Characterizing the acute heat stress response in gilts: I. Thermoregulatory and production variables

Abstract

Identifying traits associated with susceptibility or tolerance to heat stress (HS) is a prerequisite for developing strategies to improve efficient pork production during the summer months. Study objectives were to determine the relationship between the thermoregulatory and production responses to acute HS in pigs. Prepubertal gilts (n = 235; 77.9 ± 1.2 kg BW) were exposed to a thermoneutral (TN) period (P1, 24 h; 21.9 ± 0.5 °C, 62 ± 13% RH; fed ad libitum) followed immediately by a subsequent acute HS period (P2, 24 h; 29.7 ± 1.3 °C, 49 ± 8% RH; fed ad libitum). Rectal temperature (TR), skin temperature (TS), and respiration rate (RR) were monitored and BW and feed intake (FI) were determined. All pigs had increased TR, TS, and RR (0.80 °C, 5.65 °C, and 61.2 bpm, respectively; P < 0.01) and decreased FI and BW (29% and 1.10 kg, respectively; P < 0.01) during P2 compared to P1. Interestingly, body temperature indices did not explain variation in FI during P2 (R2 ≤ 0.02). Further, the percent change in BW during P2 was only marginally explained by each body temperature index (R2 ≤ 0.06) or percent change in FI (R2 = 0.14). During HS, TR was strongly correlated with P1 TR (r = 0.72, P < 0.01), indicating a pig's body temperature during TN conditions predicts the severity of hyperthermia during HS. Additionally, the change in TR (ΔTR, HS TR - TN TR) was larger in pigs retrospectively classified as susceptible (SUS) as compared to tolerant (TOL) pigs (1.05 vs. 0.51 °C, respectively; P < 0.01). In summary, thermoregulatory responses and production variables during acute HS are only marginally related. Further, changes in BW and FI were unexpectedly poorly correlated during acute HS (r = 0.34; P < 0.01). Collectively, suboptimal growth is largely independent on the thermoregulatory response and hypophagia during acute HS. Consequently, incorporating solely body temperature indices into a genetic index is likely insufficient for substantial progress in selecting HS tolerant pigs.

Figure 1.

Box and whisker plots of rectal temperature (T_R; A), skin temperature (T_S; B), and respiration rate (RR; breaths per minute [bpm]; C) distributions during period (P) 1 (24-h thermoneutral [TN] conditions) and subsequently P2 (24-h heat stress [HS]). Considerable variation exists between animals with respect to their thermoregulatory response during HS as compared to TN conditions. Whiskers denote the minimum and maximum value for each P. The bottom and top boundaries of each box represents the first and third quartiles, respectively, while the inner middle line marks the median during each P.

Figure 2.

Relationships between rectal temperature (T_R) and the change (Δ) in feed intake (FI) of pigs during period (P) 2 (heat stress [HS] conditions; A) and of HS T_R with the ΔBW (B). The ΔFI and ΔBW variables were determined by subtracting the P2 measurement from P1 (thermoneutral [TN] conditions).

Figure 3.

Correlations between average rectal temperature (T_R) and skin temperature (T_S) of pigs during period (P) 2 (heat stress [HS] conditions; A) and of T_R with respiration rate (RR; B) during P2. Just as with T_R, all T_S and RR data collected 4 h after HS initiation in P2 were condensed into single averages for each experimental unit.

Figure 4.

Correlations between average rectal temperature (T_R) of pigs during period (P) 1 (thermoneutral [TN] conditions) and P2 (heat stress [HS] conditions; A) and of TN T_R with change (Δ) in T_R (B). All TN T_R’s recorded during P1 were condensed into a single average to represent each individual pig’s basal core temperature. Rectal temperatures recorded 4 h after HS initiation were condensed into an average to represent core body temperature during P2. The difference in core body temperature (ΔT_R) was calculated by subtracting TN T_R from the HS T_R.

Figure 5.

Correlations between the percent change (Δ) in feed intake (FI) and the ΔBW.

Figure 6.

Thermoregulatory ability of pigs retrospectively classified as tolerant (TOL) and susceptible (SUS) to HS during period (P) 1 (thermoneutral [TN] conditions) and P2 (heat stress [HS] conditions). Each gilt’s tolerance to the heat load was classified on the basis of HS rectal temperature (T_R; i.e., higher and lower HS T_R were considered markers of HS susceptibility or tolerance, respectively). For each of the five replications, the 10 most TOL (n = 50) and SUS (n = 50) were identified and allocated to their respective retrospective treatments for respective retrospective treatments for T_R (A) and the percent change in BW (ΔBW; B) data analysis. ^a–dValues with differing superscripts denote differences (P ≤ 0.05) between treatments.