Effects of heat-stress on production in dairy cattle.

Environmentally induced periods of heat stress decrease productivity with devastating economic consequences to global animal agriculture. Heat stress can be defined as a physiological condition when the core body temperature of a given species exceeds its range specified for normal activity, which results from a total heat load (internal production and environment) exceeding the capacity for heat dissipation and this prompts physiological and behavioral responses to reduce the strain. The ability of ruminants to regulate body temperature is species- and breed-dependent. Dairy breeds are typically more sensitive to heat stress than meat breeds, and higher-producing animals are more susceptible to heat stress because they generate more metabolic heat. During heat stress, ruminants, like other homeothermic animals, increase avenues of heat loss and reduce heat production in an attempt to maintain euthermia. The immediate responses to heat load are increased respiration rates, decreased feed intake and increased water intake. Acclimatization is a process by which animals adapt to environmental conditions and engage behavioral, hormonal and metabolic changes that are characteristics of either acclimatory homeostasis or homeorhetic mechanisms used by the animals to survive in a new ‘physiological state’. For example, alterations in the hormonal profile are mainly characterized by a decline and increase in anabolic and catabolic hormones, respectively. The response to heat load and the heat-induced change in homeorhetic modifiers alters post-absorptive energy, lipid and protein metabolism, impairs liver function, causes oxidative stress, jeopardizes the immune response and decreases reproductive performance. These physiological modifications alter nutrient partitioning and may prevent heat-stressed lactating cows from recruiting glucose-sparing mechanisms (despite the reduced nutrient intake). This might explain, in large part, why decreased feed intake only accounts for a minor portion of the reduced milk yield from environmentally induced hyperthermic cows. How these metabolic changes are initiated and regulated is not known. It also remains unclear how these changes differ between short-term v. long-term heat acclimation to impact animal productivity and well-being. A better understanding of the adaptations enlisted by ruminants during heat stress is necessary to enhance the likelihood of developing strategies to simultaneously improve heat tolerance and increase productivity.

https://dspace.unitus.it/bitstream/2067/1536/1/FINAL%20ANIMAL%20repo_A77rkGLY.pdf

Metabolic and hormonal acclimation to heat stress in domesticated ruminants.

http://animal.ifas.ufl.edu/hansen/publications_pdf/docs/2004_hansen.pdf

Physiological and cellular adaptations of zebu cattle to thermal stress

This guide sets out the main facts and uncertainties regarding climate change, and helps provide Australians with policy-relevant, but not policy-prescriptive, advice and source material It is largely based on, and consistent with, the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC TAR) published in 2001 However, this guide has been substantially updated with relevant summaries of the latest international and Australian observations, scientific developments, and studies regarding the impacts of, and adaptation to climate change in Australia

While much progress in understanding the climate change issue has been made, uncertainties continue to exist about aspects of the climate change science, and regarding societal developments that will affect the extent of future climate change and societal vulnerability Some impacts of climate change are now inevitable However, more certainty and understanding is needed to guide decision-makers towards the most effective and cost-efficient means to adapt to climate changes in the near-term (next decade), and to avoid unacceptably large climate changes in the longer term (multi-decades to centuries) through emissions reductions measures

The high probability of at least some global warming, given the inertia in the climate and socioeconomic systems, means that some adaptation will be necessary This will be most efficient if the location- and activity-specific nature of the likely impacts is taken into account Considerable uncertainties about location-specific impacts can be further reduced by targeted research, while caseby- case assessments of adaptation strategies will be needed for many particular sectors and locations

Any emission reductions will progressively reduce the likelihood of impacts at the high end of the existing large range of emissions scenarios, and thus help to avoid the potentially most damaging climate change possibilities Thus, in order to establish minimum objectives for emissions reductions, attention needs to be given to the more extreme possibilities to which adaptation may not be possible These will determine critical greenhouse gas concentration thresholds that must be avoided if the objective of the United Nations Framework Convention on Climate Change is to be achieved Increased research is needed to quantify the probability and global and local consequences of these high impact scenarios

