Timing and temperature thresholds of heat stress effects on fertility performance of different parity sows in Spanish herds

Abstract

High temperature is an environmental factor that impairs sow fertility. In this study, we identified the critical weeks for heat stress effects on aspects of fertility performance, namely weaning-to-first-service interval (WSI) and farrowing rate (FR). We also examined the threshold temperatures above which the fertility performance deteriorated and whether there were any differences between parities regarding heat stress effects or thresholds. Performance data of sows in 142 herds from 2011 to 2016 were matched to appropriate weekly averaged daily maximum temperatures (Tmax) from weather stations close to the herds. Two types of ratios (i.e., ratio for WSI and odds ratio for FR) were used to identify the critical weeks for heat stress by comparing the respective measures for two sow groups based on Tmax in different weeks around weaning or service events. The ratios for WSI were calculated between groups of sows exposed to Tmax ≥ 27 °C or <27 °C in each week before weaning, with the Tmax cutoff value based on a recent review study. Similarly, the odds ratios for FR for the two groups were calculated in weeks around service. The weeks with the largest differences in the fertility measures between the two Tmax groups (i.e., the highest ratio for WSI and the lowest odds ratio for FR) were considered to be the critical weeks for heat stress. Also, piecewise models with different breakpoints were constructed to identify the threshold Tmax in the critical week. The breakpoint in the best-fit model was considered to be the threshold Tmax. The highest ratios for WSI were obtained at 1 to 3 wk before weaning in parity 1 and 2 or higher sow groups. The threshold Tmax leading to prolonged WSI was 17 °C for parity 1 sows and 25 °C for parity 2 or higher sows. Increasing Tmax by 10 °C above these thresholds increased WSI by 0.65, and 0.33 to 0.35 d, respectively (P < 0.01). For FR, the lowest odds ratios were obtained at 2 to 3 wk before service in parity 0, 1, and 2 or higher sow groups. The threshold Tmax leading to reductions in FR was 20, 21, and 24 to 25 °C for parity 0, 1, and 2 or higher sow groups, respectively. Increasing Tmax by 10 °C above these thresholds decreased FR by 3.0%, 4.3%, and 1.9% to 2.8%, respectively (P < 0.01). These results indicate that the critical weeks for heat stress were 2 to 3 wk before service for FR and 1 to 3 wk before weaning for WSI. The decreases in fertility performance in parity 0 to 1 sows started at temperatures 3 to 8 °C lower than in parity 2 or higher sows.

Keywords: climatic factor; cross-sectional study; estrus resumption; porcine reproduction; seasonal infertility; segmented regression.

Figure 1.

Map showing the locations in Spain of the 142 pig herds (H) and 31 weather stations (W) used in this study.

Figure 2.

(A) Predicted means and 95% CI of WSI in parity 1 sows exposed to weekly averaged daily maximum temperatures (T_max) from weaning of either ≥27 °C or <27 °C. (B) Ratios and 95% CI of WSI in different parity sows exposed to ≥27 °C T_max relative to those exposed to <27 °C.

Figure 3.

Odds ratios and 95% CI of farrowing rate in different parity sows exposed to weekly averaged daily maximum temperatures from service, of 27 °C or higher relative to those exposed to lower than 27 °C in (A) first-service and (B) return-to-service records.

Figure 4.

Predicted effect of weekly averaged daily maximum temperatures (T_max) on WSI and 95% CI. The T_max in the third week (−21 to −15 d), second week (−14 to −8 d), and first week (−7 to −1 d) before weaning were used for the analyses in parity 1, 2 to 5, and 6 or higher sow groups, respectively. The regression equations after the thresholds for parity 1, 2 to 5, and 6 or higher sow groups were log(y) = 1.8413 + 0.0098 × (x − 17), log(y) = 1.7214 + 0.0061 × (x − 25), and log(y) = 1.6785 + 0.0060 × (x − 25), respectively. The diamond, triangle, and cross symbols indicate point estimates in parities 1, 2 to 5, and 6 or higher sow groups, respectively, predicted by a generalized linear mixed-effect model.

Figure 5.

Predicted effect of weekly averaged daily maximum temperatures (T_max) on farrowing rate in first-service records and 95% CI. The T_max in the third week (−21 to −15 d) and second week (−14 to −8 d) before service were used for the analyses in parity 0 and 1 or higher sow groups, respectively. The regression equations after the thresholds for parity 0, 1, 2 to 5, and 6 or higher sow groups were logit(y) = 1.7560 − 0.0221 × (x − 20), logit(y) = 1.8787 − 0.0330 × (x − 21), logit(y) = 2.0393 − 0.0253 × (x − 24), and logit(y) = 1.8947 − 0.0155 × (x − 25), respectively. The square, diamond, triangle, and cross symbols indicate point estimates in parity 0, 1, 2 to 5, and 6 or higher sow groups, respectively, predicted by a generalized linear mixed-effect model.

Figure 6.

Scatter plots of (A) herd-level mean WSI in parity 1 sows exposed to averaged daily maximum temperatures (T_max) of either ≥27 °C or <27 °C in the third week (−21 to −15 d) before weaning and (B) herd-level farrowing rate in first-service records in sows exposed to either ≥27 °C T_max or <27 °C in the second week (−14 to −8 d) before service and those in sows exposed to. Herds above the dotted line in (A) and below the line in (B) indicate herds with an increase in WSI and decreased farrowing rate, respectively, when sows exposed to ≥27 °C.