Growth performance of male broiler chickens in different growth phases in response to amino acid concentrations in the pre-starter diet

The experimental diets, based on corn-SBM, were formulated with standardized ileal digestible (SID) AA. These diets encompassed six levels of dietary SID AA, ranging from 90% to 115% of the expected requirements for each respective AA. Dietary Lys, Thr [17] and Met [18] recommendation levels (100% AA) were adjusted based on the results of our previous studies (Table 1). To avoid any interactions between indispensable AA, levels of all dietary AA were adjusted to maintain a constant ratio to Lys. The ideal ratios for most AA (Arg, Ile, Leu, Cys, and Val) during the starter phase were obtained from the study of Hoehler et al. [19]. Ideal ratios for His, Phe, and Trp were obtained from Wu [20]. All experimental diets from d 0 to 7 were formulated to be isonitrogenous (22.5% of CP) by reducing the inclusion of glutamic acid, thereby enhancing the utilization of synthetic AA. Ten synthetic or crystalline AA and glutamic acid were supplemented to consider both the dietary indispensable and dispensable AA. We analyzed the nutrient compositions in feed ingredients, corn, SBM, as well as the commercial feeds for starter (d 8 to 21) and grower (d 21 to 28) phases used in the present study, as shown in Table 2. The amounts of minerals and vitamins in all experimental diets met the requirements for broilers reported by the NRC [21].

A total of 720-day-old male broilers (Ross 308) were used for the experiment. At the beginning of the experiment, all birds (initial body weight [BW]: 46.5 g [SD = 2.91]) were individually tagged, weighed, and allocated to six experimental diets in a randomized complete block design, with initial BW as a blocking factor using a spreadsheet [22] to minimize the weight differences between each treatment. All birds were fed ad libitum with the experimental diets for 7 d. The experimental environment was controlled with continuous lighting, and the temperature was gradually reduced from 33°C to 25°C by d 28.

The BW of all birds and the remaining feed quantity were recorded upon discovering a deceased bird within a specific pen. This procedure was implemented to refine the growth performance data. To correct the data, we used the BW and estimated individual feed intake of the deceased birds, following the modifications suggested by Sung and Adeola [23]. The calculation for determining the metabolizable energy required for maintenance (expressed in kcal/d) was conducted following the equation proposed by Noblet et al. [24]: 136 kcal × BW^0.70.

On d 21 and 28, the BW, feed supply, and leftovers per cage were recorded for each individual bird. This information was used to determine the BWG and feed intake. Subsequently, the gain-to-feed ratio (G:F) was calculated utilizing the data on BWG and feed intake. Furthermore, analyses were conducted to determine the dry matter, CP, and AA compositions in the experimental diets. The samples were ground using a Cyclotec Mill (CT293 cyclotec, Foss, Hillerød, Denmark) through a 1.0-mm screen for chemical analysis. The feed ingredients and samples of the commercial and experimental diets were dried at 135°C for 2 h (AOAC [25]; method 930.15). Additionally, dried corn, SBM, and samples of the commercial and experimental diets samples were analyzed for their AA composition (AOAC [25]; method 982.30 E [a, b, c]).

All data for each ingredient were analyzed using the GLM procedure in SAS software (SAS Institute, Cary, NC, USA). Data for the CP and AA concentrations were examined using one-way ANOVA and considered as the fixed effect. The interquartile range (IQR) was used to identify and eliminate outliers; data points with values exceeding 1.5 times the IQR were deemed outliers. The average flock uniformity of treatments was determined by calculating the coefficient of variation (CV) of BW. The dietary SID Met concentrations in the pre-starter diet were considered a fixed variable, whereas the block (replicate) was considered a random variable. Means for each treatment were computed using the least square means. Orthogonoal polynomial contrast coefficients were used to test the linear and quadratic effects of increasing AA in diets. The experimental unit was a pen, and statistical significance was set at p < 0.05.

