Adequate supplies of dietary protein and amino acids (AA) is required to achieve optimum growth performance of commercial broiler chickens. The AA digestibility can vary due to the anti-nutritional factor and chemical composition in feed ingredients. Therefore, the use of digestible AA has been recognized as more appropriate rather than total AA when formulating broiler diets (Lemme et al., 2004). In poultry, the AA digestibility values have been determined using excreta collection method with the cecectomized roosters (NRC 1994). Previous reports (Ravindran et al., 1999; Lemme et al., 2004; Ravindran et al., 2005; Kong and Adeola, 2014) have demonstrated that the risk of deamination by microbiota in the cecum because it leads to inaccuracy of AA digestibility for birds. Therefore, it has been suggested that ileal digestibility is more appropriate value for AA digestibility of broilers than total tract digestibility (Ravindran et al., 1999).
In addition, the digestible AA contents in feed ingredients can be obtained from either apparent (AID) or standardized ileal digestibility (SID). The SID of AA is calculated by the correction of the basal endogenous losses (BEL) of AA for AID. The BEL of AA are related to the dry matter intake and the relative contribution of the BEL of AA to total ileal outflow can be influenced by the AA contents. Therefore, the AID of AA increases as AA intake increase because the relative contribution of BEL of AA to total ileal AA outflow decreases with increasing AA intake. On the other hand, the SID of AA are independent on the AA intake. This may influence additivity which is the fundamental assumption of diet formulation. Feed ingredients used for poultry diets vary CP and AA contents, thus to accurately formulate poultry diet based on digestible AA contents, the use of SID of AA is recommended rather than AID.
A number of studies (Lemme et al., 2004; Ravindran et al., 2005; Huang et al., 2006; Adodokun et al., 2009; Szczurek, 2009; Kong and Adeola, 2013) for determining AA digestibility of corn and soybean meal (SBM) in poultry diets have been reported. However, there is scarce information on standardized ileal AA digestibility of diverse feed ingredients. Therefore, the objective of the present study was to determine the AID and SID of CP and AA in five feed ingredients of male broiler chickens.
MATERIALS AND METHODS
The experimental procedure was approved by the Institutional Animal Care and Use Committee at Konkuk University (KU16025).
Five feed ingredients including corn, wheat, corn distillers dried grains with solubles (DDGS), SBM, canola meal (CM) were used for digestibility study (Table 1). Inclusion rate of test ingredients was adjusted to contain approximately 20% CP in the experimental diet except for corn and wheat. An N-free diet was also prepared to estimate the BEL of CP and AA. All diets contained 1% calcium and 0.45% non-phytate P. Chromic oxide (0.5%) was added in all diets as an indigestible index for the calculation of digestibility. Vitamins and trace minerals were included in all diets to meet or exceed nutrient requirement estimates for broilers (NRC, 1994).
2 Provided per kilogram of diet: vitamin A, 18,000 IU; vitamin D3, 8,000 IU; vitamin E, 100 IU; vitamin K3, 5.0 mg; vitamin B1, 4.0 mg; vitamin B2, 12.0 mg; vitamin B6, 6.0 mg; vitamin B12, 26 μg; niacin, 100 mg; folic acid, 3 mg; biotin, 0.2 mg; panthothenic acid, 30.0 mg; Mn, 192 mg as manganese sulfate; Zn, 180 mg as zinc sulfate; Fe, 100 mg as ferrous sulfate and ferric oxide; Cu, 48.0 mg as copper sulfate; I, 2.4 mg as calcium iodate; and Se, 0.72 mg as sodium selenite.
Nine hundred sixty Ross 308 male broiler chicks (674±4.5 g) were fed a standard starter diet from d 0 to 19. On d 19, chicks were individually weighed and assigned to 6 treatments with 8 replicate cages (20 birds /cage) in a randomized complete block design using the Experimental Animal Allotment Program (Kim and Linderman, 2007). Experimental diets and water were offered ad libitum from d 19 to 23, and temperature was maintained at 28°C during this period.
At the end of the experiment (d 23), all birds were euthanized by CO2 asphyxiation. The ileal digesta from the lower two-thirds of the ileum were collected and rinsed with distilled water into plastic containers. Collected samples were immediately stored at 20°C until chemical analysis.
The test feedstuffs and diets samples were dried at 100°C for 24 h and ileal digesta samples were freeze-dried. Dried test feedstuffs, diets, and ileal digesta samples were analyzed for N by Kjeldahl procedure using a Kjeltec Auto System (Kjeltec Auto System, Buchi, Flawii, Switzerland). The samples of ingredient, diets, and ileal digesta were analyzed for the AA (AOAC International, 2005; method 982.30).
