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US Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705
ABSTRACT
Michigan dairy herds with continuous Dairy Herd Improvement Association records 1969 to 1978 were identified. Herds that were exposed to polybrominated biphenyl in 1974 at concentrations generally not causing residues in tissue or milk fat greater than .3 µg/g were compared with control herds of comparable size, breed, and location. Analysis of covariance with herd size as a covariable was used to compare measures of herd performance between groups. Relationships of exposure to response within the exposed group were evaluated by multiple regression of the measures on the mean logarithmic concentration of polybrominated biphenyl in tissue with herd size as a second independent variable. Herd size, milk production, and fat production were greater and age was less in exposed herds. Lack of interactions of year by group indicated that all differences predated exposure. Regressions of these characteristics on exposure within years were not significant. Data on other measures were available only for 1976 to 1978. Percent culls, number of culls, and number of new cows were greater for exposed than control herds, but regressions of these characteristics on exposure were negative in some years. Calving intervals were shorter for exposed herds than for control herds, but regressions on exposure were not consistent.
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