Determining Fat, Protein, and Lactose in Milk from Cows Fed Formaldehyde Protected Sunflower-Soybean Supplement

Abstract

To determine whether feeding cows protected lipid supplement influences reliability of analyses of milk composition, we measured fat, protein, and lactose in milk produced by two groups of six Holstein cows, one fed a conventional hay-concentrate ration and one a ration containing protected sunflower-soybean supplement. Analyses were of 36 samples from each group collected biweekly over 12 wk. The three constituents were measured by an Infrared Milk Analyzer. Reference methods were Babcock test for fat, Udy dye-binding for protein, and a polarimetric method for lactose. Fatty acid composition was determined by gas chromatography, and three nitrogen fractions (total, noncasein, and nonprotein) were measured by Technicon Auto Analyzer. The differences, reference method minus infrared, for cows fed the conventional and protected lipid rations, were for fat .09 and .46%, for protein −.02 and −.08%, and for lactose .06 and .23%. Thus, agreement between infrared and reference methods was closer for cows fed conventional rations than for those fed protected lipid supplement. For cows fed the supplement, the low fat by infrared was attributed to an increase in the mean molecular weight of the fatty acids. Changes in fatty acid composition of milkfat when cows are fed protected sunflower-soybean supplement causes errors in estimation of fat by infrared analysis unless the instrument is calibrated specially for the milk.