Protein and amino acids requirements of broiler chickens

Amino acids, obtained from dietary protein sources, are used by poultry to meet a variety of functions and can be divided into two main categories. The AAs which cannot be synthesised are classified as essential amino acids (EAA), and those that can be synthesised in the body are classified as non-essential amino acids (NEAA; NRC, 1994). However, there are a few AAs which are not synthesised in the body fast enough to meet maximum growth requirements. Therefore, it is necessary that these AAs be supplied in the diet. The nutritional classification of amino acids for poultry is presented in Table 1.

Broiler chickens do not require an exact level of CP; rather, they require an exact and balanced quantity of EAA, and adequate protein, either in the form of ready to use NEAA or as N for NEAA synthesis (NRC, 1994). However, broiler requirements for AA is affected by many factors, including high ambient temperature, gender, age, strain, and some diet-related factors, such as metabolisable energy (ME), AA imbalance, the effects of vitamins and coccidiostats, and amino acids antagonistic relationships (D'Mello, Reference D'MELLO2003).

Table 1

Nutritional classification of amino acids for poultry.

*Tyrosine is synthesised from phenylalanine, Cysteine from methionine, and hydroxylysine from lysine.

Source: Scott et al. (Reference SCOTT, NESHEIM and YOUNG1982)

Despite much research to investigate dietary protein requirements of birds kept under high ambient temperatures, very little, and often conflicting data, on amino acids requirements under such conditions are available. Some research has suggested unchanged lysine (Sinurat and Balnave, Reference SINURAT and BALNAVE1985; Mendes et al., Reference MENDES, WATKINS, ENGLAND, SALEH, WALDROUP and WALDROUP1997), threonine (Shan et al., Reference SHAN, STERLING, PESTI, BAKALLI, DRIVER and ATENCIO2002), and tryptophan (Ojano-Dirain and Waldroup, Reference OJANO-DIRAIN and WALDROUP2002; Shan et al., Reference SHAN, STERLING, PESTI, BAKALLI, DRIVER and ATENCIO2002) requirements at high temperatures. However, other research recommended higher lysine (Han and Baker, Reference HAN and BAKER1993; Ojano-Dirain and Waldroup, Reference OJANO-DIRAIN and WALDROUP2002) and methionine (Ojano-Dirain and Waldroup, Reference OJANO-DIRAIN and WALDROUP2002) requirements for heat-stressed birds. Conversely, some published data suggested decreased lysine (Waldroup, Reference WALDROUP1982; Corzo et al., Reference CORZO, MORAN and HOEHLER2003), methionine (Balnave and Oliva, Reference BALNAVE and OLIVA1990) and arginine (Chamruspollert et al., Reference CHAMRUSPOLLERT, PESTI and BAKALLI2004). These conflicting results could be attributed to the wide differences in the duration and severity of heat exposure among these studies.

In terms of gender, some studies have suggested that males require greater quantities of AA than females at a similar age. It has been indicated that lysine (Han and Baker, Reference HAN and BAKER1993; Reference HAN and BAKER1994; Dozier et al., Reference DOZIER, CORZO, KIDD and SCHILLING2008; Reference DOZIER, CORZO, KIDD, TILLMAN and BRANTON2009), methionine (Thomas et al., Reference THOMAS, ZUCKERMAN, FARRAN and TAMPLIN1986), threonine (Thomas et al., Reference THOMAS, FARRAN, TAMPLIN and ZUCKERMAN1987; Dozier et al., Reference DOZIER, MORAN and KIDD2001), arginine (Kessler and Thomas, Reference KESSLER and THOMAS1976), and tryptophan (Hunchar and Thomas, Reference HUNCHAR and THOMAS1976) requirements are higher for male broilers than for females. This was attributed to the higher protein and less fat deposition in male weight gain (Han and Baker, Reference HAN and BAKER1994).

It has been consistently reported that dietary requirements for all AA and CP decrease with the age of the chicken (NRC, 1994). The NRC (1994) suggested AA requirements for three age periods 0-3 (starter), 3-6 (grower/finisher), and 6-8 (finisher) weeks of age (Table 2). However, most of the present commercial broiler chicken breeders, such as Cobb 500, Ross 308, Arbor Acres and Hubbard, suggest AA specifications for shorter age periods (e.g., d 0- 10, 11- 24, 25- 39, 40- 51, and > 52 d) for more AAs utilisation efficiency.

