) + [e^{(2.4 - WS)}], and HLI_BG<25 = 10.66 + (0.28 x RH) + (1.3 x BG) - WS. Where e = the base of the natural logarithm. A threshold HLI above which cattle of different genotypes gain body heat was developed for 7 genotypes. The threshold for unshaded black Bos taurus steers is 86 and for unshaded Bos indicus (100%) the threshold is 96. Threshold adjustments were developed for factors such as coat color, health status, access to shade, drinking water temperature, and manure management. Upward and downward adjustment are possible; upward adjustments occur where cattle have access to shade (+3 to +7) and downward when cattle are sick (-5). A related measure, the accumulated heat load (AHL) model also was developed following the development of the HLI. The AHL is a measure of the animals heat load balance and is determined by the duration of exposure above the threshold HLI. The THI and THI-hours (hours above a THI threshold) were compared to HLI and AHL. The relationship between tympanic temperature and the average HLI and THI for the previous 24 h were (R² = 0.67; P < 0.001) and (R² = 0.26; P < 0.001) respectively. The R² between HLI and panting score, and AHL and PS were positive (P < 0.001). The R² were 0.93 and 0.92 for HLI and AHL respectively. The R² for THI was 0.61 (P < 0.001), and for THI-hours R² = 0.37 (P < 0.001). The HLI and the AHL are successful in predicting panting score responses of different cattle genotypes during periods of high heat load.

Key Words: bioclimatic index, beef cattle, feedlot, heat stress