Amino acid nutrition update to ensure successful low protein diets in broiler chickens

I. AMINO ACIDS: TOWARDS MORE PRECISE FORMULATION

Dietary protein reduction in swine or poultry is driven by economic, environmental and societal issues - the three pillars of sustainability. Formulating on the basis of each indispensable amino acid (IAA) instead of a minimum crude protein (CP) level allows a reduction in feed cost, a decreased dependency on imported soybean meal, and a lower pressure on animal health. In addition, the increasing availability of feed grade amino acids (AA; i.e. L-Tryptophan, L-Valine) has made possible the further decrease of dietary CP and changed the way of addressing risk management in feeds for monogastric animals. Indeed, moving from a dietary formulation based on protein to a more precise formulation that gives value to each single AA is a shift from an unpredictable risk approach to controlled risk management. Formulating and relying on the protein criteria (defined as N x 6.25) is only considering the quantity of supply of various nitrogenous components but not their quality. In contrast to N, AA are predictors of performance and the precise control of the AA levels is crucial for performance and profitability. It also gives more opportunity for innovative nutritional choice and ends in the most economical feed solution by a better adjustment of the feed recipe to the AA requirements. Increasing knowledge about AA nutrition and focusing on their metabolic roles is therefore mandatory. New insights on next limiting AA, increased availability of feed-grade AA and new knowledge about the benefits of low CP diets on the environment and the health and welfare of animals is raising the interest for reducing dietary CP in poultry birds.

II. REVIEW OF AMINO ACID REQUIREMENTS

The ideal protein is the most practical tool to express the AA requirements of animals. All IAA are expressed in ratio to Lys, as Lys is almost exclusively used for protein synthesis (Boisen, 2003). By formulating on each IAA, the protein level will be adjusted automatically by least cost formulation. The requirement is defined as the minimal amount of the studied nutrient required to obtain the optimal or the maximal performance, assuming that all the other nutrients are provided in adequate amounts (Hauschild et al., 2010).

Table 1 - Published ideal amino acid profiles for broilers and piglets (IAA are expressed in ratio to Lys, %). Gloaguen et al. (2013) proposed an ideal profile for 7-25kg piglets.

Numerous published ideal AA profiles are available and some examples are given in Table 1. In general, they are fairly consistent especially for the first limiting and most studied amino acids. For the next limiting AA (Val, Ile, Arg), there is greater discrepancy between publications and deserves further investigation. Glycine plus serine requirement, dispensable AAs, is only given in two profiles and is highly variable depending on the methodology used. Differences between pigs and broilers can be illustrated by a comparison of the ideal AA profile recommendations. Sulphur AA (SAA) requirement is higher in broilers than in swine and therefore SAA are the first limiting AA in broiler diets while they are only the 4th or the 5th in pigs. Broilers are indeed feathered animals and feathers contain high amounts of Cys. The branched-chain AA (BCAA; i.e. Val, Ile and Leu) requirements are also higher in broilers compared to pigs while Trp requirement is much higher in swine than in broilers. Arginine is not considered as an IAA in pigs and no recommendation is therefore provided.

The use of different strains, sexes, ages, dietary nutrient levels, health conditions, chosen performance criteria, AA digestibility systems or regression models in the doseresponse studies has contributed to the discrepancy in the published AA recommendations. The meta-analysis approach can integrate inter and intra variability and help to estimate nutrient responses and requirements less dependent on experimental conditions (Sauvant et al., 2008).

Threonine is the third limiting AA in broiler diets and a recent meta-analysis estimated the SD Thr:Lys requirement for optimal ADG, ADFI and G:F to be 67% SD Thr:Lys (Lambert et al., 2015a). Besides protein synthesis, Thr is crucial for optimal gut health and immune response as it is the first AA in the composition of the mucins and immunoglobulins. Star et al. (2012) suggested a higher Thr requirement in broilers challenged with an infection model (inoculation of Eimeria maxima and Clostridium perfringens at d 9 and 14 of age, respectively). In case of challenging practical conditions, Thr will be diverted from growth towards the synthesis of proteins involved in the immune response, resulting in poor performance.

