Determination of serum total protein (TP) is being used as a tool to evaluate failure of passive immune transfer in dairy calves. However, time of evaluation recommendation is not well established. The objective of this study was to evaluate these changes, aiming a better recommendation of the better time to assess TP in neonatal calves. Blood form 47 cows was drawn just after calving, as well as from the calves at 0 (before colostrum feeding), 1, 2, 4, 6, 12, 24, 48, 72, 96 and 120h after colostrum intake. Crossbred calves (Holstein × Gir and Jersey × Gir) were separated from their mothers and 3L of high quality colostrum were fed. Hematocrit was determined and samples were centrifuged for determination of TP using a refractometer and albumin and TP using a commercial enzymatic kit. Regression curves were constructed for each parameter according to time, using SAS PROC REG. Correlations between mother TP at calving and calf TP at different times were also calculated using SAS PROC CORR. Quadratic regressions showed higher R² for all parameters. Total protein concentration estimated by the protein refractometer was higher than that determined by enzymatic method, as shown by the higher intercept in both linear and quadratic prediction equation (Table 1). Correlation between cow’s TP at calving were positive and significant only at time 0 (r = 0.2989; P < 0.05) and 1h (r = 0.24591; P < 0.1). The quadratic behavior of the regression analysis of TP, by both methods, suggest that the maximum time after colostrum feeding for a better evaluation of passive immune transfer is around 12–24h after colostrum feeding. Table 1.

Prediction equation1	R2	P <
Albumin = 2.32 − 0.00038 T	0.0009	0.497
Albumin = 2.39783 − 0.00951T + 0.00008507T2	0.0418	0.0001
TPe = 5.304999 + 0.01931 T	0.2876	0.0001
TPe = 4.79507 + 0.08244 T – 0.00058577 T2	0.5176	0.0001
TPr = 6.13537 + 0.02468 T	0.2486	0.0001
TPr = 5.53535 + 0.10027 T – 0.00070451 T2	0.4233	0.0001

¹TPe = TP determined by enzymatic method; TPr = TP estimated by refractometer.