Studies have shown that exposure of animals to a high ambient temperature environment poses serious threats to their health, performance and productivity. Above a certain threshold an animal's body temperature (Tb) appears to be driven by the hot ambient temperature (Ta). For steers challenged by heat stress, the Tb-Ta relationship shows a dramatic increase in Tb per unit change of Ta and the dynamics of the Tb-Ta relationship follow a pattern which depends on whether Ta is increasing or decreasing. A delay becomes noticeable in a steer’s thermo-regulatory response to Ta when Ta is controlled to be sinusoidal in the steer’s heat stress chamber. In other words, Tb lags behind Ta. Consequently when plotted in a Tb-Ta phase diagram, a hysteresis effect appears in the form of a hysteresis loop, indicating the steer is thermally challenged. The hysteresis loop shows a rotated elliptical pattern which depends on the delay (or lag) between Tb and Ta. The angle of rotation of the hysteresis loop indicates the correlation (and lag) between Tb and Ta. The area of the elliptical hysteresis loop can be used to quantify the amount of heat stress during the period of thermal challenge. For example, results of a thermal challenge of 32±7oC applied to a Hereford steer showed, the delay is longer (4 hr lag) and ellipse is larger in an acute stage than in a chronic stage (3 hr lag). A greater delay (or lag) suggests more time is needed to dissipate the heat stress. This result suggests, steers in an acute stage require more energy to dissipate heat than steers in a chronic stage.

Cattle production plays a significant role in terms of world food production. Nearly 82% of the world’s 1.2 billion cattle can be found in developing countries. An increasing demand for meat in developing countries has seen an increase in intensification of animal industries, and a move to cross-bred animals. Heat tolerance is considered to be one of the most important adaptive aspects for cattle, and the lack of thermally-tolerant breeds is a major constraint on cattle production in many countries. There is a need to not only identify heat tolerant breeds, but also heat tolerant animals within a non-tolerant breed. Identification of heat tolerant animals is not easy under field conditions. In this study, panting score (0 to 4.5 scale where 0 = no stress and 4.5 = extreme stress) and the heat load index (HLI) [HLIBG<25°C = 10.66 + 0.28 × rh + 1.30 × BG – WS; and, HLI BG> 25°C = 8.62 + 0.38 × rh + 1.55 × BG – 0.5 × WS + e(2.4 – WS), where BG = black globe temperature (oC), rh = relative humidity (decimal form), WS = wind speed (m/s) and e is the base of the natural logarithm] were used to assess the heat tolerance of 17 genotypes (12,757 steers) within 13 Australian feedlots over three summers. The cattle were assessed under natural climatic conditions in which HLI ranged from thermonuetral (HLI < 70) to extreme (HLI > 96; black globe temperature = 40.2°C, relative humidity = 64%, wind speed = 1.58 m/s). When HLI > 96 a greater number (P < 0.001) of pure bred Bos taurus and crosses of Bos taurus cattle had a panting score ≥ 2 compared to Brahman cattle, and Brahman-cross cattle. The heat tolerance of the assessed breeds was verified using panting scores and the HLI. Heat tolerance of cattle can be assessed under field conditions by using panting score and HLI.

Angus crossbred yearling steers ( n = 168) were used to evaluate effects on performance and tympanic temperature (TT) of feeding additional potassium and sodium to steers exposed to excessive heat load (maximum daily ambient temperature exceeded 32°C for three consecutive days) during seasonal summer conditions. Steers were assigned one of four treatments: (1) control; (2) potassium supplemented (diet containing 2.10% KHCO 3 ); (3) sodium supplemented (diet containing 1.10% NaCl); or (4) potassium and sodium supplemented (diet containing 2.10% KHCO 3 and 1.10% NaCl). Overall, additional KHCO 3 at the 2% level or NaCl at the 1% level did not improve performance or heat stress tolerance with these diet formulations. However, the addition of KHCO 3 did enhance water intake. Independent of treatment effects, TT of cattle displaying high, moderate, or low levels of stress suggest that cattle that do not adequately cool down at night are prone to achieving greater body temperatures during a subsequent hot day. Cattle that are prone to get hot but can cool at night can keep average tympanic temperatures at or near those of cattle that tend to consistently maintain lower peak and mean body temperatures. In addition, during cooler and moderately hot periods, cattle change TT in a stair-step or incremental pattern, while under hot conditions, average TT of group-fed cattle moves in conjunction with ambient conditions, indicating that thermoregulatory mechanisms are at or near maximum physiological capacity.

