PS1-18. A Preliminary Analysis of the Relationship Between Surface Area-To-Mass Ratio and Thermoregulatory Response in Gilts Exposed to Variable Heat Loads During Early and Late Gestation.

Abstract The study objective was to evaluate the relationship between surface area–to–mass ratio and thermoregulatory responses in gestating gilts exposed to variable heat loads. Twenty-six pregnant gilts were assigned to either early gestation (EG; gestation d 0 to 52; n = 13; 129.6 ± 6.2 kg) or late gestation (LG; gestation d 52 to 105; n = 13; 142.7 ± 5.0 kg) groups. Within each group, gilts were exposed to one of four cyclical environments: thermoneutral (TN; 17 to 21 °C; n = 7), mild (MILD; 24 to 26 °C; n = 6), moderate (MOD; 26 to 32 °C; n = 6), or severe (SEV; 28 to 36 °C; n = 7) heat stress. Respiration rate (RR) and skin temperatures of the ear, shoulder, rump, and tail were measured daily at 0800, 1200, 1600, and 2000 h, and skin temperatures from the four anatomical regions were averaged to generate a single composite measurement (Tskin). Vaginal temperature (Tv) was continuously recorded in 15-min intervals using implanted data loggers. Body weight (BW) was measured. On the final treatment d, whole-body three-dimensional imaging was conducted to estimate individual body surface area (SA), volume (VOL), and SA:VOL. Derived ratios of SA to the mean BW (SA:meanBW) over the entire period and the final BW (SA:finalBW) were calculated. Daytime TV (dayTv; 0800-2000 h) and nighttime TV (nightTv; 2000-0800 h) were also determined. Individual gilt was the experimental unit. During EG, RR was increased (P < 0.01) in SEV (74 ± 1 bpm) and MOD (75 ± 1 bpm) compared to MILD (55 ± 1 bpm) and TN (30 ± 1 bpm) gilts. In LG, SEV gilts had the greatest RR (P < 0.01; 97 ± 2 bpm), followed by MOD (75 ± 1 bpm), MILD (63 ± 2 bpm), and TN (31 ± 2 bpm). In EG, the Tskin of SEV was greater (P < 0.01; 36.71 ± 0.16 ºC) than that of MILD (34.95 ± 0.23 ºC) and TN (29.25 ± 0.23 ºC) gilts, and MOD gilts (36.20 ºC ± 0.14 ºC) were greater than MILD and TN gilts. In the LG, SEV gilts had greater Tskin (P < 0.01; 36.20 ± 0.24 ºC) than the MILD (34.90 ± 0.41 ºC) and TN (29.46 ± 0.34 ºC) gilts. Although, TV, RR, and Tskin were not affected (P > 0.10) by SA:meanBW, SA:finalBW, or SA:Vol, downward numerical trends in Tv and RR were observed as SA ratios increased. Vaginal temperatures and RR parameters were influenced by ambient temperature (P = 0.01), whereby for every 1.0 ºC increase in temperature, Tv and RR increased by 0.03 ºC and 5.3 bpm, respectively. Temperature was the dominant driver of physiological responses, with surface area-to-mass ratios playing a minimal role.