In terms of broiler strains, significant improvements in broiler performance and carcass composition have been achieved as a result of genetic selection, and CP and AA requirements of these new genotypes have changed accordingly. The AA requirements of today's broiler chickens are relatively higher than those suggested by the NRC (1994). By comparing the recommended AA specifications for today's broiler chicken strains, differences can be observed (Table 3). It is worth mentioning that suggested AA specifications for all strains are set to support an ideal amino acid profile, where the requirement of the main amino acids that may be limited in broiler feeds are calculated, and with lysine used as the reference AA to which ratios are set for the other AAs. Unlike NRC (1994), suggested ratios for the ideal AA profile of these broiler strains are based on digestible AA.

Table 2

Protein and amino acids requirements of broilers (%).

^# Broiler chickens do not have a requirement for crude protein per se. There should be sufficient crude protein to ensure an adequate nitrogen supply for the synthesis of non-essential amino acids, however. Suggested requirements for crude protein are typical of those derived with corn-soybean meal diets, and levels can be reduced when synthetic amino acids are used.

Source: NRC (1994)

Table 3

Latest recommended crude protein and amino acids specifications for as-hatched Arbor Acres, Cobb500, Hubbard, and Ross308 broilers¹

¹Amino acids specifications in this table were obtained from the latest broiler nutrition specifications guides of Arbor Acres (2014), Cobb (2015), Hubbard (2014) and Ross (2014).

²The age periods 0-10, 11-20, 21-33, 34-42, and >42 d are considered as pre-starter, starter, grower, finisher, and withdrawal, respectively.

³Digest = digestible

NS = not specified

Protein requirements of broiler chickens under hot climate

Determining the protein requirements of broiler chickens at high ambient temperatures has been a field of research for many years. However, findings have been inconsistent and different recommendations have been made. The first was to increase dietary protein, to compensate for reduced protein intake as a consequence of decreased feed intake associated with heat stress (Temim et al., Reference TEMIM, CHAGNEAU, GUILLAUMIN, MICHEL, PERESSON, GERAERT and TESSERAUD1999; Reference TEMIM, CHAGNEAU, GUILLAUMIN, MICHEL, PERESSON and TESSERAUD2000; Gonzalez-Esquerra and Leeson, Reference GONZALEZ-ESQUERRA and LEESON2005). Temim et al. (Reference TEMIM, CHAGNEAU, GUILLAUMIN, MICHEL, PERESSON, GERAERT and TESSERAUD1999) reported significantly higher weight gain and better feed efficiency when heat-stressed broiler chickens were fed diets higher in CP (25% vs. 19.6%) from 4-6 weeks of age. Gonzalez-Esquerra and Leeson (Reference GONZALEZ-ESQUERRA and LEESON2005) reported improvements in broiler performance when high crude protein diets were fed from d 21 to 42 at 27.3°C and 31.4°C housing temperatures. Conversely, excess dietary protein reduced the growth rate of commercial, fast-growing broiler chickens raised at a constant 32°C from 4-8 weeks of age (Cahaner et al., Reference CAHANER, PINCHASOV, NIR and NITSAN1995). Cheng et al. (Reference CHENG, HAMRE and COON1997) noted adverse effects of high protein diets (22% or 24% CP) on growth performance in 3-6-week old heat-stressed broilers. Similarly, Cheng et al. (Reference CHENG, HAMRE and COON1999) found that increasing CP (22 vs. 20% CP) and AA (110%) concentrations to compensate for reduced feed intake when broilers were reared under constant (32.2°C) or cyclic (26.6°C for 16 h to 35°C 8 h) heat stress conditions, negatively affected growth performance. Suppressed growth performance when feeding excess dietary protein diets under heat stress conditions can be attributed to higher metabolic heat production because protein has the highest heat increment during metabolism or catabolism compared to other nutrients (Musharaf and Latshaw, Reference MUSHARAF and LATSHAW1999). Basically, any increase in dietary CP will be followed by an increase in nitrogen excretion, which is energetically expensive (Musharaf and Latshaw, Reference MUSHARAF and LATSHAW1999). Accordingly, the poorer performance seen when broilers were fed increased-CP diets under heat stress conditions could be explained by the higher heat production (associated with protein synthesis and from nitrogen excretion) as well as lower energetic efficiency.

Therefore, dietary CP reduction, while maintaining AA within the recommended requirements, has long been suggested as the best strategy (Waldroup, Reference WALDROUP1982; Austic, Reference AUSTIC1985; Teeter, Reference TEETER1994). Later, this approach was supported by the advantages of reducing feed cost and nitrogen excretion.