Climate change: an Australian guide to the science and potential impacts

Effect of solar radiation on dairy cattle behaviour, use of shade and body temperature in a pasture-based system

The effects of coat colours on thermal balance, behaviour and weight gain were studied. Measurements of thermal balance were made over 36 days in strong sunshine on highly contrasting coat colours of three white and three dark steers in each of two breeds, Brahman (B) and Shorthorn (S), and three light- and three medium-red steers in Brahman Hereford-Shorthorn cross breed (BX) where there was less contrast in coat colour. Each steer was measured for 30 min on six occasions randomly distributed among 6 h between 08.00 and 13.00 h, and on six occasions between 11.00 and 16.00 h, all on different days. For B, the difference between colours in mean rate of environmental heat gain at the skin was 64 ± 22·5 W/m2 and for S, 44 ± 20·5 W/m2, while the difference in mean rate of evaporative heat loss, between colours for B and S respectively was 44 ± 15·1 and 42 ± 14·3 W/m2. However, only within S was rectal temperature significantly higher in steers which were dark red and the reasons for this are discussed. Within BX the range of colour shades diminished during the experiment so thermal effects of colour were small. Colour had significant effects on growth; these effects were greatest in S where white steers gained 0·13 kg more per day than dark red ones. There were also differential effects within colour groups on activities at pasture; light steers spent more time in the sun and grazing than dark ones. In addition to colour, deep or woolly type coats negatively affected growth (P < 0·05), time spent in the sun (P < 0·05), and grazing time (P < 0·05); however, between animals within breeds the magnitude of the adverse effect of coat type on growth and behaviour was greater in dark- than light-coloured steers. These results provide evidence that coat colour is a trait which, interacting with coat type, exerts an influence on performance of cattle under heat stress.

Coat colour in cattle: effect on thermal balance, behaviour and growth, and relationship with coat type

Time spent in shade and its relationship with physiological factors of thermoregulation in three breeds of cattle

Sweating rates were analysed in relation to rectal temperatures of cattle to yield a measure of sweating response within and between animals. The measurements, performed over 36 days, were done in a natural radiant environment on six steers in each of three breeds, Brahman (B), Brahman × Hereford–Shorthorn cross-breds (BX), and Shorthorn (S). Each steer was recorded for 30 min on six occasions randomly distributed among 6 h between 08.00 and 13.00 h, and on six occasions between 11.00 and 16.00 h, all on different days. Sweating response, e.g. the linear slope of the relationship between sweat rate and rectal temperature, was greater for B (294 g/m2. h/°C) than for BX (146 g/m2.h/°C) or S (194 g/m2.h/°C) which did not differ; this helped to explain why the range and mean rectal temperature in B were lower and little affected by environmental heat. Curvilinear models of the sweating response indicated that in the environmental conditions of this study, the limit of sweating for B and BX was not reached, while for S, the sweating response approached a plateau. Between animals, the relationship of sweating response to mean rectal temperature was negative (P < 0·01) and its relationship to the time spent in the sun at pasture positive (P < 0·025). Thus this measure of sweating response was a good indicator of thermoregulatory ability of the cattle. However, the sweating response of the steers did not relate to their grazing time or growth. The reasons for this are discussed. Finally, the sweating response was found to be negatively correlated with metabolic rate between animals within breeds (P < 0·01) and this suggested that it may be difficult to combine the desirable traits of good heat adaptation and high metabolic potential in cattle.

C. R. Holmes

Papers

Coat colour in cattle: effect on thermal balance, behaviour and growth, and relationship with coat type

Time spent in shade and its relationship with physiological factors of thermoregulation in three breeds of cattle

Sweating response in cattle and its relation to rectal temperature, tolerance of sun and metabolic rate