A better understanding of pain in piglets

By Maria Lou1,2, Yuzhi Li1, and Beth Ventura2

August 2020

According to the National Pork Board, the U.S. raises over 100 million pigs each year for human consumption. Approximately half of those pigs are male, and as part of being raised for pork, young male piglets are castrated to eliminate something called “boar taint”, which improves meat quality. However, it is currently uncommon for farmers in the U.S. to provide a form of pain relief during or after this procedure, despite the fact that extensive research has shown it to be painful. As animal welfare scientists, it is our job to work with farmers to help them better identify and prevent pain in their animals. 

Pain research focuses on evaluating different methods of pain assessment, so that scientists can ultimately determine an optimal form of pain relief. However, the first step in determining an optimal form of pain relief for a procedure like castration is learning how to identify and recognize pain in pigs. Pain assessment in pigs is a big challenge because as prey animals, they do not overtly express signs of weakness as a survival tool from predators. Nonetheless, scientists are starting to identify potential candidates for reliable pain indicators in pigs. This is where our research plays a major role. In May 2019, we completed a project at the University of Minnesota’s West Central Research and Outreach Center in Morris, MN. The goal of this project was to determine whether certain behaviors exhibited by piglets during castration could reliably indicate pain. Specifically, we examined three different behavioral indicators: struggle behavior (also termed “escape behavior”), vocalizations, and a novel approach called the piglet grimace scale, which tracks how a piglet’s facial expression changes in response to pain.

A total of 88 male piglets were enrolled into the study. We created two treatments: 1) half of the piglets were surgically castrated following our university’s on-farm standard protocol (procedures were identical to common practice on most pig farms in the U.S., so that our results could be useful to real farmers), and 2) half were sham-castrated (piglets were handled exactly the same, but did not undergo the painful procedure of castration). We studied the behavior in all piglets by video and audio recording struggle behavior and vocalizations. Struggle behavior was categorized using three specific behavioral responses – curl ups (like a sit-up), kicks, and body flailing. We also recorded the duration and peak frequency (Hz) of each piglet call. Peak frequency is defined as the loudest frequency in each call, and each call was classified by either having a low (x < 1000 Hz) or high (x ³ 1000 Hz) peak frequency. Piglet calls with high peak frequencies are associated with painful stimuli, as previously determined by scientists.

Our results showed that piglets who experienced surgical castration produced a higher number of leg kicks and high frequency calls compared to piglets that were sham-castrated. Additionally, surgical castration also increased the presence of body flailing. Lastly, the high frequency calls from piglets surgically castrated tended to be of longer in duration than those who were sham-castrated. Therefore, leg kicks, high frequency calls, and body flailing may be useful behavioral indicators of pain in piglets. We are currently in the process of analyzing how these results align with changes in the piglet grimace scale, as it is important to reliably detect pain using multiple types of indicators. We are hopeful that improving the validity and objectivity of such measures will ultimately help us identify the best strategies to prevent piglets from experiencing pain on the farm.

1West Central Research and Outreach Center, University of Minnesota, Morris, MN

2Department of Animal Science, University of Minnesota, St. Paul, MN