Effects of reduced-CP diets on the performance of broilers reared at elevated environmental temperatures

The availability of synthetic AA, accompanied by the establishment of the ideal protein concept (Baker, Reference BAKER1997), have contributed to a decrease in dietary CP in broiler diets. In general, there is consensus that synthetic AA in broiler diets can replace CP to a certain level, and any reduction beyond that level has been shown to adversely affect the growth performance of broiler chickens, even when the EAA requirements are met. However, there is considerable disagreement on the exact level of appropriate reduction. It is very difficult to cover all studies on broilers’ growth performance in response to feeding reduced-CP under heat stress in a review of studies. Hence, in this review, only selected studies have been reviewed and summarised in chronological order (Table 4).

Table 4

Effects of reduced-protein diets on growth performance in broilers reared under various heat stress conditions.

HS, heat stress; CP, crude protein; EAA, essential amino acids; FI, feed intake; ADFI, average daily feed intake; BW, body weight; WG, weight gain; ADG, average daily gain; FCR, feed conversion ratio.

¹ Values represent analysed dietary crude protein or calculated crude protein if the analysed values are not presented.

² When the study investigated various CP levels with some higher than the recommended CP level for the particular age, a diet with CP level equal/close to recommended CP level was considered as the control diet.

³ Diets were fortified with the mentioned EAA to meet the requirements.

⁴ ‘‘ns’’ indicates non-significant difference between the control and reduced-CP diets; ‘‘+’’ sign indicates a significant positive response to reduced-CP diet compared to control diet;

‘‘-’’sign indicates a negative response to reduced-CP diet compared to control diet.

⁵ A natural (uncontrolled) hot and humid environment with maximum (at least 6 h per day) and minimum temperatures of 36°C and 26°C, respectively.

⁶ 32°C for 4 h , 28°C for 16 h to 23°C for 4 h.

⁷ NA indicates the difficulty to estimate the average daily temperature from the details of the environmental condition provided by authors.

Early research to evaluate the influence of reduced-CP-diet on broiler performance under heat stress conditions was carried out by Alleman and Leclercq (Reference ALLEMAN and LECLERCQ1997). In their study, heat-stressed (32°C constant heat stress) broilers were fed a diet reduced by 4% CP (16% vs. 20% CP), fortified with methionine, lysine, threonine, valine and arginine in order to meet requirements from 23 to 44 d of age. The reduced-CP diet significantly increased FCR (2.413 vs. 2.194) and decreased body weight gain (939 vs. 1118 g) compared to the control group.

Two years later, Cheng et al. (Reference CHENG, HAMRE and COON1999) again applied a 4% reduction in dietary CP (16% vs. 20% CP) which negatively affected the performance of broilers under constant (32.2°C) and cyclic (26.6°C for 16 h to 35°C 8 h) heat stress conditions from 21 to 49 d of age. Under constant heat stress conditions, the reduced-CP diet resulted in significantly lower gain/feed ratio (0.315 vs. 0.380) and weight gain (1280 vs. 945 g) compared to the control group. Under the cyclic heat stress conditions, the reduced-CP diet significantly decreased feed intake (2368 vs. 3023 g/bird) and weight gain (1045 vs. 1071 g) compared to the control group.

Faria Filho et al. (Reference FARIA FILHO, ROSA, VIEIRA, MACARI and FURLAN2005) evaluated the performance of 7 to 21-d old broilers fed a 3% reduced CP diet (18.5 vs. 21.5% CP) fortified with all EAA except Phe, Leu and His, and maintained under constant heat stress conditions of 33°C. The birds fed the reduced-CP diet showed significantly higher FCR (1.48 vs. 1.40) and lower weight gain (623 vs. 663 g) compared to birds fed the 21.5% CP control diet.

At almost the same time, Gonzalez-Esquerra and Leeson (Reference GONZALEZ-ESQUERRA and LEESON2005) evaluated the effects of reducing dietary CP from 20 to 18% CP on the performance of broilers reared under acute (constant 27.3°C) or chronic (constant 31.4°C) heat stress conditions from 21 to 42 d of age. The authors reported no effect on body weight gain, feed intake or feed:gain ratio in birds reared at 27.3°C. However, the reduced-CP diet resulted in significantly higher feed:gain ratio (2.40 vs. 2.26) in broilers reared at 31.4°C.

A study by Faria Filho et al. (Reference FARIA FILHO, CAMPOS, ALFONSO-TORRES, VIEIRA, ROSA, VAZ, MACARI and FURLAN2007) evaluated the influence of a 3% reduction in dietary CP (from 20.0 to 17.0%) via supplementation with Met, Lys, Thr, Val, Arg, Trp and Ile in broilers kept under constant heat stress conditions (32.2°C) from 21 to 42 d of age. The reduction in CP negatively affected the growth performance of broilers by increasing the FCR by 9.0% (1.93 vs. 1.77).