Dietary total indispensable AA (synthetic and crystalline AA) to the dispensable AA (glutamic acid) ratio increased from 0.22 (90% AA) to 0.84 (115% AA). This may help reduce the impact of different dietary protein levels and diminish the potential limitations in protein accretion caused by varying ideal protein ratios. The differences in digestion and absorption between protein-bound and non-bound AA are well documented by the previous literatures [27,28], however, in this study, the protein accretion might be explained by increased AA concentrations derived from non-bound AA. The average standardized ileal AA digestibility of protein-bound AA, specifically corn and SBM used in this study, was determined to be 87.1% and 88.7%, respectively, based on data from three previous studies [29–31]. In contrast, non-bound AA were found to exhibit nearly 100% digestibility [32,33]. Consequently, it is commonly assumed in the practical formulation of broiler diets that the digestibility of non-bound AAs is 100%. Furthermore, Selle et al. [34] demonstrated that variations in AA digestibility can result in imbalances at protein synthesis sites. In addition, previous research has investigated determining the ideal protein ratios for young broilers within the first three weeks of life [20,21,35]. Compared to these studies, the protein ratio employed in this study proves to be adequate in meeting the indispensable AA needs of birds in their first week of life. Nonetheless, it is important to note that the ideal protein concept, which solely considers indispensable AA, is not entirely effective in supporting the synthesis of AA within animal cells de novo [36]. In this regard, there has been a growing emphasis on providing appropriate ratios and sufficient amounts of both indispensable and dispensable AA in order to achieve efficient protein accumulation within the animal body [36]. An excess of dietary glutamic acid can serve as a nutritional resource by providing a precursor for the synthesis of AAs in cases where dietary AAs are lacking. Eariler studies have suggested the use of large amounts of glutamic acid (10 to 13 times the level of Lys) to supply indispensable AA [37,38]. However, this study provided dietary glutamic acid at relatively lower levels, ranging from 4 (115% AA) to 7 (90% AA) times the level of Lys. This contrasts with the higher levels used in previous studies. Unfortunately, in this study, the ideal protein ratio only considered indispensable AA, with L-glutamic acid being supplemented to meet the demands of dispensable AA and adjust to maintain the dietary protein level at 22.5% across the experimental diets. The addition of L-glutamic acid from 8.50% (90% AA) to 4.87% (115% AA) may be excessive for young broiler chicks, considering the recommended optimal AA ratios from Texas A&M University [20]. The reduced growth performance observed in birds fed diets with less than 100% AA may be attributed to the low ratio of indispensable to dispensable AA in the diet. Difference of approximately 20 grams in BW of birds at 7 days was observed between the treatment groups receiving 115% AA and 90% AA. This discrepancy could be resolved within a single day at this age. Furthermore, at this age, the voluntary feed intake of birds is neglible because of thier underdeveloped gastrointestinal tracts. As a result, the authors consider it challenging to conclusively determine the impcat of elevated dietary L-glutamic acid on the findings of this study. BW was observed to increase as dietary glutamic acid concentration decreased, although this outcome was predominantly might attributed by the dietary indispensable AA concentration rather than the glutamic acid concentration. It is important to note that dispensable AA cannot be converted into indispensable AA, while indispensable AA can be exchanged with dispensable AA during catabolism.

It is now widely acknowledged that the dietary concentrations of AA in broiler diets may act as a limiting factor in early-stage growth [1]. Previous studies have demonstrated varying growth rates of broilers in response to absolute dietary AA concentrations [6,7,39,40]. The diet (100% AA) utilized in this study had a Lys concentration of 1.36%, which exceed the dietary SID Lys concentration (%) reported by Rostagno et al. [41] for 7-day-old with low-standard performance male broilers weighing 194 g (1.34%). However, the recorded BW of the 7-day-old broilers in this study was similar to that at five days old, as noted by Rostagno et al. [41]. The estimated SID Lys required for male broilers at five days old was 1.35%, similar to the dietary Lys content in the 100% AA diet. Additionally, Dozier et al. [7] reported that the dietary Lys for Ross high-yielding broilers at 7 d of age is 1.36%, estimated through regression equations. However, Met, the first-limiting AA in a corn-SBM-based diet, had the lowest percentage (0.47%) compared to the values of 0.55% recommended by Rostagno et al. [41] and 0.62% recommended by Dozier et al. [7]. Based on the values from these literatures, the 100% AA diet (0.47% Met) employed in this study may be insufficient to achieve maximum growth responses.

Following hatching, the development process of the birds’ digestive organs commences as they initially adapt to solid feed and use it as a nutrient source rather than nutrients stored in the yolk. Particularly during this phase, the voluntary feed intake of young chicks is negligible compared to that of older, growing birds, making them susceptible to influence related to energy or AA intake [14]. However, as indicated by the present study, differences in BWG and G:F persisted despite improved voluntary feed intake in later stages of growth. From d 8 to 28, during which the birds were fed commercial diets following a seven-day experimental feeding period, the preceding dietary AA concentration affected both BW and feed efficiency. We hypothesized that growth responses, negatively impacted by lower AA density in the first week of age, would be rectified during the re-alimentation period with a commercial diet. Although the commercial starter diet utilized in this study contained elevated levels of indispensable (Met, Phe, Thr, and Trp) and all dispensable AA (except for Cys and Glu) when compared to the experimental diets containing 105% AA, our findings demonstrate that comparable growth patterns were observed during the entire grow-out period. Additionally, at d 28, the BW of groups fed 100% AA or dietary AA amounts exceeding their nutrient requirements ranged from 1,523 g to 1,618 g (Ross 308 male broiler standard is 1,576 g [26]).

