Press release Heat stress in poultry: risk assessment based on animal characteristics is crucial
Poultry is particularly susceptible to heat stress, and the effectiveness of measures depends heavily on the context. In laying hens, measures such as betaine supplements and temporary feed restriction have proven insufficiently effective in trials, while genetic differences between breeds do have a clear impact. In broilers, adjusted feed formulations during hot days can help limit the impact of heat stress, but those same strategies may be detrimental under normal temperatures. Cooling the environment through ventilation has proven to be extremely efficient, and combinations of measures are recommended. A correct assessment of heat stress at the individual animal level is crucial in this regard, as body weight also has a significant impact on heat stress perception in poultry. This is evident from the doctoral research conducted by Renée De Baets at ILVO and Ghent University.
Heat stress is always the sum of heat from the environment and heat generated by the animal itself. Due to climate change, we expect to face more frequent and prolonged heat waves and higher summer temperatures in Belgium. Furthermore, current breeds of broiler chickens and laying hens have been selected for productivity, giving them a high metabolic rate that contributes to increased internal heat production. Broiler chickens, in particular, experience rapid growth, during which their cardiovascular system cannot keep pace, making them especially sensitive to heat. Chickens primarily lose heat through water evaporation via respiration, but even this has its limits. In addition to temperature, relative humidity also determines how efficiently the animal can dissipate heat into the environment.
This makes them particularly vulnerable to heat. In addition to a higher risk of mortality, heat stress also has negative consequences for animal welfare, animal health, technical performance, and economic profitability. A possible consequence of heat stress in broiler chickens is slower growth and reduced meat quality; in laying hens, it results in reduced egg production and more brittle eggshells.
To prevent heat stress, various measures are already being implemented in the industry. Cooling the environment is particularly effective, but ideally, we should also be able to reduce the heat produced by the animals themselves. In her doctoral research, Renée De Baets therefore investigated the effectiveness of measures related primarily to feed and water.
Laying hens: breed is more important than feeding strategies
Betaine is a substance that regulates water balance in cells and is therefore often administered to laying hens via supplements during heat waves. However, a trial with laying hens showed that it does not provide sufficient protection against reduced egg production and brittle eggshells under heat stress conditions. Blood samples confirm that chickens that received the supplement, just like those that did not, exhibit an altered acid-base balance during hot periods.
Temporarily reducing feed intake also did not have the desired effect in the trials. Only breed—and thus genetic characteristics—appeared to have an impact: white laying hens are more sensitive to the effects of heat stress than brown laying hens. It should also be noted that white laying hens are often more resilient and efficient under normal conditions, which may explain why the decline is potentially more pronounced in white hens. Neither betaine nor feed restriction can correct this effect of breed on performance under heat conditions, although the laying hens generally suffered less from heat than, for example, broiler chickens.
Renée De Baets (Ph.D. candidate): “This demonstrates that chicken genetics do indeed influence their susceptibility to heat stress. This is a promising avenue for further research.”
Broiler chickens: potential for feed adjustments, but difficult to implement consistently across all climatic conditions
During the finishing phase, when broilers are at their heaviest and their risk of heat stress is highest, dietary adjustments do indeed appear to influence the impact of heat on their performance. By reducing the crude protein content, farmers can decrease the heat produced by the chicks during digestion. At normal temperatures, this leads to slower growth and poorer feed conversion, but that disadvantage disappears during hot days.
A higher fat content in the diet can, in turn, ensure that the chicks still get enough energy even if they eat less on hot days. The animals in the trial appeared to adapt to the changed feed under both temperature conditions. Renée De Baets therefore found a negative effect on growth and feed conversion during normal days, but not during hot days. However, there was higher mortality in the heat group.
Renée De Baets (doctoral candidate): “These results show that there is no “one-size-fits-all” approach that poultry farmers can apply. Under normal conditions, both feeding strategies have adverse effects, but they do show potential during periods of extreme heat. This makes formulating an optimal diet for varying climatic conditions particularly complex.”
Broilers: water additives are not very effective
Finally, the study also examined the impact of various additives administered via drinking water to broiler chickens, such as polyphenols, essential oils, betaine, and functional amino acids. None of these common measures proved effective in reducing heat stress or improving performance in the trial. Some treatments even had undesirable side effects, such as reduced feed intake and poorer meat quality when functional amino acids were used. Whether the effect is also absent under practical conditions, with different drinking water flow rates and dosages, is a topic for further research.
Renée De Baets, doctoral researcher: “This also indicates that the effectiveness of the measures depends heavily on the specific circumstances. That is why it is so important that we focus on ensuring that poultry farmers conduct proper risk assessments themselves, and provide them with decision-support tools that enable them to take the right measures at the right time.”
Action plan and predictive tools
That risk assessment, in the form of practical heat stress models, was also part of the doctoral research. It is important that such models be based on heat stress responses such as respiratory rate and body temperature. Renée De Baets found that body weight in broiler chickens has a significant impact on the predictive models, with heavier animals experiencing heat stress more quickly, in the form of faster-rising heat stress responses. In addition, cooling systems significantly delay the onset of stress signals in the barn. This demonstrates that there is always an interaction between environmental parameters and animal-related parameters, which must be taken into account in the risk assessment.
The doctoral research was part of the VLAIO-LA COOLCHICKS project, which developed a heat action plan in addition to various tools and calculation modules. At the Pluimveeloket (Dutch-language information center), livestock farmers can look up the risk of heat stress in their region—and specifically in a barn using their available technologies—and then determine the best possible short- and long-term measures. The results from Renée De Baets’ doctoral research will be used to further refine this action plan and the associated calculation modules.
Renée De Baets successfully defended her doctoral dissertation, titled “Heat Stress Assessment and Mitigation in Commercial Poultry under Temperate Climate Conditions,” on May 27, 2026, at 5:00 p.m., at the GUM in Ghent. Her (co)advisors are Dr. Evelyne Delezie (ILVO), Prof. Gunther Antonissen (UGent), and Prof. Jeroen Degroote (UGent).
More info
www.pluimveeloket.be/coolchicks
Contact
Nele Jacobs, ILVO spokesperson: +32 472530696, pers@ilvo.vlaanderen.be
Evelyne Delezie, ILVO co-promotor: evelyne.delezie@ilvo.vlaanderen.be
