The 24-hour IgG check is standard practice, but this study looked at what that single snapshot actually predicts down the road. Researchers linked passive immune status measured at 24 hours postpartum to long-term calf health and performance outcomes, not just short-term survival. For producers using that 24-hour number as a pass/fail checkpoint, this speaks to what it's actually forecasting beyond the first few weeks.
"Passive immune status at postpartum hour 24 and long-term health and performance of calves" — T.E. Wittum, L.J. Perino. Am J Vet Res. 1995;56(9):1149-1154.