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Chronic heat is a major stress factor in laying hens and many studies on the effect of heat stress have been published. It remains difficult, however, to draw general conclusions about the effect of chronic heat stress on performance and its relationship with genetic and environmental factors, as these studies have been done under varying experimental conditions and using various experimental designs. A meta-analysis enabled us to make a quantitative review of the results from 131 published papers. The relative effects of four factors (genotype, age, group size, and amplitude of temperature variation) and their interactions with temperature were analyzed for 13 traits. After pre-correcting the data for a random study effect, the best model for each trait was selected in a stepwise procedure based on its residual sum of squares. Shell strength, daily feed intake, egg mass, and hen-day egg production were found to be more sensitive to heat stress than the other traits as they dropped by 9.0 to 22.6% between thermo-neutrality (15 to 20°C) and heat stress (30 to 35°C) while yolk and albumen proportions or Haugh units showed nearly no variation with temperature (

Lactating sows generally have problems dissipating their body heat to the environment. Cooling the floor under the sow¿s shoulder, called the cool-sow system, is a method to increase body heat removal by conduction, thereby contributing to the thermal comfort of the sow. In this study, the effect of the cool-sow system on the performance of the sow and her piglets in the farrowing room and on the position of the sow in the farrowing crate was determined. In total, 60 sows (parity between 2 and 5) were included in the study. One room with 12 pens was used during five batches in autumn, spring, and summer. During each batch, the floors were cooled in six randomly chosen pens, while the other six pens were used as reference pens. The sows on the cool-sow system had 0.6 kg higher average daily feed intake (P <0.001). These sow¿s piglets grew 20 g per day per piglet faster (P <0.001). There was no effect on the loss of bodyweight of the sow or on piglet mortality. The sows on the cool-sow system showed a higher feed intake during all five batches, not only during summer batches. Sow position and location in the crate was hardly affected by the cool-sow system. The cool-sow system removed on average 107 W of heat per pen, of which approximately 58 W was directly removed from the sow¿s body.

The effects of relative humidity (RH) and high ambient temperature (T) on physiological responses and animal performance were studied using 12 groups (10 gilts per group) in pens inside respiration chambers. The microclimate in the chamber was programmed so that T remained constant within a day. Each day, the T was increased by 2°C from low (16°C) to high (32°C). Relative humidity was kept constant at 50, 65, or 80%. The pigs¿ average initial BW was 61.7 kg (58.0 to 65.5 kg), and their average ending BW was 70.2 kg (65.9 to 74.7 kg). Respiration rate (RR), evaporative water (EW), rectal temperature (RT), skin temperature (ST), voluntary feed intake (VFI), water-to-feed ratio (rW:F), heat production (HP), and ADG were analyzed. The animals had free access to feed and water. We determined the T above which certain animal variables started to change: the so-called inflection point temperature (IPt) or "upper critical temperature." The first indicator of reaction, RR, was in the range from 21.3 to 23.4°C. Rectal temperature was a delayed indicator of heat stress tolerance, with IPt values ranging from 24.6 to 27.1°C. For both these indicators the IPt was least at 80% RH (P <0.05). Heat production and VFI were decreased above IPt of 22.9 and 25.5°C, respectively (P <0.001). For each degree Celsius above IPt, the VFI was decreased by 81, 99, and 106 g/(pig·d) in treatments 50, 65, and 80% RH, respectively. The ADG was greatest at 50% RH (P <0.05). Ambient temperature strongly affects the pigs¿ physiological changes and performance, whereas RH has a relatively minor effect on heat stress in growing pigs; however, the combination of high T and high RH lowered the ADG in pigs. The upper critical temperature can be considered to be the IPt above which VFI decreased and RT then increased. Temperatures of the magnitude of both these IPt are regularly measured in commercial pig houses. We conclude that the upper critical temperatures for 60-kg, group-housed pigs fed ad libitum a