Effects of Genetics on Growth and Feed Intake in Response to Repeated Exposure to Heat Stress

Heat stress (HS) accounts for over $900 million loss in the U.S. swine industry annually due to poor reproduction, slow growth and reduced feed efficiency. Selection for improved lean tissue growth may increase susceptibility to HS. Study objectives were to investigate the effects of genetics on growth and feed intake in response to repeated exposure to HS. A total of 97 animals from three genetic lines (commercial, low residual feed intake (RFI), high RFI) where subjected three separate times to a 4-d HS load, 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. Body weight was recorded at the start and end of each period and ad libitum feed intake was recorded daily. Average daily body weight gain (BWG) and daily feed intake (FI) were calculated for each period. Feed efficiency was estimated as BWG/FI. HS negatively affected BWG, FI and BWG/FI in all three lines (Table 1; P<0.05). Commercial pigs grew faster than both low and high RFI pigs (P<0.0001) but only in TN periods 1, 3, 5, and 7 (P<0.0001). Pigs of the commercial line ate more than pigs of both the low and the high RFI lines (P<0.0001); pigs of the high RFI line ate more than pigs of the low RFI line (P<0.05). BWG/FI decreased from 0.32 to 0.03 in the commercial line, from 0.21 to 0.06 in the low RFI line and from 0.19 to 0.07 in the high RFI line between periods 5-TN and 6-HS (P<0.0001). Commercial pigs ate more and grew faster than the low and high RFI lines, but also appeared to be more susceptible to HS.

	FI (kg/d)			BWG (kg/d)			BWG/FI
	TN	HS		TN	HS		TN	HS
Commercial	3.15 ± 0.04	2.61 ± 0.04		1.09 ± 0.04	0.51 ± 0.05		0.35 ± 0.02	0.20 ± 0.02
LowRFI	2.57 ± 0.04	2.04 ± 0.04		0.73 ± 0.03	0.43 ± 0.04		0.28 ± 0.02	0.23 ± 0.02
HighRFI	2.71 ± 0.04	2.22 ± 0.04		0.69 ± 0.04	0.51 ± 0.05		0.25 ± 0.02	0.25 ± 0.02

Table 1. Least square means ± s.e. for FI, BWG and BWG/FI for pigs from a commercial, low RFI and high RFI line, during heat stress (HS) and thermoneutral (TN) conditions.