The best way to solve a problem is to talk about it. Unfortunately, some of the issues we deal with, particularly related to slaughter, tend to cause discomfort when approached candidly and openly. One of those issues is head-only electrical stunning of pigs.

Whenever I meet someone from a small or very small meat processing facility that slaughters pigs, I ask them about their stunning method. Many of these plants use a common, head-only electrical stunner. Research has shown that this stunning method maintains insensibility reliably for about 15 seconds. Depending on sticking method and hoist speed, it generally takes about 30 seconds from stunning to bleeding in these facilities.

My next question — “Have you had pigs waking up on the rail?” — is usually met with some resistance to answer. If you do the math, you can guess what the answer is.

My first encounter with return-to-sensibility issues was in a very small slaughter plant that was consistently receiving noncompliance records for violations of the regulations under the Humane Slaughter Act.

The plant was using a common head-only electrical stunner. The establishment’s inspector was routinely detecting corneal reflexes — closure of the eyelids in response to a touch of the eye — from the stunned pigs, but no other signs of sensibility, such as rhythmic breathing or response to a prick on the nose with a knife tip, were observed. However, if the employee performing the sticking did not induce rapid bleed-out with a good stick, pigs were occasionally returning to sensibility, which was a very serious issue.

I decided to ask Erika Voogd, of Voogd Consulting, for her input on the issue. She recommended that the stunner operator re-apply the stunner to the chest of the pig after a successful head-only stun. She had observed that this method consistently stopped the heart from beating in stunned pigs, which ensured the animals would not return to sensibility. So, I advised the plant to use this method.

The results were exactly what we expected — none of the pigs woke up on the rail. But we observed that many of the pigs still had a corneal reflex after they had been bled out. This just didn’t make sense.

At that point, I was beginning to question the validity of the corneal reflex for judging return to sensibility in electrically stunned animals. Other pork slaughter plants that use electrical stunning have reported similar issues with the corneal reflex.

To avoid confusion, the corneal reflex is an excellent indicator of stunning efficacy when captive bolt or gunshot is used.

A handful of papers in the scientific literature mention the recurrence of the corneal reflex in animals that were electrically stunned. Most of these studies were focused on the recovery of animals following electrical stunning, so the exsanguination step was not included. A couple of studies included a step to stop the heart following a head-only stun. Both studies found that calves and sheep that were stunned in a way that induced cardiac arrest did not return to sensibility, but a corneal reflex could be induced for nearly a minute after stunning.

This experience left me with several questions regarding electrical stunning that had not been fully addressed in the research I had read. So, it was time to design a study.

Dr. Temple Grandin and I worked with Voogd, Dr. Jim Claus and Claus’ lab at the University of Wisconsin-Madison to design and implement an experiment to observe how the two-stage stun we used earlier compared to the standard head-only electrical stunning method.

We conducted our experiment over three weeks at a small state-inspected plant that slaughtered approximately 30 pigs one day per week. A total of 89 pigs were observed.

Each week, we alternated the head-only and head/chest stunning methods so every other pig that was stunned by the same method. The stunner output was 313 volts and an average of 2.5 amperes of current was applied to each pig.

Following stunning, we monitored the following signs of return to sensibility: corneal reflex (eye blink from touch), dazzle response (eye blink from application of a bright light to the eye), nose twitching without stimulus, rhythmic breathing, natural blinking, eye tracking to a moving object and righting reflex. Righting reflex is a definite sign of a fully conscious animal.

We recorded corneal reflexes in more than 85 percent of the pigs. The occurrence of corneal reflexes was not statistically different between the two stunning methods. The two-stage stun abolished heartbeat, rhythmic breathing, natural blinking, eye tracking to a moving object and righting reflexes.

These signs were all observed in varying percentages of the head-only stunned pigs. We observed natural blinking in 40.8 percent of the head-only stunned pigs. We also detected a heartbeat in all of the head-only stunned pigs. As a matter of fact, the head-only stunned pigs maintained a heartbeat for an average of two minutes after stunning. From this data, it appears that the two-stage stun does a better job of guaranteeing insensibility.

We were concerned that reapplying the stunner to the body of the pig could impact meat quality, so we also compared color, pH, and drip loss in muscle samples taken from the loin of each carcass. We found no statistically significant difference in any of these measures of meat quality between the two stunning methods.

In-depth brain studies are needed to fully understand why the corneal reflex was not abolished by electrical stunning. In any case, corneal reflexes should not be used as the sole indicator of insensibility in electrically stunned pigs. Rhythmic breathing and natural blinking appear to be more suitable to judge insensibility in electrically stunned pigs.

There are two important things to take away from this article. First, our study showed that the two-stage electrical stunning method outperformed head-only stunning at preventing pigs from returning to sensibility without impacting meat quality. Second, there is a limited, but growing, body of research related to humane-slaughter methods and their implications on animal welfare and meat quality.

Communication and collaboration between industry and academia are vitally important to ensure that we continue to identify issues that need investigation and perform research that is suitable to address these issues.