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Factors Contributing to Variation in Reported Milk Component Percentage in Holstein and Jersey Milk

Department of Dairy Science, College of Agriculture and Life Sciences, Virginia Agricultural Experiment Station, Virginia Polytechnic Institute and State University, Blacksburg 24061-6999

ABSTRACT

Precision of milk fat, protein, and lactose determinations by Multi-Spec operators in the state DHIA laboratory was studied and milk fat values were compared with Babcock determinations and with pay plant reports. Also, day-to-day variation in milk component content was examined in sample herds of 24 Holstein and 24 Jersey cows for a 10-d period. Repeatabilities from 12 sets of 6 blind samples showed Multi-Spec technicians varied in precision of milk fat results (range: r = .944 to .999). Precision of Babcock results were similar to the best Multi-Spec results (r = .998). Mean Babcock results were .065% greater than Multi-Spec results. Across both breeds, mean within-cow repeatabilities for day-to-day milk fat (r = .75) was lower than for protein (r = .90) and lactose (4 = .87) and was higher for Holsteins (r = .80) than for Jerseys (r = .69). Proportions of Multi-Spec determinations of composite milk components that failed to measure between component values from morning and afternoon samples were 16.5, 32.9, and 44.2%, respectively for fat, protein, and lactose. Mean pay plant test on bulk tank milk was .02% higher than Multi-Spec test and .03% lower than Babcock results. These data indicate that variation in herd milk component tests results from variation in determinations as well as from biological variation in milk component content of cattle.