Associations between dry cow therapy, clinical mastitis, and somatic cell count score with milk and fat production in ten New Zealand dairy herds.

Cows in each of 10 dairy herds were assigned to two similar groups on age, breed, production, and previous mastitis history. Only cows in one group received dry cow therapy at the end of their 1977-1978 lactations. Production of all cows was tested at bi-monthly intervals during their 1978-1979 lactations. The composite evening and morning sample for milk fat measurement also was used for somatic cell counting. In the therapy group, 15.4% of cows had clinical mastitis detected by the herd owners during post-treatment lactations compared to a 9% incidence among untreated herd mates. Lactational average milk somatic cell count was higher in cows detected with clinical mastitis but lower in the therapy group. Untreated cows subsequently produced less milk (3706 versus 3966 liters) with this effect most pronounced among untreated cows with higher average cell count scores. Similarly, the depressive effects of detected clinical mastitis on production were most apparent among those cows in which clinical infection was associated with high lactational cell count score. Clinical mastitis depressed production more in Jersey cows than in Friesian-Jersey crossbreds. In these herds selective dry cow therapy may have been preferable to whole herd therapy.