Weaning at 30, 75 and 180 days: Comparison between immune responses of beef calves.

The interruption of the maternal-filial bond in young calves can alter the physiological responses due to increased stress at this moment. Therefore, we aimed to analyze the association between age at weaning and the immune responses of 64 beef calves weaned abruptly. The animals were distributed in three weaning (W) treatments: hyper-early (W30), early (W75) and conventional (W180). The treatments agree with the weaning ages used in cow-calf systems in South America. Blood samples were collected of all experimental animals on the day of weaning and days 1, 2 and 7 post-weaning day of the three treatments. Blood samples were used to analyze hematological variables, total plasma proteins (TPP), fibrinogen, serum amyloid A protein (SAA) and cortisol. The variables analyzed were influenced by the sampling day (P < 0.001) and the calf gender (P < 0.05), and an interaction between treatment x sampling day was observed. Over the study period, high serum cortisol levels were observed for W30 calves. The serum amyloid A protein was significantly influenced by sampling day. When compared to W180 calves, W30 and W75 animals showed prolonged concentrations in plasma fibrinogen. In addition, they presented neutrophilia and lymphopenia during the 180-187 days period, which resulted in a greater neutrophil:lymphocyte ratio (N:L ratio). Calf gender resulted in changes in the number of blood leukocytes (P < 0.05). This result provides an alert for weaning at an early age, indicating that different management strategies must be tested to minimize physiological instabilities in critical periods of the life of animals.