EFFECT OF WEANING MANAGEMENT ON HEMATOLOGICAL AND INNATE IMMUNITY PARAMETERS IN HOLSTEIN CALVES

Weaning involves the transition from a liquid diet (based on milk or milk replacer) to one that is composed of solid feed. Further, the transition from the isolated environment and social behavior to a newly grouped living space is usually stressful for calves. All these events have negative impacts on the immune responses in the calf. Stress and immune function dampening appear to contribute to reducing weight gain through weaning. Total calorie intake, due to a period of learning to eat dry matter, is also a factor. Consequently, the calves become more susceptible to infection. The weaning period is one of higher morbidity caused by multiple disease agents. The objective of this study was to evaluate the effect of weaning on the dynamics of hematological parameters, some biochemical variables, and the phagocytic activity of neutrophils. Holstein calves (n=188) were selected to be subjected to a time-based analysis over the following time windows: 69-85 days of life (pre-weaning calves - PWC); at six (D-6), four (D-4), and two (D-2) days before, and on the weaning day (D0), with 47 calves at each time point. The calves were fed with 8L of milk/day split into two feedings ending at weaning. At weaning, the calves were fed 5L/day (D-6), then the volume was reduced by 1L of milk every 3 days until only 3L per day was achieved. At this point, all milk was terminated (D0). Blood samples were harvested to allow measurement of haptoglobin (Hp); red blood count (RBC) and white blood count (WBC). Blood samples provided a source of neutrophils (PMN) for a phagocytosis assay (both rate of uptake and mean fluorescent intensity [MFI]) and to measure the production of reactive oxygen species (ROS), after stimulating with Staphylococcus aureus, Escherichia coli, and Mannhemia haemolytica. The Hp concentration decreased from D-6 to D0. The total WBC values were stable, but the absolute values of neutrophils, monocytes, and basophils oscillated over the measurement period. At D-2, there was an increase in neutrophils and a decrease in monocytes. This corresponded with a reduced lobularity index (LI). Myeloperoxidase intracellular index (MPXI) values were similar across the weaning period. Phagocytic capacity for S. aureus decreased on D0, but there was an increase in phagocytic avidity on D-2. The E. coli phagocytosis rate and MFI dropped on D-2. There was an increase in the response ratio (RR = stimulated/unstimulated cells) for ROS production in response to E. coli and M. haemolytica stepwise to D0. However, in response to S. aureus, there was a decrease from the initial measurement to the day of weaning. These measurements indicated that innate immune response was dampened by the weaning process relative to exposure to these common pathogenic bacteria of dairy calves. Phagocytosis, and the corresponding indicator of intracellular killing activities used here -ROS production and myeloperoxidase system products, represent the most accepted core mechanisms for the early elimination of pathogenic microorganisms by the calf. We concluded that the innate immune response was dampened during the period of weaning in calves submitted to the intensive milk program even in the face of the use of a slow, gradual weaning management system to minimize stress. Therefore, we postulate that measures must be taken to minimize the consequences of weaning management to avoid reducing calf health and physiological development. Among different practices to reduce stress, it is essential to adopt a nutritional plan to stimulate a gradual starter intake, reducing calves' stress at weaning.