Degree Of Heat Stress Research Articles

Heat stress affects mammary metabolism by influencing the plasma flow to the glands

BackgroundEnvironmental heat stress (HS) can have detrimental effects on milk production by compromising the mammary function. Mammary plasma flow (MPF) plays a crucial role in nutrient supply and uptake in the mammary gland. In this experiment, we investigated the physiological and metabolic changes in high-yielding cows exposed to different degrees of HS: no HS with thermal-humidity index (THI) below 68 (No-HS), mild HS (Mild-HS, 68 ≤ THI ≤ 79), and moderate HS (Mod-HS, 79 < THI ≤ 88) in their natural environment. Our study focused on the changes in blood oxygen supply and mammary glucose uptake and utilization.ResultsCompared with No-HS, the MPF of dairy cows was greater (P < 0.01) under Mild-HS, but was lower (P < 0.01) in cows under Mod-HS. Oxygen supply and consumption exhibited similar changes to the MPF under different HS, with no difference in ratio of oxygen consumption to supply (P = 0.46). The mammary arterio-vein differences in glucose concentration were lower (P < 0.05) under Mild- and Mod-HS than under no HS. Glucose supply and flow were significantly increased (P < 0.01) under Mild-HS but significantly decreased (P < 0.01) under Mod-HS compared to No-HS. Glucose uptake (P < 0.01) and clearance rates (P < 0.01) were significantly reduced under Mod-HS compared to those under No-HS and Mild-HS. Under Mild-HS, there was a significant decrease (P < 0.01) in the ratio of lactose yield to mammary glucose supply compared to that under No-HS and Mod-HS, with no difference (P = 0.53) in the ratio of lactose yield to uptaken glucose among different HS situations.ConclusionsDegrees of HS exert different influences on mammary metabolism, mainly by altering MPF in dairy cows. The output from this study may help us to develop strategies to mitigate the impact of different degrees of HS on milk production.

Thermal characterization and ventilation assessment of a battery-caged laying hen housing in the humid tropic climate.

The indoor climate to which livestock are exposed is a critical factor influencing their performance and productivity. Elevated air temperature and relative humidity could result in heat stress for laying hens. This situation results in severe adverse effects such as weight loss and mortality. Egg fertility and hatchability are also impacted. Consequently, a study was carried out in a naturally ventilated battery-caged laying hen house to measure climatic variables (air temperature, relative humidity and air velocity). The degree of heat stress was assessed using the temperature-humidity index (THI), and the index of temperature and air velocity (ITV) was also evaluated. According to the results obtained, birds reared within the study building would spend most of their productive life under stressful thermal conditions, which could significantly hamper their performance. The air velocity was below 1.0 ms-1 for most of the internal part of the housing, meaning natural air movement at the location was insufficient to provide a suitable environment for the birds. A high THI was recorded for nearly the entire study period. This high THI could indicate high relative humidity about air temperature. The observed ITV values (ITV > 25) suggest that birds throughout the building could be perpetually uncomfortable. The thermal and velocity profile within the structure could further be assessed numerically using computational fluid dynamics. This would enable engineers to make modifications to improve living conditions within the building.

Leaf photosynthetic characteristics of waxy maize in response to different degrees of heat stress during grain filling

