Disease, production and culling in Holstein-Friesian cows. IV. Effects of disease on production

The effects of common clinical and two subclinical disease conditions (mastitis and ketosis) on milk production and reproductive performance in Holstein-Friesian cows were investigated in a two and a half year study. The data used were from 1315 lactation records and results from tests performed on 6241 composite milk samples. Path analyses were used to estimate the direct and indirect impact of disease on days to conception, milk production in a standard 305 day lactation and milk production per day. Indirect effects accounted for a large percentage of the total effect of some diseases on production. The path coefficients were quite sensitive to the units used to measure production, with deviation from the herd average being the preferable measure. These units controlled confounding due to herd level variables. Effects identified by the path analyses were substantially different from those determined on the basis of unconditional associations between diseases and production parameters. Subclinical mastitis and the reproductive diseases (most of which were subclinical) have the greatest detrimental impact on milk production per day. Several disease conditions had a beneficial effect on milk production and this was attributed to the therapy provided to cows affected with these conditions. All conditions related to reproductive performance had a detrimental effect on days to conception..