Investigations about whole body composition of current broiler genotypes provide preconditions for valid evaluation of energy and nutrient requirements, namely when factorial methods are applied. In this context, both maintenance requirement and nutrient deposition data are needed for valid conclusions. Consequently, changes of the proportion between feather and feather-free body fractions during growth are of special importance for revision of such requirement data when based on factorial methods. According to our knowledge, first detailed investigations about feather and feather-free body mass both depending on genotype and age were reported by Lehmann [1] [2] . However, corresponding data of actual broiler genotypes are missed.

Several studies are indicating that feathers achieve their mature state earlier than the remaining body fractions [3] [4] [5] . Wylie et al. [6] concluded that feather growth was apparently maintained in preference to muscle and body development. Generally, the growth rate of any body tissue is resulting from balance between synthesis and degradation of proteins, but once formed feather protein is not underlying turnover processes. Therefore, inadequate nutrient supply impacted on the proportionate feather covering by lower gain of the remaining body mass. Wylie et al. [7] pointed out that dietary crude protein (CP) was preferentially partitioned for feather production rather than for muscle growth in a modern turkey line. It was concluded that feather growth is maintained as high as possible on the expense of body growth when feed supply was below the need for optimal performance data. Consequently, the nutrient consumption for feather development is a significant factor of high nutritional priority during the period of rapid growth.

As reviewed both by Deschutter and Leeson [8] and Leeson and Walsh [9] , several factors of influence like animal, nutritional, environmental and management factors may respond on feather growth and development. Accordingly, significant differences in feather development and rate of feathering between genotypes of growing chicken and laying hens were reported [1] [2] [10] - [17] . Among nutritional factors, the dietary CP and amino acid (AA) supply is the most important [17] - [25] . Reduced feather development as well as abnormal feather structure and pigmentation was observed due to suboptimal AA supply [26] - [32] . In addition, feather coverage of birds with naked-neck and fizzle gene is considerably lower as compared to their “fully-feathered” counterparts [16] [20] [33] - [40] . Furthermore, time-dependent plumage development of birds with early and late feathering genes is to be taken into account [41] [42] .

The present study aimed to generate a revised database of feather and feather-free body mass development in modern meat-type chickens which may contribute to explain variation in age and gender depending nutrient requirements.

Two consecutive growth experiments utilized 180 growing chicken (male to female birds = 1:1) and were conducted at the facilities of the Division Animal Nutrition Physiology, Department of Animal Sciences of Georg-August-University Gottingen. Each experiment lasted 5 weeks and was approved by the Lower Saxony Federal Office for Consumer Protection and Food Safety (LAVES), Germany.

Data of BW, feather weight and feather percentage in male and female birds of the two hatches are summarized in Table 2. According to the biological process of growth, both body and feather mass were significantly increased with increasing age of the birds (p < 0.001). At the end of the first week of age, feather

¹n = 15 per age period and gender. ²Body weight following 24 h feed deprivation. ³Final body weight of male birds was determined at 35 d of age.

weight and relative feather percentage data of female birds were significantly higher (p < 0.001) as compared to their male counterparts. For both criteria, highly significant two-way interaction between age and gender was identified. Means of feather percentage as related to the empty BW increased from approximately 2% at the end of the first week to about 4% at the end of the experiment.

In Figure 1, the individual feather percentage data are summarized. In spite of the high individual variation and except day 1, the rate of feathering is obviously higher in female birds. Fitted plots of feather percentage as function of empty BW depending on gender yielded very similar curve approximations. Superior adaptation of both trend-lines and highest correlation were observed when the data from day old broiler chickens were excluded and a non-linear power function was approximated.

DM related data (Table 3) show a significant increase of feather percentage from 4.6% at the end of the 1^st week to approximately 7% at the end of the 5^th week (p < 0.001). However, no significant gender effect was observed (p > 0.05) when data were DM related. This observation can be explained by high standard deviations and different DM content in feathers and de-feathered body fractions. Except at d1, the DM content of feathers varied between 40% and 60%, but only 20% to 32% DM were analysed in feather-free body fractions. Superior DM contents were found in the down feathers of birds at d1 (approximately 70%), but contents declined subsequently up to the end of the 3^rd week. A continuous increase of DM content in feathers was observed up to the end of the 5^th week. The age-dependent differences were significant (p < 0.001), but no significant gender effect was observed for DM content of feathers (p > 0.05).

