358 Comparison of analytical methods for determination of nutrient concentration in beef cow colostrum and milk.

Our primary objective was to determine the validity of Dairy Herd Improvement Association (DHIA) nutrient composition analysis for beef cow colostrum and milk by comparing DHIA with wet chemistry assays for protein, lactose, and milk urea nitrogen (MUN). A secondary objective was to analyze the change in milk nutrient composition over time in beef cattle. Colostrum and milk were collected from multiparous, fall-calving, Sim-Angus cows [n = 43; BW: 649 ± 80 (SD) kg; BCS: 5.3 ± 0.5; age: 4.3 ± 1.2 yr]. Colostrum was collected from a single rear-quarter pre-suckling, and milk was collected during 4-h milk yields at 35 and 60 d post-calving. Sub-samples were sent for DHIA analysis and frozen for colorimetric assays utilizing commercial kits run on a UV-visible light microplate reader. Analytical methods were compared using a general linear model, and nutrient composition across day was analyzed using a mixed model with day as a repeated effect. When comparing wet chemistry and DHIA colostrum analysis, r² were 0.142 (P = 0.06), 0.348 (P = 0.002), and 0.0004 (P = 0.91) for protein, lactose, and MUN, respectively. Colostrum protein analyses had a negative relationship, whereas lactose was positive. For 35 d milk, r² were 0.217 (P = 0.002), 0.128 (P = 0.02), and 0.248 (P = 0.001) for protein, lactose, and MUN, respectively. Comparing 60 d milk, r² (P < 0.001) were 0.706, 0.314, and 0.381 for protein, lactose, and MUN, respectfully. All milk relationships were positive. There was an effect of day on nutrient concentrations, where colostrum contained greater (P < 0.001) protein than 35 and 60 d milk, whereas lactose and MUN were greatest (P < 0.001) in 35 d milk, and lowest (P < 0.001) in colostrum. These data indicate that beef cow colostrum and milk require wet chemistry analysis for improved accuracy. [ABSTRACT FROM AUTHOR]