BackgroundIn the context of climate change, maize is facing unprecedented heat stress (HS) threats during grain filling. Understanding how HS affects yield is the key to reducing the impact of climate change on maize production. Suyunuo5 (SYN5) and Yunuo7 (YN7) were used as materials, and four temperature gradients of 28℃ (day)/20℃ (night; T0, control), 32 °C/24°C (T1, mild HS), 36 °C/28°C (T2, moderate HS), and 40 °C/32°C (T3, severe HS) were set up during grain filling to explore the physiological mechanism of different degrees HS affecting photosynthetic characteristics of leaves in this study.ResultsResults showed that HS accelerated the degradation of chlorophyll, disturbed the metabolism of reactive oxygen species, reduced the activity of antioxidant enzymes, and caused leaf damage. Heat stress induced the down-regulation of photosynthesis-related genes, which results in the decrease of enzymatic activities involved in photosynthesis, thereby inhibiting photosynthesis and reducing yield. Integrated analysis showed that the degree of the negative influence of three HS types during grain filling on leaves and yield was T3 > T2 > T1. The increase in HS disturbed leaf physiological activities and grain filling. Meanwhile, this study observed that the YN7 was more heat tolerance than SYN5 and thus it was recommended to use YN7 in waxy maize planting areas with frequent high temperatures.ConclusionsHeat stress during grain filling caused premature senescence of the leaves by inhibiting the ability of leaves to photosynthesize and accelerating the oxidative damage of cells, thereby affecting the waxy maize yield. Our study helped to simulate the productivity of waxy maize under high temperatures and provided assistance for a stable yield of waxy maize under future climate warming.

Alleviating heat stress effects in poultry: updates on methods and mechanisms of actions.

Heat stress is a threat that can lead to significant financial losses in the production of poultry in the world's tropical and arid regions. The degree of heat stress (mild, moderate, severe) experienced by poultry depends mainly on thermal radiation, humidity, the animal's thermoregulatory ability, metabolic rate, age, intensity, and duration of the heat stress. Contemporary commercial broiler chickens have a rapid metabolism, which makes them produce higher heat and be prone to heat stress. The negative effect of heat stress on poultry birds' physiology, health, production, welfare, and behaviors are reviewed in detail in this work. The appropriate mitigation strategies for heat stress in poultry are equally explored in this review. Interestingly, each of these strategies finds its applicability at different stages of a poultry's lifecycle. For instance, gene mapping prior to breeding and genetic selection during breeding are promising tools for developing heat-resistant breeds. Thermal conditioning during embryonic development or early life enhances the ability of birds to tolerate heat during their adult life. Nutritional management such as dietary manipulations, nighttime feeding, and wet feeding often, applied with timely and effective correction of environmental conditions have been proven to ameliorate the effect of heat stress in chicks and adult birds. As long as the climatic crises persist, heat stress may continue to require considerable attention; thus, it is imperative to explore the current happenings and pay attention to the future trajectory of heat stress effects on poultry production.

Lower air humidity reduced both the plant growth and activities of photosystems I and II under prolonged heat stress

The warming is global problem. In natural environments, heat stress is usually accompanied by drought. Under drought conditions, water content decreases in both soil and air; yet,the effect of lower air humidity remains obscure. We supplied maize and barley plants with an unlimited source of water for the root uptake and studied the effect of relative air humidity under heat stress. Young plants were subjected for 48 h to several degrees of heat stress: moderate (37 °C), genuine (42 °C), and nearly lethal (46 °C). The conditions of lower air humidity decreased the photochemical activities of photosystem I and photosystem II. The small effect was revealed in the control (24 °C). Elevating temperature to 37 °C and 42 °C increased the relative activities of both photosystems; the photosystem II was activated more. Probably, this is why the effect of air humidity disappeared at 37 °C; the small inhibiting effect was observed at 42 °C. At 46 °C, lower air humidity substantially magnified the inhibitory effect of heat. As a result, the maximal and relative activities of both photosystems decreased in maize and barley; the photosystem II was inhibited more. Under the conditions of 46 °C at lower air humidity, the plant growth was greatly reduced. Maize plants increased water uptake by roots and survived; barley plants were unable to increase water uptake and died. Therefore, air humidity is an important component of environmental heat stress influencing activities of photosystem I and photosystem II and thereby plant growth and viability under severe stress conditions.

Delineation of temperature-humidity index (THI) as indicator of heat stress in riverine buffaloes (Bubalus bubalis) of a sub-tropical Indian region.

