Effects of Genetics on Thermal Regulatory Responses to Repeated Heat Stress Exposure in Pigs

Heat stress (HS) accounts for over $900 million loss in the US swine industry annually. In addition, HS negatively impacts animal welfare. Evidence suggests that efficient, fast growing pigs may be more susceptible to HS. Study objectives were to investigate the effects of genetics on respiration rate (RR) and skin (ST) and rectal temperatures (RT) in response to repeated exposure to HS. A total of 97 animals from a commercial line and a divergent line selected for high and low residual feed intake (RFI) where subjected three times to a 4-d HS load that was preceded by a 9-d thermal neutral (TN) adaptation period and alternated by 7-d TN conditions: 1-TN adaptation, 2-HS, 3-TN, 4-HS, 5-TN, 6-HS, and 7-TN. RR, ST and RT were measured daily in each period. Mixed models with a repeated statement were fitted to describe the individual data. RR, ST and RT were higher during HS compared with TN conditions (P<0.0001; Table 1). RR was positively correlated with ST during TN (r=0.27, P<0.0001), but not during HS (r=0.03). Commercial pigs had higher RR than low and high RFI pigs during TN (P<0.05); high RFI pigs had higher RR than commercial and low RFI pigs during HS (P<0.05). During TN, ST and RT were higher in commercial pigs than in low and high RFI pigs, and higher in high RFI than in low RFI pigs (P<0.001). During HS, ST and RT were higher in commercial pigs than in low and high RFI pigs (P<0.01). Results indicate that fast growing commercial pigs had higher body temperatures that reached fever temperatures during HS. This suggests that fast growing animals may be more affected by HS, which may negatively impact their welfare. Selection for increased feed efficiency may eventually result in pigs that are less affected by heat stress.

	RR (bpm)		ST (⁰C)		RT ST (⁰C)
	TN	HS	TN	HS	TN	HS
Commercial	42 ± 1	93 ± 2	33.8 ± 0.1	37.8 ± 0.1	39.4 ± 0.02	40.0 ± 0.03
Low RFI	34 ± 1	92 ± 1	32.2 ± 0.1	37.3 ± 0.1	39.1 ± 0.02	39.4 ± 0.03
High RFI	39 ± 1	100 ± 2	32.5 ± 0.1	37.4 ± 0.1	39.2 ± 0.02	39.5 ± 0.03

Table 1. Least square means ± s.e. for RR, ST and RT for pigs from a commercial, low RFI and high RFI line, during HS and TN conditions.