The adaptive capabilities of animals and livestock production systems have been emphasized in this report. Biometeorology has a key role in rational management to meet the challenges of thermal environments. While the focus is primarily on cattle in warm or hot climates, the importance of dynamic animal responses to environmental challenges applies to all species and climates. Methods used to mitigate environmental challenges focus on heat loss/heat production balance. Under cold stress, reduction of heat loss is the key. Under heat stress, reduction of heat load or increasing heat loss are the primary management tools, although heat-tolerant animals are also available. In general, livestock with health problems and the most productive animals (e.g., highest growth rate or milk production) are at greatest risk of heat stress, thereby requiring the most attention. Risk management, by considering perceived thermal challenges, then assessing the potential consequences and acting accordingly, will reduce the impact of such challenges. Appropriate actions include: shade, sprinkling, air movement, or active cooling. Summarizing, the most important element of proactive environmental management to reduce risk is preparation: be informed, develop a strategic plan, observe and recognize animals in distress, and take appropriate tactical action.

The thermal environment plays a large role in an animal’s ability to convert feed into weight gain. A better understanding of a pig’s metabolism will help swine producers select environmental specifications for optimizing feed conversion. The objectives of this study are to 1) characterize the thermoregulatory responses of pigs during a feeding event 2) compare those responses for three thermal environmental treatments applied in a Latin Square design 3) investigate different procedures for fitting nonlinear mixed-effect models with crossed random effects (NLME function in R, %NLINMIX macro in SAS, random effects modeling in AD Model Builder: ADMB-RE). We found that the threeparameter first-order compartment model provides a reasonable representation of the tympanic temperatures of feeding pigs during feeding events. The thermal environmental treatments (28ºC + High air speed) and (18ºC + Low air speed) are significantly different from the reference treatment (28ºC + Low air speed), at the 5% level. Both NLME and ADMB-RE successfully fit the nonlinear mixed-effects model and produce similar results. The %NLINMIX macro did not converge unless restrictions were placed on the model. The estimates of fixed and random effects from the restricted model using %NLINMIX macro were generally different from those from NLME and ADMB-RE.

Heat stress in feedlot cattle can cause decreases in feed intake and growth, and in extreme cases may result in death. Providing shade during hot weather has shown inconsistent results, reducing direct and indirect losses in some areas of the United States, but not in others. The objectives of this study were to evaluate the dynamic responses of feedlot cattle to environmental conditions with and without access to shade, and to determine the most appropriate physiological measurement for monitoring feedlot cattle during hot weather as a guide for improved management. Eight crossbred steers (initially weighing 294.7 +/- 10.8 kg) were randomly assigned to one of eight individual pens, where one of two treatments were applied: shade access, or no-shade access. Respiration rate, daily feed intake, and core body temperature were collected, using automated systems during eight periods, for a total of 37 days. The data were analysed using four categories of daily maximum temperature humidity index (maximum I-TH) values (Normal for maximum I-TH < 74; alert for 74 <= maximum I-TH < 78; Danger for 78 < maximum I-TH < 84; Emergency for maximum I-TH >= 84). Shade was found to impact the physiological responses in all ITH categories, with the largest impacts in the Danger and Emergency categories. Shade lowered respiration rate and core body temperature during the peak temperature hours of the day. It was concluded that respiration rate is the most appropriate indicator of thermal stress to monitor because it was consistently affected in all ITH categories, it is easy to monitor without the need for costly equipment, and there is little or no lag associated with it. Published by Elsevier Ltd on behalf of Silsoe Research Institute.

