The production loss and death loss of calves at weaning is second only to the losses at calving. Weaning is a very stressful time and bovine respiratory disease (pneumonia, shipping fever, etc.) is a common problem. Coccidiosis and other digestive problems, such as acidosis, (grain overload) are also common.
Bagley, Clell, "Weaning Calves Successfully" (1997). All Current Publications. Paper 416.