Response of health care to selection for milk yield of dairy cattle.

A selection experiment began during 1964 to measure long-term responses to selection of dairy cattle based only on milk yield. An unselected control group of cows was maintained as part of the experimental design. The four highest sires for PTA milk that were available from active AI were mated each year to cows of all generations in the selection group. Expenses for veterinary treatment, health supplies, drugs, and labor of animal attendants were recorded for each cow and categorized as mastitis, udder, edema, locomotion, digestion, ketosis, milk fever, reproduction, respiration, and other. Cows studied were born from 1975 to 1990. Over the 16-yr period, 236 selection and 227 control cows were observed while lactating. Cows in the selection group had greater health expenses as a correlated response to increased milk yield than did unselected controls. Lactational difference for genetic groups was $28.22 from an analysis of only first lactations and $49.44 from an analysis across parities. Expenses for mastitis accounted for most of the difference between genetic groups. Most health expense occurred during the first 20 d postpartum and increased for cows with successive lactations. During the 16-yr period, expense for selection cows increased more than for controls for reproduction, digestion, and ketosis, but not mastitis, udder (nonmastitis), edema, locomotion, milk fever, respiration, and other categories. Separate analyses of heifers from birth to initial calving and of dry cows provided little evidence of differences for genetic groups during nonlactating stages of life.