Summer heat provides stressful conditions for Bos taurus feeder cattle; in extreme instances these conditions can be fatal. One management option is to provide shade structures for feedlot animals. This study was conducted during the summer of 2001 to compare physiological responses of cattle with shade access or no-shade access. Eight steers were selected from a group of 12, and assigned individual pens. Four pens were fitted with shade structures that allowed the steers to choose shade; the remainder had no shade option. The animals were rotated through pen assignments during the summer season. Continuous measures of respiration rate and body temperature were recorded as response variables to the shade treatments. Environmental conditions were monitored for the experimental period. Daytime means and standard errors were 86.0 +/- 0.39 breaths min(-1) for respiration rate shade, and were significantly lower (probability P < 0.05) than respiration rate No-shade of 102-3 +/- 0.36 breaths min(-1). Linear regression fit for daytime data showed the slope for No-shade to be 4.5 +/- 0.15 breaths min(-1) and 1.5 +/- 0.11 breaths min(-1) for Shade. Thresholds for humidity and treatment of Shade and No-shade were determined to exist between 25 and 30 degrees C. Linear regression equations were developed for respiration rate including effects of temperature, humidity, wind speed, and solar radiation for animals in either Shade or No-shade feedlot pens. Published by Elsevier Ltd on behalf of Silsoe Research Institute.

Heat stress in feedlot cattle causes reduced performance, and in the most severe cases, death of the animals, thus causing the loss of millions of dollars in revenue to the cattle industry. A study was designed to evaluate dynamics of thermoregulation and feeding activities when feeder cattle were exposed to simulated heat waves, in comparison with repeated sinusoidal hot and thermoneutral environments. Nine beef steers were randomly assigned to an individual pen in one of three environmental chambers. Each chamber was subjected to each of three temperature regimes (Heatwave simulation from Rockport, Mo., 1995, Heatwave simulation from Columbia, Mo., 1999, and Controlled heat stress treatment of 32±7°C) for a period of 18 days, according to a Latin square treatment design, with a 10-day thermoneutral period (18±7°C) separating treatment periods. Respiration rate, core body temperature, heat production, feed intake, and feeding behavior were measured on each animal for the duration of the experiment. Differences were found in all treatments for all parameters except feeding behavior. It was shown that the two simulated heat waves elicited very different thermoregulatory responses. Based on these results the heat wave centered at Rockport, Mo. in 1995 was devastating because the animals were not acclimated to hot conditions, thus causing an acute response to heat stress. The responses of cattle to conditions at Columbia, Mo. showed some acclimation to heat prior to the peak stress days, and therefore a dampened response was seen. It appears the extreme conditions at Columbia, Mo., 1999 were made severe by environmental conditions not simulated during this study (low wind speed and intensive solar radiation). Overall, it was determined while a cyclic heat stress treatment is a representative model to test heat stress in cattle, further heat stress experiments should be conducted in an actual feedlot.

The nonlinear PET model based on Newton's law of cooling can be used to estimate body temperature in cattle, T b challenged by hot cyclic chamber temperatures, T a . The PET model has four biologically meaningful parameters: K, the thermal constant; Δ, the difference between T b and adjusted T a ; Υ the proportion of variation in T b comparable to variation in Ta ; T bini, the initial body temperature. The two parameters Y and Δ are highly correlated in the current version of the model. This study looks at other ways to parameterize the PET model in an effort to reduce the correlation between parameters and improve nonlinear behaviors, such as parameter-effects curvature, bias, excess variance and skewness.

Heat exchanges with the environment are a crucial process for maintaining homeothermy by humans and other animals. These exchanges involve heat production, conservation and dissipation, and are dependent on both biological and physical factors. The complexity of these exchanges have led to many attempts to represent the environmental aspects by surrogate thermal indices as a basis for assessing the biological effect and consequent impact of the thermal environment. Resultant index values represent effects produced by the heat exchange process. For humans, comfort assessment is primary; for animals, assessing performance, health and well-being have been foremost. This dichotomy of approaches is discussed from the perspectives of past and current indices, and considerations for future efforts. Emphasis is on thermal indices useful in animal studies and applications, with a view toward strategic and tactical decisions for rational environmental management. Numerous illustrative examples are included.

