How light affects growth and feed conversion efficiency
Traditionally, broilers and turkeys have been given long daylengths or continuous illumination to maximise feed intake and growth. In the early days of the broiler industry, body weight was up 10 percent lower for birds exposed to 8 to 12 hour periods compared with birds given continuous illumination.
Those given only 6 hour days learned to eat in the dark and grew at a faster rate than birds given 14 hour photoperiods.
In the extreme, broilers transferred to continuous darkness at 7 days of age had a similar body weight at 63 days to birds given continuous illumination.
Data shows that whereas body weight gain in modern broilers increases by about 10 gramme/hour of photoperiod up to 21 days, it decreases by about 6 gramme/hour between the 22nd and 49th days for birds given less than 12 hours.
However, birds on 8-hour days continue to have the slowest growth (Renden et al., 1993). As a consequence, body weight at 49 days is similar for all daylengths greater than 8 hours.
This is because feed intake, which increases linearly by about 15 gramme/hour during the first 21 days, is not significantly different after 21 days for greater than 12 hour daylengths,
A 1 percent per hour higher energy expenditure during the photoperiod results in birds on longer daylengths having less feed available for growth and poorer FCR.
In contrast, photoperiod strongly influences bird health, with longer daylengths being associated with higher incidences of Sudden Death Syndrome, leg disorder, and general mortality.
Up to 21 days, the provision of 6-hour photoperiods reduces body weight and cumulative feed intake compared with birds grown on 23-hour photoperiods, but a transfer from short to long days results in compensatory growth so that body weights for the two groups become similar by day 42.
However, the initial 3 weeks of 6-h days reduces total mortality and the incidence of leg disorders through 42 d, and produces comparable liveability figures to birds maintained on short days,
About half of those for birds given 23-hour daylengths were able to carry the rapid growth that occurs when the birds are transferred to long photoperiods.
Although the benefits are greater for males than for females, they are proportionally the same for both sexes. Improvements in the skeletal integrity of meat-birds exposed to shorter photoperiods are really responses to longer scotoperiods.
As for the benefits to shell quality in laying hens, prolonged nocturnal melatonin secretion results in an extended release of calcitonin and parathyroid hormone, and an enhancement of calcium mobilisation, and advantageous modifications of osteoclast and osteoblast activity.
However, ultra short daylengths may adversely affect skeletal health because long periods of inactivity could compromise the blood supply to the leg-bone growth plates.
Some of the compensatory body weight gain that follows a change to long daylengths, especially those greater than 5 weeks, may be in response to an increase in plasma testosterone, as indicated by male birds having larger and brighter combs than constant-photoperiod or naturally lit controls.
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Article made possible through the contribution of the Australian Poultry Science Symposium (APSS) 2006.