Ileal digestibility and BEL of nutrients was calculated described by Kong an Adeola (2014):
Data were analyzed using the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC, USA) as a randomized complete block design. The experimental unit was a cage and the fixed effect was treatments. Digestibility values were presented as least squares means and compared using Tukey’s test. The significance was set at an alpha-level of 0.05.
RESULTS AND DISCUSSION
Tables 2 and 3 show analyzed CP and AA concentration in 5 test ingredients and experimental diets used in the present study, respectively. The analyzed CP content was the greatest in SBM (45.97%) and least in corn (7.33%). The SBM has the greatest analyzed concentration for most of AA. The analyzed AA concentrations in feed ingredients were within the range of previously reported values (Evonik, 2010; Rostagno et al., 2017). The analyzed CP and AA concentrations in the experimental diets were close to the calculated values.
Table 4 presents the AID of CP and AA for broilers fed experimental diets. The indispensable AA with the least AID was Thr for all the ingredients except for DDGS in which Lys has the least AID. The AID of Leu was greatest indispensable AA in corn and DDGS, Phe in wheat, Arg in SBM, and Met in CM and DDGS. The digestible CP and AA contents in feed ingredients derived from in vivo digestibility trial do not always match with total contents derived from chemical analyses (Lemme et al., 2004; Kong et al., 2014; Block and Dekker, 2017). Therefore, the information on the digestible AA content in the feed ingredients is needed for accurate diet formulation. In the present study, 2 cereal grains have similar indispensable AA composition but the AID of AA in corn was greater (P<0.05) than those in wheat except for Ile and Phe. Ravindran et al. (2005) reported that average AID coefficient of AA in corn (0.81) was less than that of wheat (0.83). This discrepancy may be caused by the different wheat cultivar and age of chickens used between studies. In the cereal grains and cereal byproduct evaluated, Thr and Lys were the least digestible indispensable AA and the most digestible AA was Phe in wheat, Leu in corn, and Leu and Met in DDGS. This result is in agreement with those reported by Ravindran et al. (2005). In addition, Thr was also the least digestible indispensable AA in SBM and CM. Threonine is a major AA in endogenous protein which is included as the output of protein for the AID calculation, consequently this may decrease AID of Thr (Ravindran et al., 2005; Kong and Adeola, 2013; Osho et al., 2019).
Table 5 shows the BEL and SID of CP and AA for broilers fed experimental diets. The estimated BEL of CP and AA was lower than the previously reported value (Lemme et al., 2004). The differences in estimates may be attributed to the method used to estimate BEL. Lemme et al. (2004) reported average BEL derived from five experiments using enzymatically hydrolyzed casein (EHC) method. Although the EHC method remove the uncertainty concerning the 100% digestibility of casein protein, the EHC method may increase BEL estimate when incomplete separation of EHC from endogenous protein occurs (Hodgkinson et al., 2000).
Similar to the results in the AID, the determined SID of CP and AA varied (P<0.01) among test ingredients. The SID of Leu was greatest indispensable AA in corn and DDGS, Phe in wheat, Arg in SBM, and Met in CM. The indispensable AA with the least SID was Thr for corn, wheat, SBM, and CM, whereas Lys was the least SID indispensable AA in DDGS. The low Lys digestibility in DDGS may be attributed to the Maillard reaction during the drying process in DDGS production. In this reaction, the free amino group of Lys can be bound to the reducing sugars resulting in decrease in the digestibility of Lys in DDGS (Fastinger et al., 2006).
The SID of AA is slightly less in corn used in present study than those reported by Adedokun et al. (2008). However, comparable SID for SBM were observed between studies. The SID of indispensable AA in DDGS in the present study are comparable with values in light-DDGS reported by Adedokun et al. (2008) in 21-d-old broilers. However, Osho et al. (2019) reported less SID of indispensable AA in DDGS. Theses discrepancies may be attributed to the difference in processing condition of DDGS samples and details in methods employed for digestibility determination.
The differences between AID and SID of CP ranged from 2.2 (DDGS) to 6.4 (corn) percentage points indicating that relative significance of BEL on total ileal outflow is greater for corn than for DDGS (An et al., 2020). The correction of AID for BEL can reduce this influence and consequently increase an additivity of digestibility for diet formulation (Kong and Adeola, 2013).
In conclusion, the results of the present study showed that there are variations in both AID and SID of CP and AA among feed ingredients. It is also confirmed that the digestible CP and AA contents in feed ingredients determined by using animals do not always match with total contents from chemical analyses.