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 world's 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.

The effects of early age thermal conditioning (TC) and dual feeding on responses of male broilers to a thermal challenge at 34 d of age are reported. Three hundred chicks were randomly divided into two equal groups. One group was exposed to thermal conditioning (38 C, 42% relative humidity for 24 h) at the age of 5 d, and the other group was not exposed. From 11 to 41 d of age, temperature was diurnally cyclic: 26 C (1600 to 0900 h) and 30 C (0900 to 1600 h). From Day 19, a factorial (2 x 2, TC x diet) experiment was conducted using eight pens of six broilers per treatment. Diets were control diet and dual feeding of a high protein fraction (1600 to 0900 h) and an energy-rich fraction (0900 to 1600 h). Growth and feed efficiency were slightly enhanced (+4%) by thermal conditioning and slightly reduced (-4%) by dual feeding. Mortality during thermal challenge was reduced by both factors. [In not conditioned chicks (N), 31 fed a control grower diet (NG) and 12 on a dual feeding program (NDF) died; in thermal conditioned chicks (T), 18 fed a control grower diet (TG) and 11 on a dual feeding program (TDF) died.] Before the thermal challenge period, body temperature (Tb) was consistently reduced in T. During the thermal challenge period, Tb was reduced by thermal conditioning and dual feeding. Measured at the age of 32 d, Tb gave an estimation of the ability of the chickens to cope with heat stress 2 d later. Pectoral mass was increased in TG, whereas the liver and the gizzard were significantly heavier in TDF and NDF. These results suggest that thermal conditioning at 5 d of age induces a consistent metabolic change in broiler males. The dual feeding program might have induced transitory effects on heat production during the heat stress.

Early age thermal conditioning (TC) durably improves resistance of broilers to heat stress and reduces body temperature (Tb). Three experiments on broiler chicks were conducted to evaluate the effects of TC at 5 d of age on Tb variation measured by thermometer between 4 and 7 d of age, under a tropical environment. Because manipulation of chickens to measure Tb with a thermometer may increase Tb, a preliminary experiment on 13 3-to-4-wk-old male broilers compared Tb measured by telemetry to Tb measured in the terminal colon during three successive periods at 22, 33, and 22 degrees C. During heat exposure, Tb rapidly increased by 0.9 degrees C and plateaued over 24 h. During the last period, seven of the broilers rapidly reduced Tb to a plateau lower than the initial Tb, although six broilers exhibited more variable Tb. Measurement by thermometer underestimated on average core Tb by 0.28 degrees C at 22 degrees C and by 0.57 degrees C at 33 degrees C, whereas Tb recorded by telemetry was not affected by manipulation of the chickens. TC reduced Tb 24 h later in the three experiments. Compared to unexposed control chicks (N), 12 h of TC at 40 degrees C did not significantly reduce Tb at 7 d of age, although 24 h did. TC at 38 and 40 degrees C over 24 h significantly reduced Tb variation from 4 to 7 d of age compared to N chicks, whereas 36 degrees C did not. Withdrawing feed from the chicks for 2 h prior to measurement did not significantly reduce Tb at 4 and 7 d of age, but Tb reduction due to TC was greater in fed chicks (0.28 degrees C) than in chicks without feed (0.05 degrees C). Early age thermal conditioning at 38 to 40 degrees C at 5 d of age for 24 h reduced body temperature of 7-d-old male broilers.

-Feed withdrawal during the warmest part of the day (09:00 to 16:00 h) was compared with the distribution of ground corn and commercial feed consumed ad libitum by control broilers from 28 to 42 d exposed to tropical climatic conditions (averaged Ta = 25 °C; Relative Humidity (RH) = 72%). From 16:00 to 09:00 h, a continuous illumination was in the poultry shed and the chickens received a commercial diet. Each diet was supplied to 7 pens of 14 unsexed broilers. Both feed withdrawal and corn distribution during the day period reduced growth (P < 0.01) and deteriorated feed conversion (P < 0.01) compared to the ad libitum control. At the age of 41 d, the average body temperature (Tb) at 14:00 h in the control group was 42.56 °C. It was higher (P < 0.002) than that recorded in the corn fed group (42.27 °C) and the feed withdrawal chickens (42.11 °C). These effects measured in 4-6 week-old broilers exposed to a moderate tropical climate, illustrate the limitation of feed restriction and sequential feeding under practical conditions. Further research is needed to study the diurnal distribution of corn or feed withdrawal coupled with a balanced diet richer in essential amino acids than the commercial diet during the nocturnal period. les phases les plus chaudes de la journée (09:00 à 16:00 h) a été comparé à la distribution ad libitum de maïs broyé ou d'un aliment commercial pendant la même période, chez des poulets de chair élevés en milieu tropical (Température moyenne = 25 °C ; HR moyenne = 72 %) entre 28 et 42 jours d'âge. Entre 16:00 et 09:00 h tous les poulets recevaient l'aliment commercial ad libitum et le poulailler était continuellement éclairé. Chaque régime expérimental a été distribué à 7 parquets de 14 poulets non sexés. Le retrait d'aliment comme la distribution de maïs ont réduit la croissance (P < 0,01) et détérioré l'indice de consommation (P < 0,01) par rapport aux témoins nourris ad libitum. A l'âge de 41 jours, la température corporelle moyenne mesurée à 14:00 h était de 42,56 °C chez les témoins. Elle était supérieure (P < 0,002) à celle mesurée chez les poulets recevant du maïs (42,27 °C) ou mis à jeun (42,11 °C). Ces effets mesurés entre 4 et 6 semaines d'âge chez des poulets exposés à un climat tropical modéré, illustrent les limites du retrait de l'aliment ou l'alimentation séquentielle dans des conditions pratiques. Des recherches complémentaires associant à la distribution de maïs ou au jeûne diurne, un régime complémentaire nocturne plus riche en acides aminés indispensables que le régime commercial sont souhaitables.poulet de chair / température corporelle / alimentation séquentielle / rythme / climat chaud

Early-age thermal conditioning (TC) by exposing young chicks to 40 C for 24 h reduces body temperature (Tb) and has been showed by others to improve long-term resistance of broilers to heat stress. Uncoupling oxidative phosphorylation in pectoral muscle mitochondria might be related to heat production. Fertile eggs were hatched under video control, and 161 pedigree chicks froml2 sires and 22 dams were immediately allocated to two groups (T, a group composed of 81 chicks exposed to TC at 5 d of age, and N, a control group of 80 nonexposed chicks). Body weights and Tb were measured at 2 and 7 d of age. Five pairs (one N and one T) of full sib chicks from families that exhibited the largest difference of Tb variation from 2 to 7 d of age between the two treatments were chosen for pectoral muscle sampling. Avian uncoupling protein (avUCP) messenger RNA expression was measured by reverse transcript-PCR coupled to southern blot in the pectoral muscle of 7-d-old broiler chicks. At 7 d of age, there were no BW differences between treatments and Tb was significantly reduced by TC (-0.13 C on average). Heritability of Tb variation between 2 and 7 d was 0.38 +/- 0.20 (SE) for T chicks and 0.35 +/- 0.17 for N chicks without a significant genetic correlation between the two environments. Expression of avUCP mRNA was significantly (85%) lower in T chicks than in N chicks. Uncoupling protein mRNA expression in pectoral muscle and Tb are quickly adjusted in broiler chicks 24 h after early thermal conditioning.