Genetic parameters of milk ELISA scores for Johne's disease.

The objective of this study was to estimate genetic parameters of antibody response to Mycobacterium avium ssp. paratuberculosis using routinely collected Minnesota Dairy Herd Improvement milk ELISA tests. After all edits, 25,809 tests from 21,514 Holstein cows in 282 Johne's positive herds were available for analysis. The Johne's test results were analyzed both as a binary trait (positive or negative) and as a linear trait as the transformed ELISA optical density [ln(OD)]. Significant fixed effects in the model were age at test date, days in milk, and laboratory negative control; random effects were herd test date, animal effect, and permanent environment effect. Transformed ELISA optical density increased with age at test day and days in milk. Heritability estimates ranged from 0.065 to 0.095. Percentage of variation explained by maternal effects ranged from 1.3 to 2.29%. Repeatabilities ranged from 0.380 to 0.433. Statistically significant correlations between the sire solutions for ln(OD) for 154 bulls with at least 30 daughters in the analysis and their USDA predicted transmitting abilities were as follows: fat yield, -0.199; protein yield, -0.179; productive life, -0.292; and Net Merit, -0.339. These correlations suggest that selection for productive life or Net Merit also will improve resistance to Johne's disease.