Randomized clinical trial evaluating the practical and biological limits of single-feed colostral immunoglobulin G dosing in Australian Holstein and Angus - Holstein calves

The optimal colostral IgG dose to feed calves remains unclear. We hypothesized that the biologically optimal dose (i.e., the dose that maximizes serum IgG concentration without a plateau in serum IgG response) and the maximum practically feasible dose (the maximum IgG dose that can be reliably delivered due to constraints in colostrum supply) were between 200 and 350 g of IgG. We investigated these hypotheses by comparing serum IgG concentrations in calves block-randomized to feeding groups of either 200 g (n = 45), 250 g (n = 125), 300 g (n = 122), or 350 g (n = 107) of maternal colostral IgG and by monitoring IgG harvest dynamics on a commercial dairy farm in southern Australia. Cows were milked once within 2 h of calving, colostrum was pooled and heat-treated (60°C for 60 min) in batches of 40 L, and calves were administered up to 4 L of colostrum (mean = 3.1 L, range = 1.8 to 3.9) within 3 h of birth. Colostrum samples were titrated into individual dose volumes after calculating IgG concentrations measured using radial immunodiffusion (RID; median = 93 g/L IgG, range = 71 to 121), such that each dose was either 200, 250, 300, or 350 g of IgG (volume = intended dose/measured concentration). Serum IgG was measured using RID on calf serum samples collected at ∼2 d of age. A multivariable linear model was used to compare mean serum IgG between colostrum dose groups, controlling for colostrum dose volume. We found that each increasing dose of oral IgG increased serum IgG in a linear fashion between 200 g and 350 g. The most important practical factors limiting IgG dosing were the average IgG yield per cow and the concentration of IgG in colostrum batches. For the farm in this study, the maximum practically feasible dose was 320 g of IgG per calf when feeding a maximum of 4 L colostrum in a single feed. This was based on the finding that the mean IgG from maternal colostrum available per calf during the study was 320 g (118 kg IgG shared among 368 calves) and that most colostrum batches (89%) would be able to deliver 320 g when limited by batch concentration (282-482 g total IgG in batch) and dose (≤4 L). We recommend that producers aim to maximize oral IgG intake based on practical feasibility rather than relying on a fixed target minimum dose. It is likely that the maximum practically feasible dose will vary between farms, according to their dynamics of IgG supply, demand, capacity for storage, and delivery.