The erstwhile developed temperature-humidity index (THI) has been popularly used to indicate heat stress in dairy cattle and often in buffaloes. However, scientific literature suggests differences in thermotolerance and physiological responses to heat stress between cattle and buffalo. Therefore, THI range used to indicate degree of heat stress (mild, moderate, and severe) in cattle should be recalibrated for indicating heat stress in buffaloes. The present study was carried out to delineate THI range to indicate onset and severity of heat stress in buffaloes based on physiological, biochemical, and expression profiling of heat shock response (HSR) genes in animals at different THI. The result indicated early onset of heat stress in buffaloes as compared to cattle. Physiological and biochemical parameters indicated onset of mild signs of heat stress in buffaloes at THI 68-69. Significant deviation in these parameters was again observed at THI range 73-76. At THI 77-80, the physiological and biochemical responses of animals were further intensified indicating extreme alteration in homeostasis. The in vivo expression profiling of HSR genes indicated that members of Hsp70 gene family are expressed in a temporal pattern over different THIs, whereas expressions of Hsf genes were evident during intense heat stress. Overall, the study established that amplitude of heat shock response and THI range for indicating severity of thermal stress for buffaloes are not in unison to cattle. The study also suggests skin temperature of the poll region could be used as non-invasive tool for monitoring heat stress in dairy buffaloes.

Study of the relationship between the spatial structure and thermal comfort of a pure forest with four distinct seasons at the microscale level

The local thermal environment of forest green space has a great influence on users' experience and health. Thus, it is of great value to systematically study the relationship between forest spatial structure and thermal comfort at the microscale. This study examined the relationship between the spatial structure and thermal comfort of bamboo forest using a case study of "Zhu Hai Dong Tian" Park in Dujiangyan city, Sichuan Province, China. We selected 20 in-forest study sites and three out-of-forest control sites and measured 13 spatial structure parameters and five climatic parameters (air temperature, relative humidity, wind velocity, surface temperature and global radiation) at the 23 sites for three representative days in each season. Then, the five climate parameters, human body parameters (sex, age, height, weight, clothing, and activity), and sky view factor (SVF) were input into Rayman software to calculate the physiological equivalent temperature (PET) value of each site in each season. PET is the physiological temperature value of the equivalent thermal environment based on energy balance. In this study, PET values between 15 and 22 °C indicated that the thermal environment was comfortable. The results showed that the PET values of the 20 study sites in the bamboo forest were mostly at a comfortable level in spring and autumn but had various degrees of heat stress and cold stress in summer and winter, respectively. The relationship between microstructure parameters and PET in four seasons was expressed by four models. In the spring and summer models, CPR (cover plant ratio) and CDW (canopy diameter width) were positively correlated with PET and had the greatest influence. In the autumn and winter models, DFS (degree of facing the sun's trajectory vertically) and CPR were positively correlated with PET and had the greatest influence. In addition, DBH (diameter at breast height), DB (density of bamboo forest), S (slope), CPH (cover plant height), ST (slope of the trunk), SP (spacing), H (height of the space), and IL (internodal length) were included in the models of all four seasons. The research results provide useful information for improving the thermal comfort of microscale pure forest space by modifying the forest structure. The results can be used to guide forest management to further create forest space that integrates climate comfort, landscape, and service.

A computer model for the assessment of employee performance loss as a function of thermal discomfort or degree of heat stress

This article presents an overview of different research and researchers’ attempts to derive a mathematical relationship between the performance loss and the thermal (dis)comfort of working people, expressed in the mean temperature sensation. The goal of this article and research is to present a single computer model and manageable design tool for a variety of disciplines, that is, mechanical engineering, building services and facility management, disciplines that participate in the design of the indoor environment of new or existing buildings and the improvement of working conditions in the workplace. The single computer model proposed in this article is assembled using a validated mathematical human thermophysiological model and a validated mathematical Wet Bulb Globe Temperature Index model, in combination with various mathematical performance (loss) models, based on comfort indices and heat stress indices. Although not perfect, the model is especially useful in the situation of comparative studies. For instance, in the early stages of the design process, the computer model assists in making better/the right (design) decisions regarding the thermal aspects of the indoor environment, whether or not in combination with validated building simulation models. The use of a dynamic thermophysiological model, in combination with the aforementioned models, enables the evaluation of the thermal influence of every architectural and building service and related adjustments for people and organizations. The use of a combination of models, as shown above, makes it possible to negotiate solutions and better balance investments with regard for profits and workable hours.