Several studies have investigated the impact of different feeding strategies on the growth rate and carcass traits of birds in subsequent growth phases following the provision of diets with varying levels of dietary AA density [6,16,42–45]. Corzo et al. [16] found that birds exhibited increased growth rates when fed a diet with higher AA density, noting that concentrating high AA density in the early growth stage yielded greater benefits for subsequent growth. These findings align with the results of the present study. In contrast to our findings, some studies have observed compensatory growth in birds following re-alimentation [14,42]. Certain research reports suggest that extending the feeding period of diets with lower dietary AA levels in later growth stages might mitigate the adverse effects of lower AA density in preceding phases. Eits et al. [42] also noted no evidence of a nutritional carry-over effect from previous feeding on the proportion of carcass weight in overall gain. Noy and Skaln [14] evaluated the long-term effects of feeding different dietary fat and CP concentrations in the early post-hatch period. They followed a feeding regimen similar to that in the present study, wherein birds were fed for 7 days post-hatching and then provided with the same commercial diet until they reached market weight (d 41). They found that differences in growth attributed to feeding varying dietary protein levels during the first 7 days had almost disappeared by day 18, with no significant differences observed thereafter. In the present study, during periods of feeding with the commercial diet, birds consumed similar amounts of feed, except for a week prior to the conclusion of the experiment. This implies that providing chicks with optimal dietary AA during the early growth stage may be crucial for ensuring performance on slaughter day. However, if birds were nourished for prolonged durations (as they approach slaugher weight), it is plausible that they may have exhibited compensatory growth. This phenomenon can be attributed to their augmented voluntary feed intake, which arises from expanding their digestive tract capacity as they mature. Because theoretical maximum feed intake would be set by the capacity of the digestive stystem. Unfortunately, the duration of this study was limited to 28 days, which prevented us from confirming the long-term impact of dietary AA density on growth during the later stages of feeding. Therefore, additional research is required to establish the extent of the compensatory growth responses in subsequent stages, considering growth potential of modern broilers. Such insights would be valuable additions to the feeding regimen programs for modern broilers to improve the growth responses as well as meat quality.

Evidence suggests that uniformity improves with the provision of high levels of a limiting AA or protein [9]. Corzo et al. [16] noted an enhancement in flock uniformity in terms of BW as dietary AA concentration increased. Various factors, including animal, nutritional, and environmental conditions, influence BW uniformity. Among these factors, a deficiency of dietary AAs, such as limiting AAs, results in poor growth uniformity [9]. Vasdal et al. [46] proposed that the expected level of flock weight uniformity in broilers should be below 10% CV. Additionally, Berhe and Gous [9] and Gous [11] reported a marked increase in bird growth variation when poor-quality feed is provided. Berhe and Gous [9] found that birds fed a high-protein diet exhibited lower variability in BW at 21 d of age compared to those fed a low-protein diet (11.3% in low protein vs. 9.8% in high protein). However, the estimated CV in the present study did not align with changes in dietary AA density during the overall growth periods, although CV values remained below 10%. Furthermore, we observed no differences in variability between birds fed experimental diets and those fed commercial diets. This difference could be attributed to the dietary ingredients and nutrient compositions of the experimental diets. In this study, experimental diets were administered for a 7-day feeding period, with a fixed dietary protein level of 22.5%, and AA density adjusted by supplementing nine synthetic indispensable AA, as well as L-Cys and glutamic acid as dispensable AA. Conversely, previous studies predominantly utilized two or three synthetic AA, primarily Met, Cys, Lys, and Thr, or feed-grade commercial AA. Notably, improvements in growth responses have been primarily observed when birds are fed diets substituting plant protein sources with synthetic AA. The reasons for the improved growth responses in the subsequent growth periods remain unclear, but the composition of dietary protein sources (protein-bound or non-bound AA) may contribute to the observed variations in the studies.

In conclusion, birds fed diets with increasing AA density (over 100% AA) exhibited linearly greater BW and improved feed efficiency during the early stages of life (d 0 to 7). This study demonstrates that feeding chicks diets with varying AA densities during the first week after hatching may persist in similar growth patterns in older birds. Increasing the dietary AA density from 90% to 115% of the recommended AA intake from d 0 to 7 linearly affected BW and feed efficiency until 28 days of age.