A paper delivered at the 2020 Australian Poultry Science Symposium reviewed the development of structural and storage skeletal tissue, and how this is utilised during the laying period. Whilst storage (medullary) bone levels are maintained or even increased during lay the structural bone can only decline whilst egg production continues. Therefore, to maintain skeletal strength it is essential to achieve maximum structural bone before the onset of lay to assist in the achievement of longer laying cycles and this needs to be an objective from day one of the rearing period.
The pullet skeleton is formed of 2 types of structural bone, cortical and trabecular. The former is the outer shell whilst the latter are the internal struts and are an avian adaptation to achieve extra strength whilst minimising weight and hence aid flight. With the onset of lay, due to the increase of circulating oestrogen the growth of structural bone ceases and will not recommence unless the bird stops laying as in a moult. During lay a third type of bone will be deposited which is medullary bone, this has little structural function and is a storage form of mineral reserves to meet demands when diet supply is low, as at night.
Unlike structural bone which will only be depleted with no regeneration during lay the amount of medullary bone ebbs and flows in accordance with supply and demand and can actually increase during the laying cycle. If diet and medullary bone reserves are not adequate structural bone will be depleted to meet demands of shell formation. Because this cannot be replenished there will be a gradual decline in skeletal strength. As laying cycle is increased this decline can lead to weakened bones and increase potential for bone failure and corresponding welfare and production issues.
Maintaining good body weight throughout rearing is going to be fundamental to optimising skeletal strength at onset of lay. Flock uniformity is also important so that timing of photo stimulation can be made to ensure birds commence lay when they are ready for egg production in terms of skeletal reserves. This should be at the appropriate physiological state for the birds and not an arbitrary age.
Because structural bone deposition ceases when a pullet reaches sexual maturity it is essential to achieve a good skeletal frame by the time this occurs and then to support rapid development of medullary bone.
For underweight flocks delaying photostimulation is probably the best intervention since flash feeding or midnight feeds for pullets close to onset of lay has been shown to interfere with the regulation of sexual maturity and may even bring birds in to lay earlier.
Medullary bone deposition takes place 10-14 days before first egg is laid and, in this period, either a pre-lay diet with higher calcium content or introduction of the laying ration may yield benefits. In this context flock uniformity is important because underweight birds will still be developing their skeleton and additional calcium in the diet may disrupt this process and lead to problems later on if a higher calcium diet is introduced too soon.
The efficiency of calcium metabolism decreases with age and slight increases in dietary calcium supply and a widening of the calcium to digestible phosphorus ratio is intended to counter act this. Over feeding of calcium should be avoided as this can reduce bone and egg shell quality and will adversely affect the efficacy of exogenous phytases which are now added to most layers diets.
The bulk of egg shell formation takes place 5-15 hours after the egg reaches the shell gland, this corresponds with late afternoon and through the night. If the diet contains small particle size calcium the intestinal tract may be devoid of calcium and to complete shell formation the bird will draw on skeletal reserves. Therefore, larger particle calcium source, 2-4mm limestone, which will dissolve slowly, will allow the diet to provide the necessary calcium through the hours of darkness when feed intake is low/zero.
Since structural bone formation does not occur during lay it is essential to start with the aim of developing a sound skeletal frame in the replacement pullet from day one to give the best chance of a longer laying cycle of 80 to 100 weeks.
Korver DR (2020) Proceedings 31st Australian Poultry Science Symposium – Calcium Nutrition, Bone Metabolism and Egg Shell quality in Longer-Persisting Layer Flocks