4.1. Trial 1
The addition of 4% feather meal in the diet had no effect on broiler growth performance; however, the addition of 5–8% feather meal resulted in inadequate levels of Lys, Met, His, and Trp, and limited growth performance [
29,
30,
31]. The dietary feather meal may be added at levels up to 10% without adverse effects on broiler growth, provided that amino acids are properly supplemented [
32,
33,
34].
In this trial, the content of MET and LYS in the diet was properly balanced. Although weight gain was negatively affected in the 10% group over the 1–21-day period, this result was not observed over days 22–35 and 1–35. The decline in weight gain may be attributable to the underdeveloped digestive tract of broilers during days 1–21. The inclusion of 10% EFM in the diet at 22–35 days, presumably a period with better gastrointestinal development, did not adversely affect growth performance. However, dietary inclusion of more than 15% EFM resulted in a significant impact on the growth performance over days 1–35. Therefore, the EFM dose tolerance in the broiler diet is 10%.
Feather meal has poor nutrient availability, and the amino acid composition is imbalanced [
1,
2,
4]. This may overestimate the nutrient availability of the feed formulation. These disadvantages may result in reducing broiler growth performance linearly and quadratically with the increase in dietary EFM dosage. In addition, increasing the EFM dosage in the diet also linearly and quadratically inhibited feed intake. Lower feed intake may further exacerbate nutrient deficiencies in broilers. The above issues require further research to understand the correlation of various factors with the growth performance of broilers.
4.2. Trial 2
B. subtilis var. natto N21 was selected for its high proteolytic capacity. Inoculated BS-fermented feed can generally improve broiler growth [
18]. However, feathers are mainly composed of keratin, the microbial decomposition of which presents difficulties. Feather decomposition ability for a given inoculation is usually evaluated by measuring the weight of feathers in a medium before and after fermentation [
35]. In trial 2, the feather decomposition rate without BS was lower than 4.5% after 72 h of incubation. Feather decomposition rates, however, were 33.7%, 62.0%, and 72.6%, respectively, after 24 h, 48 h, and 72 h of BS inoculation. High keratinase activity was also observed through further analysis of the filtrate derived from the medium. These results confirmed the keratinolytic ability of BS.
Bacillus subtilis natto is adapted to survive in a neutral pH environment [
36] and produces alkaline metabolites during reproduction (e.g., nattokinase), causing the cultivation environment to become alkaline [
37]. This is consistent with the observation from first-stage fermentation in trial 2, which exhibited a pH increase from 5.76 to 7.10. The pH of the EFM decreased from 7.07 to 5.45 upon second-stage fermentation since the inoculated microbe was BS or SC with great acidification ability. The drying process showed no significant effect on the pH of oven-dried BBEFM and BSEFM after fermentation. Chen et al. [
18] indicated that lactic acid and then acetic acid, neither of which is volatile, were the main organic acids in BS + SC-fermented feed. Accordingly, the drying process did not affect the pH levels in this trial.
Bacillus coagulans and B. subtilis have the capacity to form spores [
38,
39] which can resist high pressure, high temperature, and low pH values [
40]. Consequently, the drying process had no effect on the count of Bacillus-like bacteria in the BBEFM and BSEFM groups, nor the count of total lactic acid bacteria in the BBEFM group. SC and other microorganisms have limited heat-resistant capacity, and so their survival rate after the drying process was decreased.
Inclusion of BSEFM improved growth performance during the first three weeks to a level comparable to the control group (corn–soybean meal diets). On the other hand, the BBEFM group did not exhibit improved growth, not even reaching the EFM growth level. Our previous study confirmed that not all of the various two-stage fermented feeds could improve broiler growth performance [
19]. This agrees with the result obtained in this trial. The lower growth rate of the BBEFM group over days 1–21 may be attributed to the lower feed intake; however, further study is still required. The weight gain and feed conversion ratios of the BBEFM and BSEFM groups improved significantly in comparison to the control and EFM groups during the later stage of growth (22–35 days). In general, the broiler gastrointestinal tract is better developed at 22–35 days compared with 1–21 days of age. Broilers in the BBEFM group may experience compensatory growth at the later stage, which results in a comparable level of weight gain, feed conversion ratio, and PEF to the control group over the whole period. The BSEFM group showed better growth performance during both stages; therefore, the weight gain, feed conversion ratio, and PEF were significantly higher during the entire growth period than in the control group. The nutritional value of feather meal can be augmented through fermentation technology, upgrading its feeding value to be comparable to that of soybean meal [
8,
9,
10]. In this trial, the BSEFM not only partially substituted for soybean meal and full-fat soybean meal in the diet, but also improved broiler weight gain, feed conversion ratio, and PEF by 7.6%, 13.9%, and 6.1%, respectively. These results confirm that dietary inclusion of BSEFM could provide better feed value than a corn–soybean meal diet. In this trial, the weight gain of the EFM group was significantly lower than the control group during days 1–21, but not over days 22–35 or 1–35. This results are consistent with trial 1, which again confirms that the EFM dose tolerance for broiler diets is 10%.
