Biological responses of feedlot cattle to heat load

Abstract

Heat load is a significant animal welfare and cost of production issue worldwide. In the US alone heat load is reported to have an annual economic burden of g $300 million in the beef sector. Furthermore animal growth is often depressed during summer resulting in heat related decreases in weight gain of approximately 10 kg which coincides with a 7 day increase in days on feed. The reduced growth rate increases days on feed, thereby increasing the cost of production. Whilst the effects of heat load on cattle has been researched for a number of years, there is speculation as to the interactions between hot environmental conditions and livestock performance, reproduction, health and overall wellbeing. Therefore there is a need to develop a more comprehensive understanding of the dynamic responses of animals to heat load. The key focus of heat load research is to develop effective management strategies to support animal comfort and performance during hot periods. While heat load can occur in pasture raised cattle, it is mostly observed within the intensive grain feed feedlot industry. Heat load occurs where a combination of environmental conditions exceed the animals ability to regulate body temperature, thus impacting on homeostasis. However how an animal responds to heat load is also dependent on a number of individual characteristics, including genotype, coat characteristics, health status and days on feed. Therefore no two animals will respond to hot climatic conditions in exactly the same manner. There are numerous responses to heat load that can be measured and/or observed in cattle, including changes in behaviour, respiratory dynamics, blood metabolites and body temperature. The experiments within this thesis were focused on investigating the; i) Effectiveness of new technologies in determining body temperature: namely rumen temperature and infrared thermography ii) Influence of genotype and shade availability on the regulation of rumen temperature, behavioural and haematological responses of feedlot cattle From the experiments conducted the key findings were; i) Rumen temperatures are variable and appear to trend with increasing and decreasing ambient conditions (specifically ambient temperature). Small differences between rectal temperatures and rumen temperatures were observed. Additionally these results indicate that breed, ambient conditions and availability of shade influence rumen temperature, indicating that rumen temperature can be used to assess an animalrs thermal status. Overall the data suggest that rumen temperature has the potential to become a functional predictor of body temperature, and that it is possible that rumen temperature can be used as a proxy of core body temperature in feedlot cattle. ii) Infrared thermography does not appear to be a functional estimate of core body temperature as the results suggest that there was little relationship between the body surface temperature and rumen temperature. However there is the potential that the measurement of body surface temperature can be used to determine the heat flow from the animal, potentially providing an opportunity to further develop knowledge regarding thermal exchange. iii) Behavioural observations indicate that; a. Feedlot cattle appear to be consuming small portions of feed at regular intervals; b. Angus steers had the highest increase (61.3 %) shade utilisation when HLI increased from cool (HLI l 77) to very hot (HLI g 86), followed by Charolais (28.1 %) and Brahman (15.4 %) steers, further highlighting the importance of providing shade structures to feedlot cattle; c. All breed t treatment groups exhibited a notable increase in panting score as heat load increase, where HLI conditions were very hot (HLI g 86) the mean panting score of all breed t treatment groups differed (P l 0.05). iv) Haematological values obtained from feedlot cattle during summer are perplexing. a. There is large variability in the effect of heat load across studies. However elevated cytokine interleukin 6, glucose and insulin concentrations appear to be indicative of insulin resistance. b. What appears to be clear is that haematological parameters are closely interrelated and altering concentrations during exposure to stressors ensures animal survival. A key aspect in managing heat load is that feedlot personnel are able to recognise the responses of cattle to high heat load. Improvements in animal management have contributed to alleviating some of the negative effects of heat load. However summer conditions are still responsible for significant production losses and welfare concerns worldwide. Furthermore heat load cannot be completely eradicated where there are animal production operations in tropical and sub-tropical regions. Therefore the primary purpose of heat load research becomes focused on the effective use of mitigation strategies prior to and during heat related stress events, thus improving animal survivability and welfare during these events.