The inclusion of six disease traits in the Holstein Net Merit Index and three other economic indexes will likely result in only a subtle reranking of bulls. For example, the correlation off the current formula Net Merit $ index is .994 with the new index incorporating the health traits.
The six traits, launched with the April 2018 sire run, include hypocalcemia, displaced abomasa, ketosis, mastitis, metritis and retained placentas. The heritability of each trait is low: Hypocalcemia, 0.6%; DAs, 1.1%; ketosis, 1.2%, mastitis, 3.1%; metritis, 1.4%, and RPs, 1%. Nevertheless, genetic resistance to the traits is additive and permanent, so over time and over generations, disease resistance can be built up and accumulated.
The health traits will make up 2.3% of Net Merit $, 1.9% of Cheese Merit $, 2.3% of Fluid Merit $ and 2.1% of Grazing Merit $.
The economic value of each health trait was estimated averaging two recent research studies, including one study that looked at health treatment costs from eight herds in Minnesota. Also factored in were yield losses associated with health disorders that were not fully accounted for in the research studies, say officials at the Council for Dairy Cattle Breeding.
Here's the weighting of traits in 2018 Net Merit $ for Holsteins:
Daughter Pregnancy Rate—6.7%
Calving Ability $--4.8%
Somatic Cell Count—4%
Health Traits $—2.3%