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Comparative Accuracy of Small Lactometers for Determining the Total Solids of Individual Cows' Milk^{1, 2,}

Department of Dairy, Michigan State University, East Lansing

ABSTRACT

Milks were obtained monthly from approximately 350 cows of live dairy breeds. Averaged total solids measured with the small Watson lactometer did not differ significantly (except three Brown Swiss milks, 6% fat) from those measured gravimetrically. Correlations between the small Watson lactometer and gravimetric measurements exceeded 0.9 when fat levels were ignored, were lower when the data were grouped by fat percentages, and mostly were statistically significant. Standard errors of estimate ranged from 0.14 to 0.39, indicating errors in total solids equivalent to 0.12% fat by the Babcock method. Field use of the small Watson lactometer can be recommended, provided temperature and reading techniques are controlled.

Average total solids measured by the small Quevenne lactometer differed significantly from the gravimetric measurements in milk from several breed and fat level groups. Correlations and standard errors were of the same magnitude as those obtained with the small Watson lactometer. A significant interaction between methods (gravimetric vs. Quevenne lactometer) and fat levels was obtained when measurements for all breeds were combined, indicating an incapacity of the small Quevenne lactometer and/or the estimation equation to measure accurately total solids at various fat levels. This bias was positive at low fat and negative at high fat levels, indicating that the small Quevenne lactometer is not as accurate as the Watson lactometer.

¹ Journal Article No. 2196 from the Michigan Agricultural Experiment Station.

² These studies were supported in part by the Dairy Cattle Research Branch of the Agricultural Research Service, USDA.