Genetic analysis of longevity in the Canadian and New Zealand dairy herds

Two longevity measures, duration of productive life (DP) and duration of total life (DT) were investigated. Within each of these measures, two traits were defined based on the definition of failure of the cows. The first definition considered cows to have failed only if they were disposed of due to involuntary reasons giving functional duration of productive life (/DP). The second considered cows to have failed regardless of whether disposal was voluntary or otherwise yielding true duration of productive life (rDP). The same scheme was followed when defining functional duration of total life (/DT) and true duration of total life (/DT). These four traits were studied by modeling the hazard rate of daughters of Canadian and New Zealand Holstein sires. The data used was derived from a field trial involving the mating of 20 proven Holstein sires from each country to Holstein cows in 20 New Zealand and 10 Canadian herds. The New Zealand data set consisted of 898 daughters of Canadian and New Zealand sires. In Canada, 239 daughters of Canadian and New Zealand sires were available. In order to measure the cow's true and functional duration of productive and total life, Cox's proportional hazard models were constructed In both Canada and New Zealand, daughters of Native sires had lower hazard rates. The results suggested that daughters of imported sires were culled at higher rates relative to daughters of native sires. Heritabilities for the traits studied were estimated and ranged from 0.03 to 0.29. The results from analysis of genetic correlations between the four traits in the same environment suggested that it is not the measure of longevity that is important but the definition of failure in the Canadian environment. Genetic correlations between the same traits in different environments suggested the existence of genotype by environmental interactions at both macro and micro levels suggesting that sires must be evaluated for superiority of survival traits in the environment within which their progeny are expected to perform.Land and Food Systems, Faculty ofGraduat