Genomic selection for thermotolerance
What's its impact on the heat-stressed lactating sow?
By MaryKate H. Byrd, Luiz F. Brito and Jay S. Johnson
During hotter summer months, lactating sows are at risk of succumbing to heat stress due to the increased metabolic demands of lactation. As a result, feed intake and milk production is often reduced in heat stressed sows and this can decrease litter growth and weaning weights.
Heat stress mitigation efforts by our group have been focused on improving heat stress management through the development of cooling technologies (e.g., sow cooling pads) or decision support tools (e.g., HotHog smartphone application), as well as nutritional supplements that may reduce the negative effects of heat stress on lactating sows. Additionally, our recent work has focused on the use of genomic selection to identify breeding stock that are at reduced risk of suffering from heat stress without compromising productivity traits such as milk production or reproductive efficiency.
On-farm large scale phenotyping efforts were conducted to identify biological indicators of heat stress tolerance in lactating sows. In total, >1,600 sows were phenotyped and genotyped during the summer of 2021 at a commercial multiplier farm in North Carolina (Johnson et al., 2023). Following intensive, on-farm phenotyping efforts, it was determined that the rate of vaginal temperature increase as a function of increasing environmental temperatures was moderately to highly heritable in the studied population.
Using this phenotypic trait along with genomic information, genomic estimated breeding values (EBV; Fig. 1) were calculated and then sows were identified as either heat stress sensitive (SEN) or heat stress tolerant (TOL; Freitas et al., 2023). Sows from the tested population that were considered to be the most SEN and TOL were brought to West Lafayette, Indiana, where in-depth validation studies were conducted to assess the biological impact of genomic selection for thermotolerance on production and welfare relevant traits.
A total of 14 TOL and 12 SEN multiparous lactating sows (Large White x Landrace) and their piglets were enrolled in a study to assess the impact of genomic selection for thermotolerance on milk production and thermoregulatory measures. All pigs were exposed to cyclical heat stress (28 to 32° C) beginning on day 2.5 ± 1.0 post-farrowing until weaning (day 21.3 ± 1.1). Respiration rates and skin temperature (mean temperature of the ear, shoulder, rump and tail) were measured on all sows at 0800, 1200, 1600 and 2000 h daily. Vaginal temperatures were automatically recorded in 5-min intervals daily utilizing thermochron temperature readers attached to a blank controlled internal drug-releasing implant. Total heat production (THP) was measured on 11 TOL and 9 SEN sows and their litters on day 4, 8, 14 and 18 of lactation. The sow THP was separated from the litter THP and was used as an indirect indicator of sow milk production as previously described (Fig. 2; Johnson et al., 2019, 2022).
Overall, TOL sows had increased THP when compared to SEN sows (Fig. 3). Despite this, no feed intake differences were detected between TOL and SEN sows (Fig. 4), likely indicating that the greater THP was driven by increased metabolic rate associated with milk production rather than the heat of nutrient processing. Although THP (and likely milk production) was greater for TOL sows under heat stress conditions, no vaginal temperature differences were detected between TOL and SEN sows (Fig. 5A). This response was likely due to the increase in both overall and maximum daily respiration rate for TOL versus SEN sows (Fig. 5B, C) as a proactive measure to dissipate excess body heat. This increase in respiration rate may also help explain the decrease in skin temperature for TOL relative to SEN sows (Fig. 5D) as they may have been relying more on latent (i.e. increased respiration rate) versus sensible (i.e. heat loss through the skin) heat loss mechanisms.
While additional analyses are currently ongoing, these data provide initial evidence that genomic selection for thermotolerance and improved welfare under heat stress conditions may be possible without sacrificing production relevant traits. Additional research to understand the phenotypic variability and biological mechanisms underpinning the genomic and epigenomic background of thermotolerance in pigs and its influence on the gestating fetus is currently ongoing.
AcknowledgementsThis research was supported by an Agriculture and Food Research Initiative Competitive Grant (2020-67015-31575) from the USDA National Institute of Food and Agriculture. We would also like to acknowledge the other co-authors of this research including Hui Wen, Pedro Freitas, Sharlene Hartman, Jacob Maskal, Yijian Huang, Francesco Tiezzi, Christian Maltecca and Allan Schinckel.
ReferencesFreitas, P. H. F., J. S. Johnson, H. Wen, J. M. Maskal, F, Tiezzi, C. Maltecca, Y, Huang, A. E. DeDecker, A. P. Schinckel, and L. F. Brito, 2023. Genetic parameters for automatically measured vaginal temperature, respiration efficiency, and other thermotolerance indicators measured on lactating sows under heat stress conditions. Genet. Sel. Evol. 55: 65. doi: 10.1186/s12711-023-00842-x.
Johnson, J. S., T. L. Jansen, M. Galvin, T. C. Field, J. R. Graham, R. M. Stwalley, and A. P. Schinckel, 2022. Electronically controlled cooling pads can improve litter growth performance and indirect measures of milk production in heat-stressed lactating sows. J.Anim. Sci. 100: skab371. doi: 10.1093/jas/skab371.
Johnson, J. S., H. Wen, P. H. F. Freitas, J. M. Maskal, S. O. Hartman, M. Byrd, J. R. Graham, G. Ceja, F. Tiezzi, C. Maltecca, Y. Huang, A. DeDecker, A. P. Schinckel, and L. F. Brito, 2023. Evaluating phenotypes associated with heat tolerance and identifying moderate and severe heat stress thresholds in lactating sows housed in mechanically or naturally ventilated barns during the summer under commercial conditions. J. Anim. Sci. 101: skad129. doi: 10.1093/jas/skad129.
Johnson, J. S., S. Zhang, G. M. Morello, J. M. Maskal, and N. L. Trottier, 2019. Technical note: Development of an indirect calorimetry system to determine heat production in individual lactating sows. J. Anim. Sci. 97: 1609–1618. doi: 10.1093/jas/skz049.
Byrd is a graduate research assistant and Brito is an associate professor of animal science, both at Purdue University. Johnson is the supervisory research animal scientist of the USDA-ARS Livestock Behavior Research Unit in West Lafayette, Indiana.