Heat stress reduces performance and, in the most severe cases, death of feedlot cattle resulting in major economic losses. A study was designed to evaluate the dynamics of thermoregulation and feed intake of feeder cattle when exposed to either a simulated heat waves or a standard cyclic environment. Nine crossbred steers were randomly assigned to individual pens in one of three environmental chambers. Three temperature regimes (heat wave simulation from Rockport, MO 1995; heat wave simulation from Columbia, MO 1999; and Cyclic heat stress treatment of 32+/-7degreesC) were administered to each of three chambers for a period of 18 days according to a Latin square treatment design, with a 10-day thermoneutral period separating treatment periods (18+/-7degreesC). Respiration rate (RR), core body temperature (T-core), heat production (HP), and feed intake (FI) were measured on each animal for the duration of the experiment. From this study, it appears that RR and FI, observable parameters, give an accurate insight into the thermoregulatory status of the animal during heat challenges.

SUMMARY The implications of high heat load (HHL) on feedlot cattle can be costly. Its impact is felt economically and consumer perceptions of the beef industry can be tainted. Animal assessments such as dry matter intake, body temperature, respiration rate and behaviour can be useful in providing information on how well cattle are coping. However, used in conjunction with microclimatic factors such as ambient temperature and humidity it has the potential to be a useful management tool from which informed decisions can be made to alleviate the effects of HHL.

Researchers studying acclimation of cattle to heat stress want to know if exposure to heat stress in controlled chambers will help cattle adjust to climatic conditions in the field. The four parameter nonlinear PET model is used to study the relationship between core body temperature and ambient temperature. This model works well when cattle are challenged by heat stress but the model is less useful for thermoneutral conditions. Both proc Nlin and Nlmixed are used to compare and contrast the field parameters between the controlled and the potentially acclimated group. Simulation studies were used to compare the effectiveness of proc Nlin versus proc Nlmixed. The results are helpful, not only for researchers who study acclimation, but also for those who study sensitivity, tolerance and robustness of cattle during heat stress.

Studies of the relationship between animal body temperature and air temperature suggest body temperature is essentially unresponsive until a threshold is reached, then it responds dramatically to increasing air temperature. The goal is to estimate the threshold between the thermoneutral plateau and the beginning of the heat stress challenge. One approach is to fit a polynomial to estimate the knot position and use spline functions to perform linear least squares piecewise polynomial fitting. Another alternative is to use nonlinear regression to estimate the knot or an inflection point of a nonlinear function. In both approaches the cyclic nature of body temperature is ignored. This paper explores the use of nonlinear regression to estimate the knot position and handles the hysteresis effect resulting from the cyclic nature of body temperature. Models are fit to data collected from cattle in chambers subjected to semicontrolled sinusoidal air temperature at the University of Missouri-Columbia Animal Science department and a procedure for estimating the heat stress threshold is proposed.

Research on the effects of thermal environments requires monitoring and control based on temperature and humidity measurements. There are many approaches to both temperature and humidity measurements, but few offer high reliability in hostile environments with acceptable accuracy and relatively low cost. The Biological Engineering Research Unit at the U.S. Meat Animal Research Center (USMARC) evaluated two separate units that were developed with commercially available sensors for meeting the cost/performance criteria: 1) a linear temperature sensor designed around a National Semiconductor LM35CA, and 2) an Ohmic Instruments Company ABS 300 sensor This article details sensor calibration and performance.

A study was designed to investigate the thermoregulatory responses of feeder cattle to both acute and chronic exposures to elevated environmental temperatures. Rectal temperatures (RT) and respiration rate (RR) showed significant differences between temperature treatments. Both RT and RR had a diurnal pattern, which followed the diurnal pattern of the ambient conditions with some lag. Heat production at thermoneutral conditions was significantly higher than at the heat stress treatments. Heat production and respiratory quotient were the only two parameters shown to change with acclimation to heat stress.