Technical note: colorimetric methods for accurate determination of nutrient composition in beef cow colostrum and milk.

Our objectives were to develop colorimetric methods to accurately measure nutrient concentrations of beef cow colostrum and milk, to determine if the yield of colostrum from a single rear quarter is representative of complete collection of colostrum in beef cows, and to compare data from our developed colorimetric methods with Fourier transform infrared spectroscopy (FTIR) analysis to determine the accuracy of FTIR for beef cow colostrum and milk. In Exp. 1, colostral weight and volume of the most full rear quarter were compared with complete collection of colostrum from post-calving, unsuckled beef heifers. Both volume and weight had r2 = 0.85 (P < 0.001) between single-quarter and 4 quarter yields. In Exp. 2, colostrum (n = 35) and milk at d 35 (n = 42) and d 60 (n = 38) of lactation were collected from multiparous, fall-calving, crossbred beef cows. Subsamples were submitted for FTIR analysis and frozen for colorimetric analysis. Colorimetric analyses were developed for lactose, triglycerides (measure of fat), protein, and urea N. To validate method accuracy, spike recoveries were determined for lactose (96.8% average) and milk protein (100.1% average), triglyceride concentration was compared with fat concentration determined by the Mojonnier method (r2 ≥ 0.91, P < 0.001), and colostral or milk urea N was compared with serum urea N from the same sampling day (r2 ≥ 0.72, P < 0.001). Coefficients of determination between colorimetric methods and FTIR were determined for colostrum, d 35 milk, and d 60 milk. Colostral lactose concentration from FTIR was positively associated (r2 = 0.24, P = 0.01) with colorimetric analysis, but there was no relationship (r2 ≤ 0.09, P ≥ 0.14) between methods for colostral fat, protein, or urea N. Milk nutrient composition was positively associated for all nutrients measured at d 35 (r2 = 0.28 to 0.58, P < 0.001), and coefficients of determination strengthened for all nutrients measured at d 60 (r2 = 0.38 to 0.82, P < 0.001). In conclusion, colostrum yield of a single rear quarter can be used to indicate complete collection of colostrum for beef cows, and colorimetric methods developed have adequate accuracy for beef cow colostral and milk nutrient analysis. Based on our analyses, nutrient composition of beef cow colostrum was not accurately analyzed by FTIR. Accuracy of FTIR for beef cow milk varies with component and may be affected by the day of lactation.