Durability of Traits: Nonlinear Association between Initial and Subsequent Measures

Subjective linearized scores were recorded for milking speed, fore udder smoothness, shoulder looseness, and udder depth on 7357 and 3730 Holstein cows during first and second lactations. Random sire effects in threshold models were estimated for each trait and lactation. There were 95 sires that had estimated effects for all the traits in both lactations. For each trait, sire effects for first and second lactations were used as independent and dependent variables, respectively, in quadratic regression. Sire effects corresponding to milking speed in second lactation had linear relationship to sire effects corresponding to milking speed in first lactation. Similar relationships for udder smoothness, shoulder looseness, and udder depth appeared nonlinear. The quadratic terms associated with prediction equations for shoulder looseness and udder depth were significant. Nonlinear associations between genetic evaluations in first and second lactations may have resulted from aging.