Interpretive Summaries

Seeking Optimal Broiler Depopulation Method

by Clayton Gill

Compared to broilers, the poultry industry has had much more experience depopulating turkeys and layers due to outbreaks of highly pathogenic avian influenza (HPAI). However, because of reduced restaurant demand and labor shortages during the human COVID-19 pandemic, broiler producers had to depopulate a large number of birds. Also, the pandemic broiler depopulations mostly occurred in regions not usually impacted by HPAI. 

So the pandemic experience begs the question: When it comes to broilers specifically, how do approved methods of rapid depopulation compare? 

Researchers investigated three approved methods or modes of action -- ventilation shutdown with heat, ventilation shutdown with heat and humidity, increased carbon dioxide atmosphere -- looking for effects on broiler stress parameters and behavior. They found that under controlled environmental conditions, ventilation shutdown with heat and humidity may be superior to other methods, but this method required further study under industry-like conditions.

In 2015 and 2022, HPAI outbreaks caused massive losses to the poultry industry. Yet broilers made up only about 6% of the 96 million birds dead from disease or sacrificed. By contrast, in 2020, the COVID-19 outbreak resulted in the widespread shutdown of sit-down and fast-food restaurants. The drop in demand for chicken products along with labor shortages due hygiene mandates led to the depopulation of around two million broilers. 

Researchers at North Carolina State University note that current poultry depopulation methods approved by the American Veterinary Medical Association include water-based foam application, carbon dioxide atmosphere (by CO2 carts), and various forms of "ventilation shutdown plus" (VSD+). Foam application tends to be labor intensive and a large disease outbreak can cause shortages of depopulation supplies, including CO2. 

"While ventilation shutdown plus heat (VSDH) and ventilation shutdown plus CO2 (VSDCO2) are approved," the researchers state, "there is still a need for quicker, less-stressful methods." 

A previous study with layers found that adding steam (heat plus high relative humidity) as a VSD+ method resulted in significantly faster first hen mortality and complete mortality compared to VSD plus heat by itself. In any case, any use of VSD+ must be approved by the Veterinary Medical Officers overseeing the depopulation. 

In this study, NC State researchers evaluated the addition of higher relative humidity (Rh) to VSDH, resulting in VSDHRh. They compared VSDHRh to VSDH and VSDCO2 for effectiveness in rapid, controlled-stress depopulation of broilers. Experimental data included time-of-death, stress parameters (electroencephalograms, blood chemistry, corticosterone, gene expression), and bird behavior. Their hypothesis: "Potentially, adding relative humidity may result in a reduction in time to death."

The researchers conducted a two-phase experiment using randomly selected, mixed sex broilers from the Poultry Teaching Unit of the Prestage Department of Poultry Science. The birds had been raised to 42 days under the same, conventional broiler grow-out conditions. To control experimental conditions, the researchers used 4.75 cubic-foot, partially insulated Plexiglass® chambers in order to add heat, humidity, or CO2 for both phases of the experiment.

The VSDH treatment started at 29.58°C (85.24°F) and rose to 38.81°C (101.86°F) with 85.55% relative humidity (Rh). The VSDHRh treatment started at 30.00°C (86.00°F) and rose to 41.78°C (107.20°F) with 82.70% Rh. The VSDCO2 treatment started at 0.28% CO2 (not significantly different from other treatments) and rose to 16.85% CO2.

The first phase analyzed the effects of the depopulation method (VSDH, VSDHRh, VSDCO2) on stress parameters and the concentration of Hsp70 (heat shock protein 70, a "molecular chaperone" expressed in response to stress) at time of death. The second phase examined the progression of the stress parameters over time for each method.

An interesting feature of this experiment was the use of individual electroencephalogram (EEG) monitors to measure each bird's brain electrical output in millivolts (mV). This technique allowed correlation of EEG with bird behavior per mode of action during the depopulation process along with accurate determination of time-of-death.  

The researchers found the most rapid depopulation via VSDCO2 (20 min. to 100% depopulation), followed by VSDHRh (60 min.) and VSDH (64 min.). 

They noted that the results "appear to indicate similarity among these methods as effective broiler flock depopulation methods with respect to their effects on each parameter measured over time."

At the lower or upper EEG ranges, the different methods caused no significant differences between conscious and unconscious behaviors. "However, around the midway point of each treatment there was a noticeable shift toward unconscious behaviors."

The NC State researchers concluded that ventilation shut down with heat and relative humidity (VSDHRh) "may be a viable alternative method" for broiler depopulation considering how similar it is to ventilation shut down with heat alone (VSDH). Also, they suggested that VSDHRh may cause less stress on broilers due to lower Hsp70 levels. Nonetheless, they cautioned that "more research needs to be conducted to fully understand how this treatment works in a non-environmentally controlled setting."

What does this study mean for producers?

• To depopulate a broiler house, ventilation shut down with heat and relative humidity -- for example, using steam -- may work better than ventilation shut down with heat alone

• Heat with added humidity may help reduce stress on broilers during depopulation.

• Any ventilation shut down "plus" (VSD+) method must be approved by the Veterinary Medical Officers overseeing the depopulation.

The full paper, titled "The comparative effects of ventilation shutdown with heat (VSDH), relative humidity (VSDHRh), or CO2 (VSDCO2) on broiler electroencephalogram (EEG), blood chemistry and gene expression" can be found in Applied Poultry Research and online here.

DOI: 10.1016/j.japr.2025.100541