RESUMEN
An experiment was conducted to investigate the reliability of milk fat measurement by the mid-infrared spectroscopic method when analyzing milk fat containing greater than normal amounts of unsaturated fatty acids. Sixteen mid-lactation Holstein cows were divided into four treatments including a control (C), control with bovine somatotropin (C+), bovine somatotropin and added dietary fat from sunflower seeds (Sun+), or bovine somatotropin and added dietary fat from safflower seeds (Saff+). Milks were sampled weekly for 16 weeks (n=256). Unsaturated fatty acid percentages in milk fat were 25.0, 28.4, 39.6, and 37.9 for C, C+, Sun+, and Saff+ treatments, respectively. Milk fat percentages measured by the Mojonnier fat extraction and mid-infrared spectroscopic methods were 2.99, 2.97; 3.06, 3.01; 2.73, 2.56; and 2.86, 2.74 for C, C+, Sun+, and Saff+ treatments, respectively. Results indicate the mid-infrared spectroscopic method underestimates the fat content in milk which is higher in unsaturated fatty acids. Dairy producers feeding diets with added fat from unsaturated fat sources may be underpaid for milk fat content when the milk is analyzed by the mid-infrared spectroscopic method. A possible remedy for this problem may be to have milk plants calibrate the mid-infrared spectroscopic instrument with milk samples containing higher than normal amounts of unsaturated fatty acids in milk fat.