Our previous studies showed that feeding a broiler BS + BC- or BS + SC-fermented feed could enhance the relative weight of the proventriculus and gizzard [
18,
19]. However, there was no significant difference in carcass traits among treatments. Although BBEFM and BSEFM are also produced from the same two-stage fermentation technology, the substrate and conditions of the fermentation differ from those of the previous fermented feed, and so the influence on carcass traits may also be different. In addition, the fermented feather meal in this study accounted for only 10% of the diet, a lower ingested amount of fermented feather meal than the fermented feed in the previous study. Therefore, BBEFM and BSEFM had no significant influence on carcass traits.
Glutamate oxaloacetate transaminase and lactate dehydrogenase are widely distributed in chicken liver, heart, kidney, and muscle, and creatine kinase is an enzyme specific to the heart and muscle. While rising glutamate oxaloacetate transaminase and lactate dehydrogenase levels with constant creatine kinase levels signal damage to the liver and kidney, an increase in all three enzymes signals both heart and muscle damage [
41]. Amylase is found in the liver, bile, saliva, and pancreas, with the pancreas being the major source. Amylase activity, therefore, can be used to diagnose pancreatic function [
42]. γ-glutamyl transpeptidase is a cell membrane enzyme related to glutathione metabolism and amino acid absorption in the glomerulus and small intestine [
43]. γ-glutamyl transpeptidase is an enzyme specific to poultry kidney [
41]. Alkaline phosphatase is an important enzyme involved in bone mineralization [
44], and along with serum calcium and phosphorus is an indicator of bone characteristics. No significant differences among carcass traits and serum biochemical constituents were observed in trial 2, indicating that diets including 10% EFM, BBEFM, or BSEFM did not show any detrimental effect on liver, heart, kidney, muscle, pancreas, or bone.
4.3. Trial 3
In order to determine the level of endogenous nitrogen in the gastrointestinal tract of the broiler, no nitrogen-containing materials were included in the NFD diet, which exhibits very low CP and amino acid content. Casein was the only source of nitrogen-containing material in the HDP diet and was assumed to be completely absorbed by birds. This thereby provides a basis for the assessment of endogenous amino acids induced by protein intake [
23]. Several publications have shown that the amino acid content in feathers was limiting in Met and His, with high Glu and Ser [
45,
46], which is consistent with the results of our trial. After fermentation, the amino acid composition ratio of BSEFM was still similar to that of EFM; however, the CP and total amino acid content increased by 11.7% and 21.1%, respectively. Shi et al. [
47] indicated that solid-state fermentation of rapeseed cake with
Aspergillus niger significantly increased total amino acids by 24.3% and essential amino acids by 28.5%. The solid-state fermentation of cottonseed meal with
Candida tropicalis increased total amino acids by 11.9% and essential amino acids by 12.0% [
48]. Our previous study produced two-stage BS + BB-fermented feed using a corn–soybean meal diet as the substrate; this fermentation was also found to increase the total amino acid content by 6.7% [
19]. Observations from the present study are also consistent with the above references. The increase in CP and amino acid content are also accompanied by loss of carbohydrate content through fermentation [
47,
49]. However, the increase in amino acid composition of BSEFM requires further study.
Recent publications have reported the potential for improvement in solubility and digestibility of feather meal through an enzyme or fermentation technique [
10,
11], techniques which were applied to EFM in this study. The use of SIAAD to evaluate the endogenous ileal amino acids is more accurate than the traditional AIAAD measurement methods [
23,
50]. Similar results, obtained using AIAAD and SIAAD on NFD or HDP diets in this trial, confirmed the amino acid digestibility improvement in feather meal via the BS + SC fermentation process.
The amino acid digestibility of feather meal varies greatly [
51]. Bandegan et al. [
45] indicated that the poorest AIAAD values among amino acids in feather were for Cys and Asp. Thr, Lys, and sulfur-containing amino acids showed the poorest values while Ile, Phe, and Val showed the highest values among the essential amino acids [
45]. Our results in this trial also showed Cys and Asp to have the poorest AIAAD values. However, the Thr, Leu, Val, and Arg values were poorest while Met, Phe, and Ile showed the best AIAAD values among the essential amino acids. The abovementioned literature is partly consistent with the results of this study. Ileal amino acid digestibility values are influenced by the analytical method and animal species [
52,
53,
54]. In addition, processing methods also affect amino acid composition and digestibility [
29,
46,
51,
55], which may contribute to the different results.
Summarizing the results of trials 2 and 3, the BSEFM group showed better nutritional digestibility under the same amount of feed intake, and therefore showed an improvement in weight gain and feed conversion ratio. Protein from feather meal might be decomposed during BS + SC two-stage fermentation, thus producing microbial protein or metabolites that improve growth performance of broilers.