The second moult usually begins at around 4-5 weeks and for the modern broiler chick- en, this is the only moult of significance.


heating issues tend to arise much later and even +10ºC overheating in the hatcher has been shown to slightly re- duce growth rate and breast feather numbers in market weight broilers.


Feather cover Contrary to popular belief, the feathers do not grow random- ly or uniformly over the entire skin surface but rather in dis- tinct tracts. The fact that most birds appear to have a uniform feather cover is due to the discrete angles and layering of feathers which is designed to give the bird all-over protection for the skin. About 25% of the bird’s skin surface is devoid of feather tracts. The most notable featherless region is beneath the wing – hence the birds spread their wings when heat- stressed in an attempt to expose the skin for cooling. The in- side of the drumsticks and the underside of the wings are also naturally devoid of feather follicles. All birds undergo predictable feather loss (moulting) both as juveniles and as adults. As various generations of feathers grow and are shed, they are eventually replaced by adult feathers. Early feather growth can be retarded by caustic litter treatments. Healthy broilers that exceeded management standards had breasts almost devoid of feathers due to excess calcium hydroxide treatment of litter in the brooding area.


Genetics of feather growth Choice of broiler strain can have a profound effect on feather- ing because most breeding companies offer a choice of


32 ▶ POULTRY WORLD | No. 8, 2021


feather-sexed birds. It has been recognized for many years that a single recessive sex-linked gene influences the rate of feather growth, especially in the very young chick. After intro- ducing this gene into primary breeding stock, the two sexes have different rates of feathering that allow for simplified de- termination of sex in the newly-hatched chick. The females will be fast feathering and can be differentiated from the males simply by the extra length of their primary feathers at hatch. The genes involved essentially delay the emergence of the feather but by 5-7 weeks of age the feathering in males and females is similar. If there are differences at later ages, this will invariably be seen in the length of the tail feathers because late feathering males do not normally show exten- sive tail feather development until 17-20 days of age. Feather colour is also affected by sex-linked genes and so this trait of- fers potential for chick sexing. It’s intriguing to think that to- day colour sexing could be automated, perhaps even in the older embryo?


Hormonal balance Thyroid hormones and sex hormones have a major influence on feather growth and development. Thyroidectomy, or the incorporation of thyroid antagonists in the diet, causes cessa- tion of feather growth, although with the latter the effect is reversible if normal nutrition resumes. In situations involving an atrophied thyroid, young birds can be almost naked. Thy- roid imbalance is not likely to occur or be diagnosed in com- mercial poultry, although the role of thyroid antagonists,


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