Do Pigs Selected for Feed Efficiency Traits Respond Differently to Stress?

Improving feed efficiency is essential for increasing sustainable food production and profitability for producers. Residual feed intake (RFI) is a way to genetically select pigs for feed efficiency. The RFI model used at Iowa State University (ISU) is defined as the difference between observed and expected feed intake, based on a given amount of growth and backfat. Two divergent genetic lines were selected for RFI over eight generations: 1) low-RFI pigs, which are more feed-efficient, and 2) high-RFI pigs, which are less feed-efficient.

It was recently suggested that breeding for improved feed efficiency may decrease the animal’s stress adaptation, thus resulting in adverse effects on livestock behavior, management and welfare (Rydhmer and Canario, 2014). Therefore, the objective of this study was to evaluate the effect of genetic selection for RFI on swine welfare in regards to stress responsiveness.

The first experiment looked at adrenocorticotropin hormone (ACTH) challenge and the physiological stress response. Following a stressful event, pigs secrete ACTH from the anterior pituitary gland, which then stimulates the secretion of cortisol from the adrenal cortex. Therefore, ACTH can be exogenously administered to induce a stress response, and the responsiveness can be measured through cortisol. Cortisol prepares the body for a fight-or-flight response and, therefore, influences animal behavior and welfare.

Six low-RFI and six high-RFI Yorkshire gilts weighing 149 lb. ± 11.4 lb. body weight (BW) were challenged with ACTH. Gilts were housed in individual metabolism pens and were acclimated to this housing for one week prior to the start of the study. After acclimation, gilts were fitted with a non-surgical catheter in the jugular vein. Three days later, the gilts were fasted overnight and challenged with 0.2 IU/kg BW of exogenous porcine ACTH. Serial blood samples were collected at intervals ranging from 30 minutes before to 90 minutes after the ACTH administration. Plasma cortisol concentrations were determined using a DPC Immulite assay.

Results of Experiment 1

Prior to the ACTH administration, the average baseline cortisol concentration tended to be 26% lower in the low-RFI compared to the high-RFI line (P=0.08). This suggests that innately, prior to eliciting a stress response, low-RFI gilts tend to undergo less stress than high-RFI gilts. Both lines of pigs showed a stress response to the ACTH challenge. Low-RFI gilts tended to reach a peak plasma cortisol concentration 18% lower than high-RFI gilts (P=0.08). Furthermore, low-RFI gilts tended to have a 28% lower difference between baseline and peak cortisol concentrations compared to high-RFI pigs (P=0.08). Low-RFI plasma cortisol concentrations returned to baseline 60 minutes post-ACTH, whereas high-RFI plasma cortisol concentrations returned to baseline 90 minutes post-ACTH. These results show that low-RFI (more feed efficient) gilts tend to innately undergo less stress, tend to be less responsive to the stress challenge, and recover to baseline cortisol concentration more quickly compared to the high-RFI (less feed efficient) gilts.

Experiment 2

Researchers next conducted a human approach test to measure the behavioral response to an unfamiliar human stressor. The human approach test investigates swine stress that occurs during human-animal interaction and can have implications on animal welfare and handling. During this 10-minute test, pig behavior can be observed to measure the amount of stress the pig is undergoing.

Forty low-RFI and 40 high-RFI eighth-generation Yorkshire barrows (102.3 lb. ±18.92 lb. BW) were tested. Barrows were housed in a conventional confinement unit within one room containing 12 mixed-sex and mixed-line pens of 15 to 16 pigs/pen. The pigs were moved to this facility 10 days prior to the start of the experiment.

The human approach test was conducted in a rectangular arena (4.9 m long x 2.4 m wide with 1.2 m high, black corrugated plastic sides) separate from the home pens. Pig testing occurred five days per week between 1 and 5 p.m. A testing session consisted of a 10-minute period during which each pig individually underwent the human approach test within the experimental arena. The human was the same for all tests, wore unfamiliar clothing (orange coveralls), and had not previously interacted with the pigs.

During the testing, the human stood silently at the center of the wall opposite of pig entry and did not interact with or move toward the pigs. Barrow behavior was collected on video and was analyzed by one person using Observer software to decode activity, number of defecations, escape attempts, freezing behavior and pig interaction with the human.

Results

Low-RFI barrows were less active within the arena (P < 0.0001), defecated fewer times (P = 0.002), performed fewer escape attempts (P = 0.001), froze fewer times (P < 0.0001) and spent 2% less time freezing (P = 0.05) compared to high-RFI barrows. These behaviors suggest that low-RFI (more feed efficient) barrows were less stressed during the human approach test compared to high-RFI (less feed efficient) barrows. Interestingly, low-RFI barrows took 48 seconds longer to first make contact with the human compared to the high-RFI barrows (P = 0.04), which may suggest that low-RFI barrows are slower-moving or spend more time investigating the arena prior to interacting with the human. Similar to the ACTH challenge, the behavioral results suggest lower-stress responsiveness to the human approach test in the low-RFI (more feed efficient) barrows compared to the high-RFI (less feed efficient) barrows.

Overall, these findings suggest that low-RFI pigs may be better at coping with stressful events, which in turn likely contributes to their increased feed efficiency. Furthermore, these findings are important to the swine industry because they show that pigs can undergo genetic selection for improved feed efficiency without negatively impacting swine welfare in regards to the stress response.

Researchers: Jessica D. Colpoys, Anna K. Johnson, Nicholas K. Gabler, Department of Animal Science, Iowa State University, Ames, IA. Contact Colpoys at (515) 230-4327 or email [email protected]

 

Acknowledgements

This project was supported by Agriculture and Food Research Initiative Competitive Grant no. 2011-68004-30336 from the USDA National Institute of Food and Agriculture.

Literature cited:

Rydhmer, L., Canario, L., 2014. Behavioral genetics in pigs and relations to welfare. In: Grandin, T., Deesing, M. J. (Eds.), Genetics and the Behavior of Domestic Animals. Academic Press, San Diego, CA, pp. 397-434.    

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