Chickens are sensitive to changes in the duration of illumination, and these will influence the age of sexual maturity. In addition, feed consumption is greatly influenced by the duration of day length. Lighting programmes have, therefore, different objectives.
During rearing, they allow us to encourage growth and to control the birds' sexual maturity. For this reason, we consider it to be essential to achieve the recommended bodyweight at 5 % lay, in order to obtain an egg weight which conforms with the target from start of lay, and to achieve high overall production.
Lighting programme and growth:
In addition to the influence of growth, the light programme plays a determinant role for 3 essential reasons:
- progressive growth of the digestive system.
- gradual adaptation to a body clock ( above all , anticipation of a dark period ).
- lack of night time energy supply when dark periods are too long.
The observation of the feeding behaviour with the water consumption shows a first peak of food intake in the 2 to 3 hours that precede a dark period, and a second peak shortly after lights come on. The crop is used during these peaks of consumption as a storage organ.
The introduction of a dark period from start of the rearing period is important to progressively develop the crop capacity, which plays a role of food reserve. However the amount of food stocked remains insufficient for the nocturnal energy needs.
Buyse (1993) found that with pullets subjected to a 10-hour dark period, the amount of food stored in the digestive tract was only 75% of the energy needs for those 10 hours.
Other authors have found similar results. Thus the feeding behaviour of poultry is an attempt to satisfy night time energy needs. It is a reasonable to suppose that the night energy deficit is proportional to the length of the dark period
Light duration and growth:
A rapid decrease in light length is used to slow the growth of broilers and broiler breeders when young. Conversely any increase in light duration will favour growth.
The trial (24th Random Sample Test- Eickelborn) shows clearly show the relation - Light
The duration of light must be taken into account when planning light programmes while bearing in mind that the objective is not to break records for growth but to follow the established growth curve.
Control of sexual maturity
The purpose of light programmes is to control the age at point of lay and above all to avoid the influence of the variations in natural day length.
Role of bodyweight
- Photo stimulation is not necessary to stimulate production even when the pullets are reared under very short day lengths.
- A trial carried out by Lewis (1996) shows that with a light length greater or equal to 10 hours, the age at 50% lay does not vary, or only a little. On the other hand, a light length held at 8 hours appears to delay sexual maturity by one week. This delay of maturity with 8 hours at the plateau is explained by the lower growth obtained compared to 10 or + hours of light programme.
- These observations are confirmed in latitudes close to the Equator. With very little change in day length, we have seen that sexual maturity is mainly activated by obtaining adequate body weight.
According to the latitude, differences in sexual maturity between summer and winter are more and more important when latitude is important.
- The variation of light duration greatly influences sexual maturity. Under certain conditions, we can observe a response to a light stimulation from 6 weeks old. The more sensitive period is between 10 and 12 weeks old.
- According to the programme used, the age at 50 % can vary by at least 6 weeks.
Light stimulation will change bird weight at sexual maturity and adult weight, as a consequence the egg weight, which is directly related to the bodyweight of the bird at first egg.
Bird weight at sexual maturity will be 75 g lower when light is advanced one week. Egg numbers will be greater but egg weight will be reduced by about 1 g. Total egg mass produced does not seem to be affected by reasonable variations in the age of sexual maturity (Lewis 1997).
For this reason, it is suitable to determine time of light stimulation according to bodyweight instead of age of the bird.
Light intensity in rearing
Little information is available. However some work has shown that light intensity can be very low. Morris (1996) showed that intensity greater than 1 lux did not modify sexual maturity.
Ideal light intensity will be determined in practice by the following needs:
- Light required to inspect the birds well.
- The degree of darkness of the building (light leaking in).
- The intensity to be used during laying period.
Lighting programmes have to be adapted to the rearing facilities (dark or open house systems), to conditions of production, to climate and to egg weight profile demanded by the market.