By Dr John Hardiman and Dr Manouchehr Katanbaf, Cobb, USA More than 8,000 feathers cover broilers although their number differ by breed and genetic line. Feathers are involved in thermoregulation and play an important role in providing protection against outside challenges and physical injuries. Therefore attention should be given to maintaining good feathering through management and nutrition. Feathers make up anywhere from three to six percent of the live weight of a healthy chicken depending on breed, sex, weight, age and muscularity. In fact, there are more than 8,000 individual feathers on today’s six week old commercial broiler. Over 3,000 of these are located on the neck, head and face with over 4,900 feathers elsewhere on the body of a five week-old-broiler chicken. The total weight of feathers on a five pound pedigreed broiler is about 74 grams or 2.6 ounces (about 3.3% of body weight). Most commercial broilers have been selected for white feathers with a few exceptions found in specialty markets. Individual feather types vary widely in length, width, shape, texture, density and structure and there are three main types of feathers on broilers (Photo 1). The most visible to us are the contour feathers that cover most of the surface of the bird providing a smooth appearance. Contour feathers have a shaft, vanes and interlocking barbules. Flight feathers on the wing and the tail are the largest contour feathers. The smallest, softest and fluffiest feathers are the down feathers. Down feathers are most evident at hatch but are sparsely distributed under contour feathers throughout each broiler’s life and even at breeding age. These feathers have a shorter shaft and their barbules do not interlock. The filoplumes are the third major type of feather on broilers. These are fine, hair-like feathers with a long shaft with few barbs at their tip. The vanes extend to each side of the shaft and lock together with barbules and maintain the characteristic shape of the feathers. Feather growth Although feathers are made up of the same protein structure as human hair, keratin, they are not uniformly distributed throughout the bird’s body. Rather they are arranged in rows to produce distinct feather tracts. Each tract follows a specifi c arrangement with distinct angles which gives broilers a common pattern in feather cover. About 75% of a bird’s skin surface is covered with feather tracts with the uncovered areas being mostly beneath the wings and on parts of the breast. The follicles are formed during embryo development and fi xed once the chick is hatched. During a broiler’s life of six weeks, each bird undergoes two molts. The fi rst occurs during the fi rst two weeks of age where the down feathers are mostly shed and replaced with juvenile feathers and the second at about 4-5 weeks of age where new feathers push the fi rst set of feathers out. The head and neck of broilers are the last area of the body to go through molt. It is normal to see broilers with the fi rst set of down feathers still on their neck and head area at 5-6 weeks of age. Functions of feathers on broilers Feathers are living and growing structures within follicles that set birds apart from other animals. Jungle fowl can use feathers for flight. Although modern broilers don’t fl y, their wings and feathers can facilitate bird movement while in escape posture. Feathers are involved in thermoregulation and play an important role in providing protection against outside challenges and physical injuries. Good feather cover contributes to better energy utilisation. Excessive feather loss adversely affects feed conversion as birds must allocate some of the available feed energy to compensate for heat loss. In adult birds, feathers are used for courtship displays and their pattern of structure also aids in sex differentiation of broilers. Contour feathers protect the broiler against the sun, wind, rain and injury in range type settings. In more industrialised settings with controlled environments, contour feathers protect broilers against scratches that could result in infections or bruising of the tissues. Down feathers make it possible to trap air in an insulating layer next to the skin providing much needed protection against heat or cold. Filoplumes are thought to have sensory functions giving the bird ability to respond to air fl ow and direction of body feathers. Differences among breeds Feathers serve to both protect and insulate birds. Although feathers provide protection from damage including skin scratching and possible skin infections such as cellulitis, they can also limit a bird’s thermoregulation through evaporative cooling in hot environments. The existence of genetic stocks which differ in exposed skin surface area and degree of feather cover provides alternative breeder products. For example, minimum feathering can be observed on chickens selected for the scaleless (sc) gene and this has been associated with less high heat related problems and mortality as well as higher relative carcass yields and meat quality. Reductions in feathering can be seen also in naked neck (Na) chickens which show very little feathering on the neck area and a 20-40% reduction in feathering on the body depending on whether the chicken has one or two copies of the Na gene. Therefore there is increased surface area for evaporative cooling in these birds. Feathers as an aid to chick sexing Feather sexing is a form of autosexing which allows broiler chicks to be sexed at day old depending on the phenotype or length of their primary feathers and overlying coverts. Broiler chicks with relatively short coverts and long primaries