In poultry incubation it is widely accepted that turning is absolutely necessary in the first week, a good idea in the second week and not really necessary anymore in the third week. That has mainly to do with the formation of the membranes under the shell. By turning the eggs the membranes form a complete coverage under the shell, and that helps the embryos to uptake enough oxygen and release enough carbon dioxide and moisture. If we turn regularly, once every hour, we see a good development of the membranes which can be recognized after hatch by looking at the blood vessels in the point of the empty shells.

It is also widely accepted that there is a certain minimum angle over which to turn. Ideally we turn over 45 degrees to both sides, but as long as the turning angle is 38 degrees or more to both sides, we do not see negative effects.

However, there is one factor that is sometimes forgotten. Its not only important to turn over a high enough angle and frequently enough, but also that the change in position during turning is severe enough. One can imagine that if the turning is done in little steps, one at a time, that the embryo is less able or forced to grow the membranes all over the shell.

Normally we assume that the turning goes from 45 degrees (or minimum 38 degrees) to one side, in one move to 45 degrees to the other side. This is normally the case but there are systems that have the opportunity to create a pause moment in a horizontal position. As a horizontal position creates more air flow over the eggs, that can be an advantage, for instance when we warm the eggs up at the start of incubation. However, when we turn the eggs from one side to horizontal, we don’t turn over 90 degrees but actually only over 45 degrees. After that we complete the turning to the other side with again 45 degrees, but if the horizontal time is too long, the embryo will not see it as turning over 90 degrees but as two times over 45 degrees.

It is difficult to establish what is too long for that horizontal position, but we do see that if we have a 30-30-30 pattern (30 minutes turned to the left, 30 minutes horizontal, 30 minutes turned to the right etc), a 60-30-60 or even a 30-10-30 pattern, we see more problems than when we have 60-0-60 (or 60-1-60 as some machines require). Those problems show in more early, mid and late mortality. What the limit is for the time in horizontal position is as far as I know not known, but I would be careful if the horizontal position exceeds 10 minutes. This is especially true for the first week. After the first week it will be less of an issue, and in the third week it will not be relevant at all anymore. This holds for chicken eggs, it might be that with longer incubation like in duck or goose eggs, this periods need to be adjusted.