Zaman et al. (Reference ZAMAN, MUSHTAQ, NAWAZ, MIRZA, MAHMOOD, AHMAD, BABAR and MUSHTAQ2008) conducted an experiment to evaluate the performance of broiler chickens reared in a hot climate and fed diets varying in CP (20.8 vs. 18.8% CP) and metabolisable energy from seven to 28 d of age. When the birds were fed the standard energy level (12.55 MJ/kg), a 2% reduction in CP did not affect weight gain, feed intake or feed:gain ratio of broiler chickens. Similarly, Laudadio et al. (Reference LAUDADIO, DAMBROSIO, NORMANNO, KHAN, NAZ, ROWGHANI and TUFARELLI2012) reported similar growth performance when broilers were fed a reduced-CP diet (2% reduction; from 20.5 to 18.5% CP) under summer environmental conditions from 14 to 42 d of age. In both the Zaman et al. (Reference ZAMAN, MUSHTAQ, NAWAZ, MIRZA, MAHMOOD, AHMAD, BABAR and MUSHTAQ2008) and Laudadio et al. (Reference LAUDADIO, DAMBROSIO, NORMANNO, KHAN, NAZ, ROWGHANI and TUFARELLI2012) studies, reduced-CP diets were fortified with the limiting amino acids Met, Lys and Thr to meet requirements.

Awad et al. (Reference AWAD, FADLULLAH, ZULKIFLI, SOLEIMANI and LOH2014a) used 1 to 21-d-old broiler chickens to investigate the effect of feeding different ideal EAA supplemented, reduced-CP diets (19.2 or 17.9% CP) under tropical climates, i.e. a hot and humid environment with maximum (at least 6 h per day) and minimum temperatures of 35°C and 26°C, respectively. Compared to a control group of broilers fed 20.6% CP, the reduction in CP by 1.4% did not affect performance of broilers. However, the reduction in CP by 2.7% resulted in significantly lower weight gain (707 vs. 778 g) and higher FCR (1.48 vs. 1.39) compared to the 20.6% CP group.

In a different study in a hot tropical environment maximum (for at least for 6 h per day) and with minimum temperatures of 34.5°C and 26°C, respectively, Awad et al. (Reference AWAD, ZULKIFLI, SOLEIMANI and LOH2015) evaluated the effect of 5.5% reduction in CP, from 22.3 to 16.8% C,P fortified with all EAA to meet their requirements plus the NEAA Gly, on the growth performance of broilers from 1 to 21 d of age. The authors found that feeding the Gly fortified, reduced-CP diet resulted in significantly lower feed intake (1119 vs. 1186 g/bird), poorer body weight (844 vs. 923 g) and higher FCR (1.41 vs. 1.35) compared to the 22.2% CP diet.

A study by Liu et al. (Reference LIU, FENG, CHAO, CHEN, WEI, WANG, SUN and ZHANG2016) examined the effect of a 3.3% reduction in CP (from 18.2 to 14.9% CP) supplemented with Met, Lys, Thr and Trp on the growth performance of broiler chickens reared under cyclic heat stress conditions (32°C for 4 h , 28°C for 16 h to 23°C for 4 h) from 28 to 42 d of age. The authors reported a that, when birds were fed the reduced-CP diet, FCR was significantly higher (2.30 vs. 2.12) compared to their counterparts fed the 18.2% CP diet.

Considering the previously available data, Awad et al. (Reference AWAD, ZULKIFLI, SOLEIMANI and ALJUOBORI2017) evaluated the effect of a 5% reduction in dietary CP in the starter (22.3 vs. 17.3% CP) and finisher (19.4 vs. 14.5% CP) diets with EAA fortification plus Gly on growth performance of male and female broiler chickens reared under a hot tropical environment (maximum and minimum temperatures of 35.7°C and 25.8°C, respectively). Regardless of gender, feeding the reduced-CP diet negatively affected growth performance during both the starter (d 1 to 21) and overall (d 1 to 42) periods. During the starter period, the reduced-CP diet resulted in significantly lower feed intake (1075 vs. 1166 g/bird) and weight gain (754 vs. 846 g) and higher FCR (1.43 vs. 1.38) compared to control group. During the overall period, the group fed reduced-CP diet showed significantly lower feed intake (3768 vs. 4073 g/bird) and weight gain (2091 vs. 2251 g) compared to the control group.

