Checking Colostrum Quality
Have you used a Colostrometer or Brix refractometer?
They are on-farm tools for assessing the antibody concentration in bovine colostrum. Neither of them directly measure antibodies. Rather, they use highly correlated indexes to make rough estimates of actual antibody concentrations.
In a recently published article (Bartier, A. L. and Others, "Evaluation of on-farm tools for colostrum quality measurement" Journal of Dairy Science 98:1878-1884 April 2015), the authors reported that both of these instruments were effective for identifying poor quality colostrum.
That is, they worked quite reliably to exclude low concentration colostrum from first feeding for newborn calves.
As an aside, this study again reported just under thirty percent of the samples measuring as "poor" quality - less than 50g antibodies per liter (that is the internationally recognized lower threshold for colostrum acceptable for first feeding for heifer dairy calves).
By lactation the average immunoglobulin type G (IgG) concentrations were:
First lactation 62g/l
Second lactation 60g/l
Third+ lactation 70g/l
These averages emphasize for me that we should be checking colostrum from first lactation dams - a lot of that colostrum is just as antibody rich as colostrum from older dams.