23 Sustainable Livestock Breeding with a Focus on Heat Stress

The genetic selection is a form of optimization. Traits in explicit or implicit selection index are improved, while other traits change according to genetic correlations among the traits. After long selection, usually production traits improve while fitness traits deteriorate. At the gene level, it seems that genes associated with positive effects on major traits tend to fixation, and remaining genes with large effect exhibit pleiotropy. While selection for fitness traits is moderately successful because of small heritability, constantly improving management counterbalances the deterioration of fitness traits. Treating the selection as an optimization raises a question about selection limits, particular with feed conversion. Latest experiences suggest that eventually strong selection for residual feed intake reduces fertility and increases late mortality. The environment changes, with climate change having a profound effect. The industry faces a choice of either breeding for new conditions or devising new managemental modifications. The choice depends on species as the environment can be more easily improved for animals in confinement or raised in intensive conditions (pigs, broilers, dairy cattle), but less for animals on pasture (beef cattle and sheep). The choice depends on financials and speed. An interesting example of such a trade-off is in dairy industry approach to mitigate heat stress, which results in plummeting fertility, decreased milk yield, and premature culling leading to insufficient replacements. To counter these challenges, a methodology for genetic evaluation for heat tolerance was developed and applied to US national data for Holsteins. Instead of genetics, the industry implemented managemental modifications: improved cooling devices, timed AI increased fertility, and the use of sexed semen. In pigs and chicken, that the response to stress is less in nucleus and stronger in commercial environment, and selection for commercial performance is paramount. In summary, mitigations of stress involve a combination of genetic and environmental factors.