A commercial turbidimetric immunoassay (ARS Foal-IgG Test; Animal Reproduction Systems, Chino, CA), designed for measuring IgG concentrations in foal serum, was evaluated as a means for quantifying IgG concentrations in the colostrum of 43 mares (28 post-partum/pre-suckle samples and 15 pre-partum samples). This TIA test was found to be unsuitable for determining IgG concentrations in either untreated or centrifuged (fat-free) mare colostrum. The IgG concentrations estimated by TIA were significantly higher than those quantified by the radial immunodiffusion (RID) assay, and there was no overall correlation between the two tests.\ud By contrast, the BRIX (sugar) refractometer proved to be an accurate, fast and easy technique for estimating colostral IgG concentrations. The BRIX percentage score correlated highly with colostral IgG concentrations measured by RID (pre- and post-foaling samples: r=0.78, serial pre-foaling samples: r=0.88, serial post-foaling samples: r=0.99).\ud Pre-partum colostral IgG concentrations were measured daily in 3 mares during the final 14 days of gestation. During this period, 2 mares maintained relatively constant IgG concentrations whereas the third mare showed a rise in colostral IgG starting around 12 days before parturition.\ud Colostral IgG concentrations were also evaluated in 2 mares during the first 24 hours post-foaling. IgG concentrations dropped by 78% in the first 4-6 hours after birth, and then reached a plateau that persisted for the next 10 hours
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