Genetic association between direct and indirect measures of IgG concentration in bovine colostrum

In cow colostrum, immunoglobulins G (IgG) are important for the passive transfer of antibodies from dam to calf. Direct determination of such trait in colostrum through the gold standard, i.e. radial immunodiffusion, is highly demanding in terms of time and costs. For this reason, the optical refractometer is commonly used at farm level for the indirect determination of total solids, which include also IgG. In fact, it has been showed that density of colostrum is positively associated with IgG concentration. In the present study, 672 colostrum samples (1 per cow) were collected within 6 h after calving in 9 Holstein farms located in north-east of Italy. The refractometric index (RI, on the BRIX scale) was assessed using an optical portable refractometer and in the same samples IgG (g/l) was determined in laboratory through radial immunodiffusion kit specific for bovines, according to the manufacturer instructions. A bivariate analysis was carried to estimate (co)variance components and genetic parameters for IgG (g/l) and RI. Parity (1, 2, 3, 4, and ≥5), calving season (4 classes), calving year (2019 and 2020), and herd were the fixed effects; the additive genetic animal and the residual were the random effects. The pedigree file (6,714 individuals) consisted of all sampled cows and 6 generations of their ancestors. The IgG and RI averaged 93.24±36.44 and 23.24±4.28 and were heritable (0.18±0.13 and 0.23±0.14), showing an additive genetic variance of 200.60±151.97 and 3.56±2.20. The phenotypic correlation (0.78±0.02) estimated between the two traits mirrored the genetic correlation (0.74±0.21). Findings support the use of the refractometer device for screening and management purposes, e.g. assessment and appropriate administration of the farm colostrum bank. As cheap and easy to be used, this tool could facilitate the collection of phenotypes on a large scale in a near future for definition and improvement of potential breeding strategies to enhance colostrum quality in the major Italian dairy cattle breed.