Disease-induced variability of genetic correlations: Ascites in broilers as a case study

Breeding against a production disease is complicated by multiple relationsh ips between productivity, disease, and environment. Ascites in broilers is such a disease. The combination of the reasonably well understood etiology (a physiological/pathological cascade due to inadequate oxygen supply) and the practical relevance makes ascites a relevant case for demonstrating and partly revealing these complex relationships. Chickens (n = 2,788) were te sted in an ascites-challenging (cold) environment. Genetic analysis of mort ality and pathology in combination with performance and physiological trait s (especially blood gas traits) revealed ample opportunities for selection against ascites expression. The genetic correlation structure indicated tha t different mortality traits and pathology traits roughly represent one com mon characteristic. Direct selection against pathology is more effective th an selection on the basis of growth or blood gas traits. The observed negat ive correlation (-0.26) between productivity and ascites was unexpected. Fr om the etiology of ascites (inadequate supply of oxygen relative to the dem and), a positive (unfavorable) correlation was expected. To demonstrate tha t the actual disease occurrence caused this apparent contradiction, the dat a from the undiseased subpopulation were reanalyzed. In the undiseased subp opulation, the genetic correlation between productivity and ascites was pos itive (0.29). This discrepancy was confirmed by comparing regression of asc ites expression on actual performance with regression of ascites on indepen dently assessed performance breeding values. The lability of the genetic co rrelation was explained from complex interactions between productivity, dis ease susceptibility, and actual occurrence of the disease. The revealed mec hanism can be generalized to other production-related diseases and results in systematically lower genetic correlations between disease and productivi ty. It was inferred that genetic correlations between productivity and such diseases will always be prone to the demonstrated environmental sensitivit y, which complicates index selection against production-related diseases.