are genetically female while chicks with approximately equal length primaries and coverts are genetically male. Furthermore, fast feathering female broiler chickens show relatively more feather cover than slow feathering male broilers at early broiler ages. The genetic differences vital to this process were reported by Dr Don Warren in 1925 with feather sexing via relative wing feather growth described in 1930. Warren identified the fact that in chickens an early or fast feathering gene was normally limited to White Leghorn breeds with the late or slow feathering gene generally associated with heavy breeds of chickens. Thereafter, feather sexing in layer crosses was developed by creating layer female lines with the late or slow feathering gene crossed in from heavy breeds and correspondingly feather sexing in broiler crosses employed early or fast feathering genes crossed in from Leghorn lines. Breeders today maintain pedigree lines chosen and selected for either slow feathering genotypes (KK males, K_ females) or fast feathering genotypes (kk males, k_females). When parent males from fast feathering lines (kk) are crossed with parent females from slow feathering lines (K_) the resulting broilers are either fast feathering females (k_) or slow feathering males (Kk). However, the accuracy of this process depends on continuous quality checks and selection of chicks both within commercial pedigree lines and also within each level of the production pipeline for proper feather type. Variation in degree of feathering The smallest to the largest feather on an individual broiler varies by a factor of several hundreds if not a thousand. Difference in any of a variety of factors such as genetic background, nutritional requirements, environmental conditions, husbandry practices, sexual dimorphism, season of the year, age at processing, weight at processing, health status and flock stress levels may contribute to variation in the degree of feather cover in a broiler flock. Feather cover variation is noticeable on the wings and tail between slow and fast broilers up until about 5-7 days of age. By the time of processing at 5.0 lbs (2.26 kg) there is little difference in feathering. Within any fl ock of broilers or pedigreed pure lines of various commercial genetic backgrounds, one can observe a variation in degree of feathering as seen in subjective grading scores assigned to individuals within a flock (Figure 1). In general, female broilers have better feather cover than males and older/ heavier broilers have better feather cover than younger broilers. Selection for feather development Today’s broilers are crossbred progeny of three or four pure lines. Each of these pure lines exhibits their own unique feather growth characteristics at the same age or weight (Photo 2). Feather volume has a similar degree of inheritance as that of growth which means that it can be improved by selective breeding. Primary breeders have developed visual scoring systems to evaluate individual variations in degree of featherings within selected lines. Since thigh area is one of the broiler’s body surface regions with less dense feathering, it is more prone to the presence of scratches. A special scoring system has been developed to facilitate selection for better feathered individuals within each pure line (Photo 3). Additionally, selections at pure line level continuously purge undesirable genetic mutations affecting feather cover, including Singed down, Club Down, Frizzled feathers and Sunsuit. Management and nutrition A critical management aspect in feather development is the use of correct brooding temperatures during the first two weeks of bird growth. If the temperatures are too cold, protein consumed in the feed will be used for temperature control rather than internal development and feather development. Temperature control, especially avoiding too low temperatures or excessive ventilation levels, are important to make sure that the protein consumed by each bird (containing the necessary sulphur amino acids for example) are used for feather growth and not unnecessarily for temperature control. In addition, many husbandry aspects which may induce stress or negatively influence feed absorption can impact broiler feathering. These include issues such as incubation problems, high stocking density, meal feeding, poor litter management (wet litter), heat and cold stress, fi eld disease challenges (RSS, GD, IP, mal-absorption), poor feed quality (ingredient separation, improper vitamins or mineral content) and even feed delivery problems. Nutritional decisions, of course, also can play a role in poor broiler feathering. Adequate amounts of protein, amino acids, vitamins and minerals are essential to the healthy development and maintenance of feathers. The broiler industry has developed several feed formulations to address different nutritional requirements at various growth stages. Due to the high content of protein in feathers, feeds with higher protein levels are shown to encourage faster feather development and shedding. The major amino acids involved in the synthesis of feather keratin are the sulphur containing amino acids, cysteine and methionine. Imbalances of various amino acids in the diet could cause feather abnormalities with a “rough” feather appearance. Also, insufficient levels of vitamins and minerals in the diet can adversely influence feathering. In the future, maintaining good feathering broiler chickens will require adequate management, nutrition and eff ective genetic selection especially as broiler crosses continue to improve in growth rate and broilers are marketed at earlier ages.