Valine requirement was also recently updated by meta-analysis (Corrent et al., 2017). Among 28 dose-response studies to Val, eight experiments were selected for modelling with non-linear regression models. Estimated Val requirements varied between 78.6 and 95.1% standardized digestible (SD) Val:Lys depending on the model used and the growth performance criteria. By evaluating the different requirements and responses, it is concluded that 80% SD Val:Lys ratio is sufficient to ensure optimal growth and feed efficiency of broilers. Results of the meta-analysis were confirmed by recently released in-house trials (Figure 1). The variability of the requirement and the response to Val can be partly explained by the amount of dietary Leu in experimental broiler diets. In contrast to broilers, in piglet AA nutrition, the interaction between BCAA is well described showing increased catabolism of Val and Ile when dietary Leu is in excess, reducing the availability of Val and Ile for protein deposition and growth (Wiltafsky et al., 2010). Based on Corrent et al. (2017) and a total of 37 new trials, interaction between Val and Leu was confirmed in broiler chickens, demonstrating that requirement for Val was not influenced by dietary Leu, but the response to Val was greatly increased when feeding higher levels of dietary Leu (AEL Internal Research Report). These findings were confirmed by Ospina-Rojas et al. (2017) who indicated that ensuring adequate dietary Val avoids a depression of performance in case of excessive secondary limiting AA provision. In addition, increasing dietary Val linearly improved body weight and breast meat uniformity (AEL Internal Research Report). Valine is also known to potentially improve Ca bone mineralization and the immunomodulation response to Newcastle virus (Farran and Thomas, 1992; Foroudi and Rezamand, 2014).

Figure 1 - Gain to feed (GF) according to the dietary standardized digestible (SD) Val: Lys level. Dashed lines = 6 newly released in-house Val trials, solid line = curvilinear-plateau model from Corrent et al. (2017).

Compared to Thr and Val, Ile requirement was much less investigated in recent years. Average of the published recommendation and dose-response trials suggests an Ile requirement of 67% SD Ile:Lys to maximize growth performance. However, this requirement must be fine-tuned in order to formulate efficient low CP diets. Internal desk study on Ile based on 19 trials concludes that the requirement and response to Ile is highly dependent on the experimental diet composition (AEL Internal Research Report). Most of the trials are indeed containing raw materials imbalanced in AA (blood cells, blood meals, corn gluten meal), leading to imbalanced experimental diets. Van Milgen et al. (2012) suggested that, in piglet diets without blood products, the Ile requirement appears to be lower than the currently recommended requirement. These findings also support the idea that Ile requirement is much higher (> 67% SD Ile:Lys) for breast meat deposition than for growth performance. The underlying mechanisms still need to be explored and warrants further attention in the future.

In terms of Arg requirement, more than 50 dose-response peer-reviewed papers are available. Contrary to Thr, for instance, it is more difficult to integrate these data into a meta-analysis as the variability between trials is much greater. Some trials tested extreme levels of Arg, or Arg in interaction with extreme dosage of other nutrients, or Arg under extreme conditions (heat stress, altitude, LPS challenge). General recommendation for Arg is 105% SD Arg:Lys but, depending on the nutrient composition and the farming conditions, Arg requirement can vary substantially. Main factors affecting Arg requirement are the dietary Lys content (Schedle et al., 2015), heat stress (Brake et al., 1998), coccidiosis (Laika and Jahanian, 2017) and altitude (Khajali and Wideman, 2010). Arginine requirement is also highly dependent on age (AEL Internal Research Report).

Although not a strict indispensable AA, Gly is becoming a very studied AA. Glycine requirement is always expressed as the sum of the Gly and Ser requirements as Ser to Gly conversion is equimolar and reversible. Recent published meta-analyses (Akinde, 2014, Lambert et al., 2015b; Siegert, 2016) indicate that it is theoretically possible to estimate Gly+Ser requirement but many dietary factors influence its response and requirement level. The requirement is only valid for young broilers (0-22d) as young broilers are known to have a higher requirement (Coon et al., 1974). Low dietary CP levels were used and convincing evidence suggested a higher Gly requirement at low CP (Dean et al., 2006). Gly is rich in N and increasing dietary levels of Gly logically increased dietary CP levels resulting in diets being not isonitrogenous. Dietary Cys level was low and response to Gly is increased at low dietary Cys (Powell et al., 2011). In contrast to mammals, the majority of the degraded Thr is converted to Gly in broilers and many trials have demonstrated that Thr is capable of sparing dietary Gly in broilers (Ospina-Rojas et al., 2013a,b; Corzo et al., 2009; Siegert, 2015). Evidence from a recent trial confirmed that Gly supplementation does not improve performance of broilers fed a reduced CP diet when Thr is adequately supplied (Lambert et al., 2015c).

III. TACKLING THE CHALLENGE OF DIETARY CRUDE PROTEIN REDUCTION

In Pesti (2009), growth performance of broilers was found to be negatively correlated with dietary CP level, confirming the depressing effect of dietary CP on performance. In piglets, Gloaguen et al. (2014) proved that dietary CP can be drastically reduced down to 14% CP for 1.0% standardised ileal digestible (SID) Lys without any effect on growth performance, indicating that controlling IAA levels allows to reduce dietary CP in piglet diets. Pigs and broilers are monogastric animals and have a comparable digestive system. However, in terms of metabolism, they are completely different animals and they do not share the exact same metabolic pathways. Mammals such as pigs are ureotelic species; the urea cycle is the primary pathway of N excretion. Arginine is the substrate for urea synthesis via the liver arginase and can be de novo synthesized via the arginine-ornithine cycle (Morris 2004). In chickens, the urea cycle is inactive in the liver, Arg being not produced via the urea cycle pathway in broilers must be dietary supplied as any IAA. Avian animals are thus uricotelic animals and uric acid is therefore the end product of N excretion. The uric acid pathway is highly demanding on digestible amino acids (DAA - Gln, Gly, Asp) which suggests that some DAA can become limiting for this pathway. More and more knowledge has been pointing at DAA as very important AA in broilers and more research is needed to understand the differences between swine and poultry fed low CP diets.