Effects of temperature-humidity index and chromium supplementation on antioxidant capacity, heat shock protein 72, and cytokine responses of lactating cows.

Heat stress adversely affects the productivity and immune status of dairy cows. The temperature-humidity index (THI) is commonly used to indicate the degree of heat stress on dairy cattle. We investigated the effects of different THI and Cr supplementation on the antioxidant capacity, the levels of heat shock protein 72 (Hsp72), and cytokine responses of lactating cows. The study used a total of 24 clinically healthy uniparous midlactation Holstein cows, which were randomly divided into 2 groups (n = 12 per group), and was conducted in 3 designated THI periods: low THI period (LTHI; THI = 56.4 ± 2.5), moderate THI period (MTHI; THI = 73.9 ± 1.7), and high THI period (HTHI; THI = 80.3 ± 1.0). The 2 groups of cows were fed corn and corn silage based basal diet supplemented chromium picolinate to provide 3.5 mg of Cr/cow daily (Cr+) or basal diet with no Cr (Cr-). The experiment was a 3 × 2 factorial design. The numbers of leukocytes (P < 0.05) and serum levels of glucose (P < 0.001) were lower; however, the serum levels of blood urea nitrogen (BUN; P < 0.001) and creatinine (P < 0.001) were greater in the MTHI and HTHI than in LTHI. The total antioxidant capacity in the serum was unaltered; an increase in superoxide dismutase activity (P < 0.001) and in serum malondialdehyde concentration (P < 0.001) was observed in the MTHI and HTHI compared with the LTHI. The high THI led to increases in serum concentrations of tumor necrosis factor-α (TNF-α; P < 0.001) and IL-10 (P < 0.05). Cows supplemented with Cr had lower (P = 0.009) serum concentrations of cholesterol but greater (P < 0.001, respectively) serum levels of Hsp72 and IL-10 compared with those without Cr supplementation in the HTHI. Western blot analysis revealed that cows supplemented with Cr had greater (P = 0.038) expression of the inhibitor of nuclear factor kappa B α (IκBα) in peripheral blood mononuclear cells (PBMC) compared with those without Cr supplementation in the HTHI, whereas the expression of Hsp72 in PBMC was unaltered. Data indicate that there is a decrease in glucose and increases in BUN and creatinine in the serum of midlactation cows under hot conditions during the summer and that these cows have a lowered oxidative capacity but an elevated antioxidant capacity. In addition, Cr may play an anti-inflammatory role in lactating cows by promoting the release of Hsp72, increasing the production of IL-10, and inhibiting the degradation of IκBα under hot conditions during the summer.

Application of Microclimate Modelling and Onsite Survey in Planning Practice Related to an Urban Micro-Environment

Numerical simulations of human thermal comfort conditions were carried out by means of the urban microclimate model ENVI-met in a popular children’s playground located in Szeged, Hungary. Bioclimatic conditions were quantified by the Physiologically Equivalent Temperature (PET). Based on the PET values, thermal stress maps were created in two different periods of typical summer and autumn days. The study aims to reveal the seasonal and diurnal spatial patterns of the simulated thermal conditions and thus the degree of heat stress in different parts of the playground. Furthermore, we analysed the momentary spatial distributions of the visitors triggered by the microclimatic conditions of the area. According to the simulation, remarkable differences in the thermal conditions were found depending on the sun elevation and the resulting shaded conditions as well as the radiation of the heated surfaces. The spatial distribution of the visitors seems to be highly influenced by the patterns of the thermal conditions but the location and the preference of the children’s playground equipment also affects it. In order to reveal the possible causes of the people’s behaviour, an onsite questionnaire survey was conducted on their opinions and possible modification requirements related to the design of the playground.

Study on the pathological changes of the lung and brain in mice during heat stress

Objective To prepare mouse model with heat stress and determine its pathological changes of the lung and brain during heat stress. Methods BALB/c mouse were randomly (random number) divided into two groups, control group and heat stress group. The animals in the control group were sham- heated at a temperature of ( 25 ± 0.5) ℃ and humidity of (35 ± 5 ) %. The animals of heat stress group were placed in a prewarmed incubator maintained at (35.5 ± 0.5) ℃ and relative humidity of (60 ± 5) %. Rectal temperature (Tc) was monitored, and when Tc respectively reached 39 ℃, 40 ℃ , 41 ℃ and 42 ℃, those study animals were killed. The other animals were removed from the incubator and allowed to cool at an ambient temperature of (25 ±0. 5)℃ and humidity of (35 ±5)% , respectirvely for 12 and 24 hrs when Tc reached 41 ℃ , and for 6 hrs when Tc reached 42 ℃. The lung and brain of all the animals were isolated. Hematoxylin and eosin stain and light microscope were used to detect their pathological changes. Results All the animals displayed uniform response to the heat stress. Low degree of heat stress could induced obviously pathological changes of the lung, progressively greater damage to lung with further congestion of lung matrix, asystematic hemorrhage of alveolar space, abscission of alveolar epithelial cell and disappear of pulmonary alveolus tissue structure were detected with the rise of Tc to 42 ℃. However, absorption of congestion and hemorrhage and recovery of pulmonary alveolus tissue structure could also be seen with cooling at ambient temperature. With low degree of heat stress, the brain only showed moderate edema. Neuronal denaturation and necrosis were detected when Tc reached to 42 ℃. Interestingly, the lesions of brain further aggravated even through cooling treatment after Tc reached to 42 ℃ , but recovery could been observed after cooling treatment followed with Tc of 41 ℃. Conclusions The pathological changes of the lung and brain showed distinctive lesions to heat stress and cooling treatment, and these changes were correlated with the timing and time of cooling treatment, which provide the experimental basis to further study the mechanisms between the heatstroke and multiple organ dysfunction syndrome (MODS). Key words: Heat stress; Heatstroke; Lung; Brain; Pathological change; Mice; MODS; Cooling treatment

Modelo de Thom para o zoneamento bioclimático de Mato Grosso do Sul

The objective of this study was to evaluate the climate conditions of Mato Grosso do Sul, using the index of temperature and humidity (ITU), and present the bioclimatic mapping relating to thermal comfort. The ITU values were estimated from data of temperature and relative humidity for the period 1979 to 2008. Four intervals of ITU were used to classify human performance (ITU 84: extremely hot), and two intervals to classify the Livestock (79≤ ITU 84: emergency). We conclude that the prevailing mood is one of two alternating seasons throughout the year: hot and humid summer and dry winter with warmer temperatures, the results showed a higher risk of thermal discomfort during the period from October to April, compared to the period between May and September. The most critical period occurred between December and March. Although it was observed that with ITU values between 77.9 and 83.9, humans and animals can suffer some degree of heat stress during the hottest hours of the day, negatively affecting both.

Relationship Between Accumulated Heat Stress Prepartum, Body Condition and Postpartum Performance in Dairy Cattle

Avendaño-Reyes, L., Fuquay, J.W., Moore, R.B., Liu, Z., Clark, B.L. and Vierhout, C. 2009. Relationship between accumulated heat stress prepartum, body condition and postpartum performance in dairy cattle. J. Appl. Anim. Res., 35: 33–38. To estimate the relationship between heat stress during the last 60 d prepartum, body condition score and certain productive traits in the subsequent lactation of Holstein cows, 564 multiparous and 290 primiparous cows from four dairy herds in Mississippi state were used. Maximum temperature-humidity index (MTHI) was calculated to quantify the degree of heat stress and was included in multiple regression models. Animals were scored for body condition at dry off, at calving and at 60 d postpartum. In multiparous cows, prepartum MTHI (MPTHI) had a negative effect on milk production at 200 d postpartum. One point of decrease in BCS from calving to 60 d postpartum in multiparous cows was associated with 246 kg more milk during the first 100 d of lactation. One point of increase in BCS from calving to 60 d postpartum was associated with 1.85 kg of less milk at peak of milk production. There was no effect of MPTHI on postpartum production performance in primiparous cows. Heat stress prepartum, considering ambient temperature and relative humidity, reduced milk production.

Short Communication: Genotype by Environment Interaction Due to Heat Stress

Heat stress was evaluated as a factor in differences between regional evaluations for milk yield in the United States. The national data set (NA) consisted of 56million first-parity, test-day milk yields on 6million Holsteins. The Northeastern subset (NE) included 12.5million records on 1.3million first-calved heifers from 8 states, and the Southeastern subset (SE) included 3.5million records on 0.4million heifers from 11 states. Climatic data were available from 202 public weather stations. Each herd was assigned to the nearest weather station. Average daily temperature-humidity index (meanTHI) 3 d before test date was used as an indicator of heat stress. Two test-day repeatability models were implemented. Effects included in both models were herd-test date, age at calving class, frequency of milking, days in milk×season class, additive genetic (regular breeding value) and permanent environmental effects. Additionally, the second model included random regressions on degrees of heat stress (t = max[0, meanTHI–72]) for additive genetic (breeding value for heat tolerance) and permanent environmental effects. Both models were fitted with the national and regional data sets. Correlations involved estimated breeding values (EBV) from SE and NE for sires with ≥100 and ≥300 daughters in each region. When heat stress was ignored (first model) the correlations of regular EBV between SE and NE for sires with ≥100 (≥300) daughters were 0.85 (0.87). When heat stress was considered (second model), the correlation increased by up to 0.01. The correlations of heat stress EBV between NE and SE for sires with ≥100 (≥300, ≥700) daughters were 0.58 (0.72, 0.81). Evaluations for heat tolerance were similar in cooler and hotter regions for high-reliability sires. Heat stress as modeled explains only a small amount of regional differences, partly because test-day records depict only snapshots of heat stress.

Evidence of a major gene influencing hair length and heat tolerance in Bos taurus cattle.

Evidence was found that supports the existence of a major gene (designated as the slick hair gene), dominant in mode of inheritance, that is responsible for producing a very short, sleek hair coat. Cattle with slick hair were observed to maintain lower rectal temperatures (RT). The gene is found in Senepol cattle and criollo (Spanish origin) breeds in Central and South America. This gene is also found in a Venezuelan composite breed, the Carora, formed from the Brown Swiss and a Venezuelan criollo breed. Two sets of backcross matings of normal-haired sire breeds to Senepol crossbred dams assumed to be heterozygous for the slick hair gene resulted in ratios of slick to normal-haired progeny that did not significantly differ from 1:1. Data from Carora x Holstein crossbred cows in Venezuela also support the concept of a major gene that is responsible for the slick hair coat of the Carora breed. Cows that were 75% Holstein: 25% Carora in breed composition segregated with a ratio that did not differ from 1:1, as would be expected from a backcross matinginvolving a dominant gene. The effect of the slick hair gene on RT depended on the degree of heat stress and appeared to be affected by age and/or lactation status. The decreased RT observed for slick-haired crossbred calves compared to normal-haired contemporaries ranged from 0.18 to 0.4 degrees C. An even larger decrease in RT (0.61 degrees C; P < 0.01) was observed in lactating Carora x Holstein F1 crossbred cows, even though it did not appear that these cows were under severe heat stress. The improved thermotolerance of crossbred calves due to their slick hair coats did not result in increased weaning weights, possibly because both the slick and normal-haired calves were being nursed by slick-haired dams. There were indications that the slick-haired calves grew faster immediately following weaning and that their growth during the cooler months of the year was not compromised significantly by their reduced quantity of hair. In the Carora x Holstein crossbred cows there was a positive effect of slick hair on milk yield under dry, tropical conditions.

