Heritabilities of and genetic relationships between salmonella resistance traits in broilers

Using experimental infections, three traits for salmonella resistance were studied: mortality, survival time (in animals that died by infection), and quantitative cecal salmonella carriage at the end of the rearing period (in animals that did not die). In total, 548 animals were used; mortality was 29.2%, mean survival time was 5.97 d (n = 160), and the mean (10)log of col ony forming units per gram of cecal contents was 1.62 (n = 387). Genetic pa rameters were evaluated in bivariate threshold-linear models to account for the selective measurement of survival time and cecal carriage. Heritabilit ies were .06 for survival time, .09 for cecal carriage, and .12 for mortali ty. The genetic correlation between mortality and cecal carriage was weak ( .26), which suggests that these traits are largely controlled by different genes. The genetic correlation between mortality and survival time was rela tively strong (-.68). Simultaneous study of multiple traits seems to be of particular importance in judging epidemiological consequences of a possible selection for resistance. Results here indicate that selection on decrease d mortality could be unfavorable for the spread of salmonella because the r esulting correlated increase in survival time, implying longer shedding by infected animals, is relatively stronger than the correlated decrease in le vel of cecal carriage. Selection to reduce the level of cecal salmonella ca rriage could be done while keeping survival time constant, if so desired, b ecause the correlation between these traits is weak (-.15).