¹n = 3 pooled samples of each five birds per age period and gender. ²Body weight following 24 h feed deprivation. ³At 1 d of age only one pooled feather sample of each gender due to the low feather yield per bird. ⁴Final body weight of male birds was determined at 35 d of age. ⁵Standard deviation.

Consequently, all feather DM data related to the empty BW were plotted in Figure 2. Superior correlation between DM content of feathers and empty BW was achieved by application of a quadratic function, but only when data from d1 and d8 were excluded. Due to the limited quantity of collected down feathers at d1 and only one sample available for DM analysis per gender, these data need further validation. In addition, including data of male and female birds at d8 yielded a strong decline of the coefficient of the regression function (r = 0.7308).

Contrary, the DM content both in the feather-free and whole empty body (Table 3) increased continuously from 20% - 21% at start up to 31% - 32% at the end of the experiment (p < 0.001). The body of female birds contained significantly more DM as compared to the males (p < 0.001). The gender-specific correlations between DM content in the feather-free body and the empty BW are summarized in Figure 3.

The nonlinear increase of feather growth with increasing age of actual fast growing meat-type chickens seems to be higher as compared to previous genotypes which yielded about 50 g feather per bird at market age, as reviewed by Leeson and Walsh [48] . According to our results (Table 2), a total of 80 to 90 g feathers per bird can be expected at the commercial slaughter age of 5 weeks. Higher individual feather yields between 110 and 130 g were observed in older birds (Table 4).

Wheeler and Latshaw [49] observed the beginning of rapid feather growth up to the end of the 2^nd week of age. Sheridan and McDonald [41] stated the competition between feather and body growth as most pronounced during the first weeks of life. However, to our knowledge reliable quantitative data on feather weight of male and female chicken up to the 2^nd week of age are scarcely reported. The growth rate of feathers is reported to be the highest during the first 6

¹Lowest and highest average body weight (BW) and feather weight data of the experimental groups. ²Feather weight expressed as a percentage of …. ³Empty body weight (BW). ⁴2.86% - 3.01% and 2.60% - 3.03% feather dry matter (DM), respectively. ⁵Calculated values based on published body weight and feather weight data.

weeks of age [19] [20] [50] . Consequently, Yalcin et al. [15] stated full feathered birds at 7 wk of age. As reviewed by Leeson and Walsh [48] , most estimates of feather percentage in adult birds range between 3% and 6% of BW. In laying hens, 6.4% to 7.0% feather DM of BW is reported [33] . Depending on age growing turkeys yielded 2.0% to 6.3% feathers as related to BW [51] or empty BW [52] .

According to our present results, a very high individual variability of feather percentage can be expected in day old growing chickens. Similar data (Table 5) were calculated from earlier results as published by Lehmann [1] , Stilborn et al. [50] and Sklan and Noy [53] [54] . In terms of fresh feather weight percentage related to the empty BW, feather proportion in fast growing chickens was

¹Calculated, based on reported body weight and feather weight data. ²Feather weight expressed as a percentage of the empty BW (Means of three different genotypes). ³Feather dry matter weight expressed as a percentage of the BW. ⁴Feather weight expressed as a percentage of the BW (Means of two different genotypes). ⁵Feather weight expressed as a percentage of the BW (Means of six different genotypes).

elevated non-linearly with increasing age and ranged between 1% and 3% at d8 to 3% and 5% at d36, respectively (Figure 1). These results are in general agreement with literature data (Table 5). According to Sklan and Noy [54] , the feather proportion of BW in male broiler chickens is approximated by following equation: Feathers (% of BW) = 0.26∙BW^0.38.

Generally, literature data in normally feathered growing chickens vary markedly. In addition to individual differences, both the absolutely certain description of bird`s age and basis of reference (BW resp. feather-free BW) are the main factors responsible for high variation of feather percentage data. In consequence, data comparability is very limited. In majority of reported experiments feather weight was estimated by difference between BW before and after plucking. However, this procedure implies inaccuracies due to dry or wet de-feathering. Furthermore, the process of feather drying or the feather DM analysis and/or the analyses of feather-free body fraction are factors of influence. Table 2 and Table 3 provide an additional overview about the importance of reliable reference data impacting on derived feather percentage.

