The objective of this study was to determine the transfer of passive immunity of dairy heifer calves in 4 provinces of Costa Rica. The data presented correspond to total serum protein (TSP) measurements obtained in 50 dairy farms. A total of 2500 heifer calves were sampled. Dam breeds were classified into Holstein, Jersey, Holstein × Jersey and other. Blood samples were collected between d 1 and 7 of age into serum Vacutainer tubes, refrigerated overnight, centrifuged, and the serum separated from clot within 24 h of collection. A hand-held refractometer was used to measure TSP. For the purpose of this study, failure of passive immunity was considered when TSP concentration was less than 5.5 g/dL. GLM procedure was used to establish differences between parity and breed of the dams. Descriptive statistics were generated to define percentage of failure of passive transfer by breed and parity of the dam. TSP concentration ranged from 2.0 to 10.0, with an overall mean of 5.9 g/dL. Of all the calves evaluated, 38.8% presented failure of passive transfer of immunity. Calves born to Jersey and Holstein × Jersey crosses had significantly (P < 0.05) higher TSP concentrations than calves born to Holstein and other breeds. When considering parity of the dam, there were no significant differences (P > 0.05) on TSP concentration; however, offspring born to first lactation heifers showed the lowest percentage of animals with inadequate transfer of immunity. Calves that were allowed to suckle their dams showed a 44% failure of passive immunity against 33% of calves that were given colostrum by bottle. The findings of this study suggest that minimizing the risk of calves with failure of passive transfer of immunity is an important task to perform in dairy herds of Costa Rica. Table 1. Effect of dam breed over total serum protein (TSP) concentration in 2500 heifer calves from 1 to 7 days old in 50 dairy farms located in four provinces of Costa Rica¹

Dam breed	n	TSP, g/dL	Animals with FPTI, %
Holstein	769	5.8 ± 0.06b	39b
Holstein × Jersey	287	6.4 ± 0.09a	25a
Jersey	743	6.3 ± 0.07a	27a
Other	740	5.4 ± 0.07c	55c

¹n = number of animals; FPTI = failure of passive transfer of immunity.