A Climatological Perspective on Feedlot Cattle Performance and Mortality Related to the Temperature-Humidity Index

Abstract

This paper explores the climatology of high temperature-humidity index (THI) values (THI ≥ 84) in the U.S. Central Plains. The THI is derived from the dry bulb and the dewpoint temperatures. High THI values are associated with increased stress among feedlot cattle, which leads to a decrease in the rate of weight gain. When high THI values are sustained for several days, the result can be an increase in feedlot cattle mortality. We found that high THI values have a general southeast to northwest gradient across the region. We also introduce the concept of a THI-hour as a measure of the magnitude of a heat stress event. The THI values for a major heat-wave in 1995 are presented as well as a climatology of recovery hours (THI ≤ 74) during a heat-wave. The length of the recovery time during a heat-wave can mean the difference between life and death for feedlot cattle. Research Question Heat waves often result in thermal conditions of an intensity and duration that tax the ability of feedlot cattle to cope. Thermoregulation and feeding behavior of cattle are strongly affected. Heat waves occurring before cattle become acclimated to summer weather can lead to decreased performance, and impaired health and well being, or even heatstroke and death of vulnerable animals. Literature Summary There is an impact of heat waves on thermoregulation, feeding behavior, and death of ab-lib-fed feedlot cattle. The July 1995 heat wave in the central USA illustrates (i) the characteristics of heat waves as assessed by thermal indices; (ii) the dynamics of thermoregulatory and feeding behavior responses of cattle during exposure to hot environments; and (iii) thermal conditions associated with cattle deaths. The heat waves can be characterized in terms of their intensity and frequency of occurrence by use of climatological data. These derived historical or climatological probabilities of heat waves are the best indication of the future frequency of occurence that planners require for use in climatic risk analysis associated with feedlot siting and environmental management strategies and tactics to limit animal stress, and subsequent morbidity and mortality. Study Description This study develops a climatology in the Central Plains for: (i) the total events with temperature humidity index (THI) ≥ 84; (ii) the duration of events; (iii) the magnitude of events (hours with THI ≥ 84); and (iv) the degree of recovery during an expected hot period (the number of hours with THI ≤ 74). Applied Questions Which locations in the Central Plains average the greatest number of days with critical THI (THI ≥ 84)? Our research indicates that there is a northwest-southeast gradient of the average total THI ≥ 84 d/yr across the central plains. Which locations in the Central Plains have the longest duration of critical THI events? The duration of high THI days and the probability of a run of high THI days also follows the same northwest-southeast gradient. The magnitude of high THI events is primarily along a west-east gradient. Which locations in the Central Plains have the greatest number of recovery hours during critical THI events? Recovery hours (THI ≤ 74) during a high THI event are least in northeast Kansas, southeast Nebraska, northwest Missouri, and the High Plains of Nebraska and Colorado. Recommendations To minimize the deleterious effects of high temperature and humidity on feedlot cattle operations in the central plains, we recommend that feedlots should be located in the High Plains of Colorado, Kansas, or Nebraska. Nonetheless, the location of feedlots involves more than reducing the impact of high temperature and humidity. Other environmental factors that also must be considered include winter impacts, dust, and ground and surface water protection issues. Such logistical factors as the availability of cattle, feed and markets should also be considered.