In line with Deschutter and Leeson [8] , Moran [19] and Hancock et al. [55] , feather growth of growing chicken also depends on gender (Table 2, Figure 1). The observed higher feather weight and feather percentage in female birds is in agreement with several reports [10] [41] [56] and gender-specific data [15] [19] [38] [42] [50] [55] [57] as summarized in Table 4 and Table 5. However, age dependent also similar or even less feather yields of female growing chickens are reported [3] [50] . But, due to the lower feather-free body mass and the lower surface area of female birds their feather mass per unit BW was higher as compared to their male counterparts of equal age. Otherwise, according to sex-linked different rate of feathering, feather loss of female birds during growth was observed to increase [57] [58] .

As reviewed by Leeson and Walsh [9] , the nutrient supply impacts on feather growth and development. Stimulated feather growth was observed with increasing dietary CP and SAA supply [18] [20] [22] , mainly due to varying body proportion of feathers, connective tissue and muscle tissue [59] . Additionally, more feathers were found on the litter following a high protein diet [58] indicating faster feather development with subsequent moult. Otherwise, deficient feed and CP supply or imbalanced dietary AA composition created opposite effects on the rate of feathering of young birds [6] [7] [9] [21] . Feather percentage was significantly higher in restrictive fed birds as compared to birds fed on free choice level [6] .

At d1, an elevated DM content in the feathers of freshly hatched chickens was observed (Table 3). Lehmann [2] indicated that down feathers are exceptional as related to age-depending changes of feather composition due to their high DM contents between 73% and 80% [1] . Smith and Bath [60] suggested that individual feathers of growing chickens reached maximum weight when the dry feather mass was 50% of the total mass. At 2 weeks of age all the feathers were half feather (50% to 55% DM) and half pulp (13-15% DM) with approximately 38% to 40% DM content in the entire feather [60] . According to Lehmann [1] [2] and Hancock et al. [55] the DM content of the feathers increased steadily throughout the growing period from 38% at week 1% to 63% DM at week 8 - 10 indicating an apparent drying out of feathers with age [14] . Several reports [12] [35] [37] [61] underline our observed age-depending effect (Figure 2), but missing related data about the influence of gender on DM content in feathers.

Carcass DM content in the modern strain birds was significantly higher than in previous genotypes [62] [63] [64] . Significantly different DM contents of the feather-free or whole body of varying genotype broiler chickens are reported [1] [2] [42] [65] [66] . Increased DM content in fat-type chickens can be expected [67] . Accordingly, elevated body DM contents were observed with increasing age [1] [2] [61] [63] [65] [68] [69] [70] and in female versus male birds [3] [42] [62] [64] .

Finally, energy and protein supply as well as dietary AA balance impact on DM content of the feather-free or total body of broiler chickens [42] [67] - [73] , but eliminated in the current study by standardized diets.

Within the growth period of growing chickens, the feather coverage as a proportion of the total or empty body mass increases with increasing age of the birds. Due to larger relative feather percentage in female vs. male birds, gender-specific differences were observed and also validated by literature data. With increasing age, the DM content in feathers and in feather-free or whole empty body raised continuously, but DM content of feathers differed not significantly between male and female birds. However, the whole body of female birds contained significantly more DM as compared to their male counterparts. The observed age and gender-specific proportion between feather and feather-free body fractions and modulation of DM contents during growth are of special importance for evaluating energy and nutrient requirements based on factorial procedures. For this purpose, additional data are needed to specify the database. In consequence, further detailed data about nutrient composition of feathers and feather-free body, but also about nutrient deposition depending on age and gender in modern genotype growing chickens are required. Actually, a consecutive publication is prepared to provide such a database for re-evaluation of requirement data in modern genotype meat-type chickens.

Wecke, C., Khan, D.R., Sünder, A. and Liebert, F. (2017) Age and Gender Depending Growth of Feathers and Feather-Free Body in Modern Fast Growing Meat-Type Chickens. Open Journal of Animal Sciences, 7, 376-392. https://doi.org/10.4236/ojas.2017.74029