Several hypotheses were discussed over time to find out why there is a limit in dietary CP reduction in broilers. Aftab et al. (2006) reviewed the possible explanations including dietary electrolyte balance, non-pair feeding regimes, dietary content of DAA, ratio DAA:IAA, inaccurate AA requirement and excess of synthetic AA. From Cirot et al. (2017), based on 367 trials, convincing evidence showed that 97% of published trials where dietary CP was reduced were either not constant in dietary Lys or deficient in one or more IAA. As previously discussed, among the DAA, glycine seems to be the most pivotal one and its dietary content can be in some cases insufficient to maintain the fast-growing pace of modern broilers fed vegetable diets (Dean et al., 2006). Regarding the impact of free AA supplementation, the general thought is that free AA follow different digestive dynamics compared to protein-bound AA. They do not have to be digested and might be directly absorbed in the most upper part of the small intestine. When replacing AA from protein supply by free AA, it is suspected that great amounts of the first limiting AA will be absorbed earlier in the digestive tract compared to AA coming from protein. The rate of protein synthesis would therefore be impaired by an imbalance in AA in the blood circulation. However, a recent INRA trial indicated that broilers can be fed dietary free AA content up to 45kg/T without any significant adverse effect on performance or carcass characteristics while the average practical inclusion in today’s broiler diets is around 6kg/T (Belloir, 2016). In addition, Selle et al. (2015) even indicated that increasing the use of free AA would probably decrease protein-bound AA catabolism in the distal part of the intestine, leading to AA being used primarily for protein synthesis instead of energy supply.

Figure 2 - Average daily gain (ADG), average daily feed intake (ADFI) and gain to feed (GF) according to the dietary Crude Protein (CP) level. References available on request.

In recent trials, low CP could be drastically reduced without affecting growth performance and feed efficiency when dietary Lys was kept constant, dietary Thr was adequately supplied to reduce the need for dietary Gly and main IAA (Met, Val, Ile, Arg) respected the correct requirements (Figure 2). Under these conditions, it could be concluded that no minimum of dietary CP level is needed in formulation (lowest dietary CP reached without compromising performance was 16%), no minimum of dietary SD Gly+Ser was needed in formulation (lowest SD Gly+Ser:Lys was 130%) and no maximum of feed-grade AA supplementation is necessary to be applied in formulation (highest reached was 12kg/T).

IV. BENEFICIAL EFFECTS OF LOWER DIETARY CRUDE PROTEIN CONTENT

Reducing the dietary CP content in broiler diets is an efficient nutritional strategy in order to improve the overall sustainability of the broiler meat chain. In order to quantify the effect of low CP diets, a meta-analysis study in partnership with Laval University (Cirot et al., 2017) and a life-cycle assessment in partnership with INRA (Méda et al., 2017) were conducted. By reducing dietary CP content and controlling dietary content of IAA, SBM is gradually replaced by cereals and feed-grade AA. Nitrogen intake and therefore excretion can be substantially reduced. Cirot et al. (2017), based on a database of 125 trials, showed that N excretion is linearly and significantly reduced by 0.06 g/day or 0.24 g/day per broiler per point of dietary CP reduction for starter or grower-finisher broilers, respectively (Figure 3). This indicates that nutritional intervention by reducing dietary CP is four times more efficient in reducing N emissions to the environment in the last phase of broiler production than in the first phase. The meta-analysis study also shows that there is still potential to increase the N efficiency of modern broilers and that a plateau value seems to be reached at 75%, compared to 55 to 60% for today’s broilers.

Figure 3 - Nitrogen excretion (gram/day) according to dietary Crude Protein (CP) level for a) 0 to 21 days of age broilers (left, N=41 trials) and for b) 21 to 42 days old broilers (right, N=52 trials).

In Belloir et al. (2017), where dietary CP was reduced from 19 to 15% in 21-35d broilers, combined synergetic effect of lower N excretion (-2g/g BWG per CP percentage point) and lower manure moisture content (-2 points per CP percentage point) resulted in lower N volatilization (e.g. ammonia emissions, -5 points per CP percentage point). Based on published results from Belloir et al. (2017), