S. Yahav

Adaptation to hot climate and strategies to alleviate heat stress in livestock production

Despite many challenges faced by animal producers, including environmental problems, diseases, economic pressure, and feed availability, it is still predicted that animal production in developing countries will continue to sustain the future growth of the worlds meat production. In these areas, livestock performance is generally lower than those obtained in Western Europe and North America. Although many factors can be involved, climatic factors are among the first and crucial limiting factors of the development of animal production in warm regions. In addition, global warming will further accentuate heat stress-related problems. The objective of this paper was to review the effective strategies to alleviate heat stress in the context of tropical livestock production systems. These strategies can be classified into three groups: those increasing feed intake or decreasing metabolic heat production, those enhancing heat-loss capacities, and those involving genetic selection for heat tolerance. Under heat stress, improved production should be possible through modifications of diet composition that either promotes a higher intake or compensates the low feed consumption. In addition, altering feeding management such as a change in feeding time and/or frequency, are efficient tools to avoid excessive heat load and improve survival rate, especially in poultry. Methods to enhance heat exchange between the environment and the animal and those changing the environment to prevent or limit heat stress can be used to improve performance under hot climatic conditions. Although differences in thermal tolerance exist between livestock species (ruminants > monogastrics), there are also large differences between breeds of a species and within each breed. Consequently, the opportunity may exist to improve thermal tolerance of the animals using genetic tools. However, further research is required to quantify the genetic antagonism between adaptation and production traits to evaluate the potential selection response. With the development of molecular biotechnologies, new opportunities are available to characterize gene expression and identify key cellular responses to heat stress. These new tools will enable scientists to improve the accuracy and the efficiency of selection for heat tolerance. Epigenetic regulation of gene expression and thermal imprinting of the genome could also be an efficient method to improve thermal tolerance. Such techniques (e.g. perinatal heat acclimation) are currently being experimented in chicken.

Physiological responses of chickens and turkeys to relative humidity during exposure to high ambient temperature

1. 1. At 35°C, the response functions of body weight gain and feed intake of 5–8 week old chickens to relative humidity (RH) were bell shaped with maxima at 60–65%, corresponding to minima in core and skin temperature, blood pH, and to maxima in blood pCO2. 2. 2. In 5–8 week old turkeys, weight gain and feed intake declined only with the increase from 40–45% to 50–55% in RH. Core and skin temperature, and blood acid-base were not affected by RH. 3. 3. Plasma (T3) concentration was positively correlated with feed intake.

Thermal Manipulations During Broiler Embryogenesis: Effect on the Acquisition of Thermotolerance

Rapid growth rate has presented broiler chickens with serious difficulties when called on to thermoregulate efficiently in hot environmental conditions. Altering the incubation temperature may induce an improvement in the acquisition of thermotolerance (AT). This study aimed to elucidate the effect of thermal manipulations (TM) during the development of the thyroid and adrenal axis of broiler embryos on the potential of broilers to withstand acute thermal stress at marketing age. Cobb broiler embryos were subjected to TM at 39.5 degrees C and 65% RH from embryonic day 7 to 16 (inclusive), either continuously (24 h) or intermittently (12 h). After hatching chicks were raised under standard conditions to 35 d of age and then subjected to thermal challenge (35 degrees C for 5 h). Continuous TM caused a significant decline in hatchability, coupled with significantly lower BW and body temperature at hatching. The intermittent (12-h) chicks showed results similar to the controls but had significantly lower body temperature. Thermal challenge at marketing age demonstrated a significant improvement in AT in both the 12- and 24-h TM-treated broilers, which was characterized by a significantly lower level of stress (as evidenced by the level of plasma corticosterone) and rate of mortality. It was concluded that TM during the portion of embryogenesis when the thyroid and adrenal axis develop and mature had a long-lasting effect and improved the AT of broiler chickens. Whereas intermittent TM had no significant effect on hatchability and performance parameters, continuous TM negatively affected these parameters.

THERMOREGULATION IN NAKED NECK CHICKENS SUBJECTED TO DIFFERENT AMBIENT TEMPERATURES

1. Heterozygous (Na/na) naked neck chickens and their normally feathered (na/na) sibs, were exposed to constant ambient temperatures (Ta) ranging between 15 and 35 degrees C and 12h: 12h diurnal high:low temperatures of 15 degrees C:35 degrees C. 2. No significant effect of genotype was obtained in weight gain and food intake. However, the naked neck birds tended to gain somewhat more weight at high Ta and consume more food at low Ta. 3. At 35 degrees C Na birds showed better regulation of body temperature (Tb) and demonstrated considerably higher radiation from the neck. 4. The greater food intake of the naked neck chickens at 15 degrees C was associated with significantly higher packed cell volume, haemoglobin concentration, heart and liver size. These appear to involve both higher heat production and haemodynamic changes to accommodate the higher oxygen demands of the naked neck chickens at low Ta. 5. The results indicate the ability of the naked neck chickens, on the one hand to thermoregulate at low Tas and, on the other their slightly better capacity to maintain Tb at high Tas. However, no genotype advantage was obtained under diurnal cyclic temperature conditions.

Thermal manipulations in late-term chick embryos have immediate and longer term effects on myoblast proliferation and skeletal muscle hypertrophy

We investigated the cellular and molecular bases for the promotion of muscle development and growth by temperature manipulations (TMs) during late-term chick embryogenesis. We show that incubation at 39.5 degrees C (increase of 1.7 degrees C from normal conditions) from embryonic days 16 to 18 (E16 to E18) for 3 or 6 h daily increased diameter of myofibers as of day 13 of age and enhanced absolute muscle growth relative to controls, until day 35 of age. TMs had immediate (E17) and later (up to 2 wk posthatch) effects in elevating muscle cell proliferation relative to controls. This was indicated by higher DNA incorporation of thymidine and a higher number of cells expressing PCNA in intact muscle, accompanied by higher Pax7 levels, all reflecting a higher number of myogenic cells, and suggesting that the increased hypertrophy can be attributed to a higher reservoir of myogenic progeny cells produced in response to the TM. IGF-I levels were higher in the TM groups than in controls, implying a mechanism by which heat manipulations in chicks affect muscle development, with locally secreted IGF-I playing a major role. Whereas hypertrophy was similar in both TM groups, cell proliferation and Pax7 levels were more robust in the 6-h muscle, mainly posthatch, suggesting a differential effect of various TM periods on cell reservoir vs. hypertrophy and a high sensitivity of myoblasts to relatively small changes in heat duration with respect to these processes, which is manifested in the short and long term.

Effects of diurnally cycling versus constant temperatures on chicken growth and food intake

1. Male broiler chickens were exposed in separate experiments to: (a) constant ambient temperatures (Ta) ranging between 10 and 35 degrees C; (b) 12 h:l2 h diurnal high-low temperature of 10:30 degrees C, 15:35 degrees C and 15:30 degrees C in different trials; and (c) 12 h of 30 or 35 degrees C and 12 h of variable lower temperature. 2. A progressive decline in weight gain and food intake was obtained as Ta increased from 18 to 35 degrees C. Under diurnally cycling temperature, weight gain and food intake were lower than in the average corresponding constant temperature, with the exception of chickens exposed to 15 : 30 degrees C, where weight gain and food intake were not significantly different from those of the constant average temperature. 3. As the diurnal cold period was made colder, chickens exposed during half of the diurnal cycle to high temperature (30, 35 degrees C) demonstrated a significant (P <= 0.05) increase in weight gain and food intake. 4. There was a good correlation between plasma triiodothyronine (T3) and Ta. The correlation between T3 and food intake was highly significant, but that between T3 and weight gain was poor. 5. The results indicate the difficulties in predicting the performance of broilers exposed to diurnally cycling temperatures from knowledge of the average temperature.

Thermal manipulations of broiler embryos—The effect on thermoregulation and development during embryogenesis

This study aimed to elucidate the effects of thermal manipulations (TM) of broiler embryos, during the development of the thyroid and adrenal axis, on embryo development and metabolism. Cobb eggs were divided into 3 treatments: control, 24H-continuous TM at 39.5 degrees C and 65% RH from embryonic day 7 to 16 inclusive, and 12H-intermittent TM for 12 h/d in the same period. Only the 24H treatment negatively affected embryo growth and development, with lower relative weights of embryo, liver, and pipping muscle. During TM, eggshell temperature, heart rate, and oxygen consumption were elevated as embryos were in their ectothermic phase, but from the end of the TM until hatch, these parameters were significantly lower in both treatments than in the control. Moreover, plasma concentrations of the thyroid hormones were significantly lower in the 2 treatments during and after TM, until hatch. Plasma corticosterone concentration of the TM-treated embryos was significantly lower after the TM but significantly higher at hatch. It was concluded that TM during the development of the thyroid and adrenal axis lowered their functional set point, thus lowering metabolic rate during embryogenesis and at hatch.

Thermal manipulation of the embryo modifies the physiology and body composition of broiler chickens reared in floor pens without affecting breast meat processing quality.

Selection in broiler chickens has increased muscle mass without similar development of the cardiovascular and respiratory systems, resulting in limited ability to sustain high ambient temperatures. The aim of this study was to determine the long-lasting effects of heat manipulation of the embryo on the physiology, body temperature (Tb), growth rate and meat processing quality of broiler chickens reared in floor pens. Broiler chicken eggs were incubated in control conditions (37.8°C, 56% relative humidity; RH) or exposed to thermal manipulation (TM; 12 h/d, 39.5°C, 65% RH) from d 7 to 16 of embryogenesis. This study was planned in a pedigree design to identify possible heritable characters for further selection of broiler chickens to improve thermotolerance. Thermal manipulation did not affect hatchability but resulted in lower Tb at hatching and until d 28 post-hatch, with associated changes in plasma thyroid hormone concentrations. At d 34, chickens were exposed to a moderate heat challenge (5 h, 32°C). Greater O2 saturation and reduced CO2 partial pressure were observed (P < 0.05) in the venous blood of TM than in that of control chickens, suggesting long-term respiratory adaptation. At slaughter age, TM chickens were 1.4% lighter and exhibited 8% less relative abdominal fat pad than controls. Breast muscle yield was enhanced by TM, especially in females, but without significant change in breast meat characteristics (pH, color, drip loss). Plasma glucose/insulin balance was affected (P < 0.05) by thermal treatments. The heat challenge increased the heterophil/lymphocyte ratio in controls (P < 0.05) but not in TM birds, possibly reflecting a lower stress status in TM chickens. Interestingly, broiler chickens had moderate heritability estimates for the plasma triiodothyronine/thyroxine concentration ratio at d 28 and comb temperature during the heat challenge on d 34 (h(2) > 0.17). In conclusion, TM of the embryo modified the physiology of broilers in the long term as a possible adaptation for heat tolerance, without affecting breast meat quality. This study highlights the value of 2 new heritable characters involved in thermoregulation for further broiler selection.

Skin surface temperature of broiler chickens is correlated to body core temperature and is indicative of their thermoregulatory status

Extreme thermal conditions may dramatically affect the performance of broilers and other domestic animals, thereby impairing animal welfare and causing economic losses. Although body core temperature is the parameter that best reflects a birds thermal status, practical and physiological obstacles make it irrelevant as a source of information on the thermal status of commercial flocks. Advances in the technology of infrared thermal imaging have enabled highly accurate, noncontact, and noninvasive measurements of skin surface temperature. Providing that skin surface temperature correlates with body temperature, this technology could enable acquisition of reliable information on the thermal status of animals, thereby improving diagnoses of environmental stress in a flock. This study of broiler chickens found a strong positive correlation between body core temperature and facial surface temperature, as recorded by infrared thermal imaging. The correlation was equally strong at all ages from 8 to 36 d during exposure to acute heat stress with or without proper ventilation and after acclimation to chronic heat exposure. A similar correlation was found by measurements in commercial flocks of broilers. Measurements of blood plasma concentrations of corticosterone, thyroid hormones, and arginine vasotocin confirmed that metabolic activity was low after acclimation to chronic exposure to heat, whereas ventilation was at least as efficient as acclimation in reducing thermal stress but did not impair metabolism. In light of these novel results, commercial benefits of infrared thermal imaging technology are suggested, especially in climate control for commercial poultry flocks. The application of this technique to other domestic animals should be investigated in future experiments.

αMUPA mice: a transgenic model for longevity induced by caloric restriction

Caloric restriction (CR) is currently the only therapeutic intervention known to attenuate aging in mammals, but the underlying mechanisms of this phenomenon are still poorly understood. To get more insight into these mechanisms, we took advantage of the alphaMUPA transgenic mice that previously were reported to spontaneously eat less and live longer compared with their wild-type control mice. Currently, two transgenic lines that eat less are available, thus implicating the transgenic enzyme, i.e. the urokinase-type plasminogen activator (uPA), in causing the reduced appetite. This phenotypic change could have resulted from the ectopic transgenic expression that we detected in the adult alphaMUPA brain, or alternatively, from a transgenic interference in brain development. Here, we have summarized similarities and differences so far found between alphaMUPA and calorically restricted mice. Recently, we noted several changes in the alphaMUPA liver, at the mitochondrial and cellular level, which consistently pointed to an enhanced capacity to induce apoptosis. In addition, alphaMUPA mice showed a reduced level of serum IGF-1 and a reduced incidence of spontaneously occurring or carcinogen-induced tumors in several tissues. In contrast, alphaMUPA did not differ from wild type mice in the levels of low molecular weight antioxidants when compared in several tissues at a young or an old age. Overall, the alphaMUPA model suggests that fine-tuning of the threshold for apoptosis, possibly linked in part to modulation of serum IGF-1 and mitochondrial functions, could play a role in the attenuation of aging in calorically restricted mice.