RESPONSE OF HEALTH-CARE TO SELECTION FOR MILK-YIELD OF DAIRY-CATTLE

A selection experiment began during 1964 to measure long-term response s to selection of dairy cattle based only on milk yield. An unselected control group of cows was maintained as part of the experimental desi gn. The four highest sires for PTA milk that were available from activ e 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 197 5 to 1990. Over the 16-yr period, 236 selection and 227 control cows w ere 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 a nalysis across parities. Expenses for mastitis accounted for most of t he difference between genetic groups. Most health expense occurred dur ing the first 20 d postpartum and increased for cows with successive l actations. During the 16-yr period, expense for selection cows increas ed more than for controls for reproduction, digestion, and ketosis, bu t not mastitis, udder (nonmastitis), edema, locomotion, milk fever, re spiration, and other categories. Separate analyses of heifers from bir th to initial calving and of dry cows provided little evidence of diff erences for genetic groups during nonlactating stages of life.