If the calves are handled roughly, deworming with a pour-on can cause stress too. By performing the typical health practices while the calves still have their dam to return to for comfort reduces the impact of the physical stress on the calves. This is much more than just the weather. Environmental stress can include changes in diet, space and location. While calves can do fine in a limited space such as a lot, if it is new to them it will cause additional stress.
Weather is the obvious environmental stressor.
It is the one stressor over which you have no control. Weather during weaning is the only stress factor over which you have no control. With the day to day variability in weather during the fall, your best strategy at weaning is to minimize all other stress factors and accept the weather as it is. Most producers use a weaning management program in which calves are moved to drylot and fed a weaning ration. By implementing a weaning program that minimizes these stress factors, sickness should be reduced and performance improved during weaning.