Factors associated with colostral specific gravity in dairy cows

The objectives of this study were to identify factors associated with colos tral specific gravity in dairy cows, as measured by a commercially availabl e hydrometer (Colostrometer), Colostral specific gravity was measured in 10 85 first-milking colostrum samples from 608 dairy cows of four breeds on a single farm during a 5-yr period. Effects of breed, lactation number, and m onth and year of calving on colostral specific gravity were determined, as were correlations between colostral specific gravity, nonlactating period l ength, and 305-d yields of milk, protein, and fat. For 75 multiparous Holst ein cows, relationships between colostral specific gravity, colostral IgG(1 ), protein, and fat concentrations, and season of calving were determined. Colostral specific gravity values were lower for Brown Swiss and Ayrshire c ows than for Jersey and Holstein cows, and lower for cows entering first or second lactation than third or later lactations. Month of calving markedly affected colostral specific gravity values, with highest values occurring in autumn and lowest values in summer. In multiparous Holstein cows, colost ral specific gravity was more strongly correlated with colostral protein co ncentration (r = 0.76) than IgG(1) concentration (r = 0.53), and colostral protein concentration varied seasonally (higher in autumn than summer). Our results demonstrate that colostral specific gravity more closely reflects colostral protein concentration than IgG(1) concentration and is markedly i nfluenced by month of calving. These results highlight potential limitation s of using colostral specific gravity as an indicator of IgG(1) concentrati on.