Broody hens provide optimum conditions for embryos developing in the eggs they are sitting on. The brood patch provides heat from one direction only, and the eggs at the side of the patch are cooler than those in the middle of the nest. However, because the broody hen regularly turns and moves the eggs in the nest, uniform egg temperature is achieved.

In commercial incubation, we try to mimic the natural conditions in the nest. From the point of view of uniform egg temperature
turning of eggs seems to be of less importance in modern incubators. Are there other reasons for turning eggs?

As summarized by Deeming (2002), egg turning is essential to normal development for several reasons.
Egg turning:

  • Prevents adhesion of the embryo to the inner shell membrane.
  • Stimulates the rate of development of the area vasculosa (the membrane which grows around the yolk and is rich in blood vessels). The area vasculosa is important for sub-embryonic fluid formation, as well as for yolk uptake later in incubation.
  • Allows normal transfer of albumen proteins into the amniotic fluid, promoting optimum use of the albumen.
  • Supports the growth of the chorioallantois (the blood vessels right under the shell) to maximise oxygen absorption.
  • Embryos in unturned eggs grow at a lower rate compared to embryos in eggs turned each hour over 90°.
  • Facilitates movements of the embryo into the normal hatching position and reduces the incidence of malpositions
    in unhatched embryos.
  • Recently Elibol and Brake (2004) confirmed the finding of New (1957), that the most critical period for turning broiler hatching
    eggs is during the first week of incubation. Elibol and Brake observed differential effects due to an absence of turning between 0 to 2 days (primarily increased early mortality) versus 3 to 8 days (primarily increased late mortality). The effect of not turning during the first half of incubation is only seen during the second half of incubation, but by then it is too late to take corrective actions. Turning failures during the second half of incubation  will generally have less dramatic  effects, although the growth rate of the embryo can be affected, depending on the  moment and duration of the turning failure.
    The angle through which the eggs are turned is important. Hatch of fertiles was significantly better in eggs turned over an
    angle of 45° either side of the short axis of the egg, as compared to turning of 30° and 15°. Hatched chicks from eggs turned
    45° weighed more and had less dry matter in the residual yolk. (Cutchin et al, 2007)
    Advice
  1. Check the turning device before the start of each incubation cycle, as turning failures, depending on the moment of occurence, are detrimental to results.
  2. Check and maintain the turning device regularly, to prevent a breakdown during incubation.
  3. Make sure that turning does not produce shocks or jolts, as this adversely affects hatchability and chick quality.
  4. If necessary, check and adjust the turning angle: 45° is optimal.
  5. Not turning for the first 12 hours in the setter is advised, based on our practical experience and especially when eggs
    are transported to the hatchery on the same day as setting. Eggs need some rest time to restore their ‘internal balance’.
  6. Turning is not absolutely necessary after 15 days of incubation. Especially in incubators with insufficient cooling capacity, it can be beneficial to leave the eggs in a horizontal position to facilitate increased air flow (cooling over the eggs). In some modern setters, there is also the option of turning to three different positions, e.g. 45 minutes each in right, horizontal and
    left position).