Correlation of Photoplethysmography, Doppler Velocimetry, Digital Blood Pressure and Skin Temperature in the Hand Under Conditions of Warming

Finger skin temperature (FST) and digital systolic blood pressure (SBP) have long been used as measurements of blood flow in the hand. More recently, attempts have been made to quantify hand blood flow through the use of photoplethysmography (PPG) and Doppler ultrasound (US). Correlation among these measurements has not been documented, either in normals or in persons with pathology. The purpose of this study was to establish the correlations among FST, SBP, PPG, and US under conditions of warming in healthy adults. Ninety-three healthy subjects, 75 females and 18 males aged 18 to 25 years, volunteered to participate in this study of correlation among these variables under resting conditions and various degrees of heat stress. Subjects were systematically divided into a control group that sat quietly, a group that received paraffin dip-and-wrap (av. FST increase of 2.87°F) and a group that received paraffin dip-andreimmerse (av. FST increase of 5.63°F). Measurements were taken on the palmar aspect of the second digit before and after 15 min treatment sessions by a researcher who was blind to the subject's group assignment. Percent change was calculated for each variable under each treatment condition. Correlation analyses revealed no significant correlations among variables in the control group; a weak negative correlation between SBP and FST and a moderate positive correlation between FST and US peak amplitude in the dip-and-wrap group; a strong positive correlation between PPG amplitude and FST and a moderate correlation between PPG amplitude and US peak amplitude in the dip-and-reimmerse group. This leads us to conclude that these variables are not closely related to each other nor do they behave similarly under control conditions. As blood flow is increased by warming, positive correlations among these variables begin to become apparent. The specific measurement of blood flow used should be chosen based on the information required and the conditions under which it will be used.

Nutritional considerations of poultry during heat stress

SummaryThe term “heat-stress” is often used to define the response of the bird to most warmer environments where a different abnormal physiological response is observed. There are likely to be two or three different degrees of heat stress where the birds' response is different and where goals may change from consideration for production through to survival. In view of these different physiological situations any attempt at manipulation of nutrient-intake must be flexible and oriented towards the birds' most immediate needs. Fasting of chicks and/or use of carbohydrate-free diets dramatically increases survival time under conditions of acute heat-stress; both these mechanisms are thought to induce a metabolic acidiosis that helps maintain pH in situations of respiratory alkalosis. The so-called thermic effect of feed should not coincide with the period of highest ambient temperature, and last feed intake should occur at least 3–8hr before maximum temperature is expected. Cooling of drinking water has also been shown to be beneficial during heat stress, where increased performance of laying hens occurs as a result of increased feed intake. Various mechanisms of dietary manipulation are discussed which could lead to improvement in performance of broilers and layers housed under heat-stress conditions.

Physiological acclimatization of fowls to a hot humid environment

1. The resting heart rate of cocks and hens was measured in varying degrees of heat stress, and during acclimatization to heat.2. When the rectal temperature was below 110°F. (43·3° C), the heart rate varied inversely with the severity of the climate up to an air temperature around 99° F. (37·2° C).3. At air temperatures of 99° F. (37·2° C.) and above the heart rate was nearly constant over a wide range of rectal temperatures below 110° F. (43·3° C). It averaged about 50 beats/min. less than in an equable climate.4. Above 110° F. (43·3° C.) there was an increase of about 30 beats/min./° F. (17 beats/min./° C.) rise in rectal temperature. The rate equalled that found in an equable climate, when the rectal temperature reached 111–112° F. (43·9–44·4° C). Over this zone of body temperature the fowls were often agitated.5. In severe heat stress there was a slight fall in the heart rate on acclimatization, but this was small compared with the effect of climate.6. In severe heat stress the heart rate during the day was slightly higher than that at night.7. The relation of these findings to the mechanism of temperature regulation is discussed.