Recently, Zulkifli et al. (Reference ZULKIFLI, AKMAL, SOLEIMANI, HOSSAIN and AWAD2018) investigated the effect of reducing dietary CP from 19.1 to 16.2% CP (-2.9 %) with supplemental Met, Lys, Thr and Trp on growth performance of 22 to35 d old broiler chickens reared under cyclic heat stress conditions (33°C for 6 h to 23°C for 18 h). The reduced-CP diet adversely affected performance of broilers by significantly reducing weight gain (700 vs. 1055 g) and increasing FCR (2.03 vs. 1.79).

At the same time, Law et al. (Reference LAW, ZULKIFLI, SOLEIMANI, LIANG and AWAD2018) evaluated the influence of reducing dietary CP from 21.0 to 18.5% CP in the starter (d 1 to 21) diet and from 19.0 to 16.7% CP in the finisher (d 22 to 35) diet, with Met, Lys, Thr and Trp supplementation, with or without protease on growth performance of broiler chickens reared in a hot and humid tropical environment (at least 6 h of 36°C maximum temperature and 26°C minimum temperature). Birds fed the reduced-CP diet had significantly lower weight gain (1721 vs. 1894 g) and higher FCR (1.87 vs. 1.73) compared to their counterparts fed the control diet.

With this previous published data in mind, Awad et al. (Reference AWAD, IDRUS, SOLEIMANI FARJAM, BELLO and JAHROMI2018) evaluated the effect of a 5.1% reduction in CP (19.4 vs. 14.3%) with supplementation with all EAA to meet their requirements plus Gly, on the performance of broiler chickens raised under cyclic (34°C for 7 h to 23°C for 17 h) or constant (34°C) heat stress conditions. The reduced-CP diet had no effect on growth performance of broilers under cyclic heat stress conditions. However, under constant heat stress conditions, feeding the reduced-CP diet resulted in significantly lower feed intake (1547 vs. 2158 g/bird), lower weight gain (584 vs. 975 g) and higher FCR (2.65 vs. 2.21) in comparison with those fed the control diet.

More recently, Law et al. (Reference LAW, ZULKIFLI, SOLEIMANI, LIANG and AWAD2019) investigated the effects of protease supplementation in a 2.3% reduced-CP diet (19 vs. 16.7% CP), supplemented with Met, Lys, Thr and Trp on performance of broiler chickens reared under cyclic heat stress conditions (34°C for 7 h to 23°C for 17 h) from 22 to 42 d of age. Regardless of protease supplementation, feeding birds with the reduced-CP diet did not impede growth performance of broilers compared to birds fed the control diet.

Factors affecting the response of broiler chickens to reduced-CP diets under heat stress conditions

Based on the findings from the above studies, it is complicated to compare the data dealing with feeding reduced-CP diets at high environmental temperatures, because numerous factors may be involved. These include energy level (Zaman et al., Reference ZAMAN, MUSHTAQ, NAWAZ, MIRZA, MAHMOOD, AHMAD, BABAR and MUSHTAQ2008; Law et al., Reference LAW, ZULKIFLI, SOLEIMANI, LIANG and AWAD2019), the severity of heat exposure (Cheng et al., Reference CHENG, HAMRE and COON1999; Gonzalez-Esquerra and Leeson, Reference GONZALEZ-ESQUERRA and LEESON2005; Awad et al., Reference AWAD, IDRUS, SOLEIMANI FARJAM, BELLO and JAHROMI2018), supplemental EAA compositions (Balnave et al., Reference BALNAVE, HAYAT and BRAKE1999; Balnave and Barke, Reference BALNAVE and BARKE2002) and supplemental NEAA (Awad et al., Reference AWAD, ZULKIFLI, SOLEIMANI and LOH2014b). Other factors, such as protease supplementation (Law et al., Reference LAW, ZULKIFLI, SOLEIMANI, LIANG and AWAD2018; Reference LAW, ZULKIFLI, SOLEIMANI, LIANG and AWAD2019) and sex (Awad et al., Reference AWAD, ZULKIFLI, SOLEIMANI and ALJUOBORI2017) must be considered when investigating the effect of feeding reduced-CP diets at elevated environmental temperatures, making it difficult to define an exact point for dietary CP reduction under such conditions. Three factors (the severity of heat exposure, supplemental AA compositions and age) which appeared to play a major part in determining the level of CP reduction in broiler diets under heat stress conditions will be further discussed.

Acknowledgments

This work was supported by the Malaysian Ministry of Higher Education [Long-term Research Grant Scheme].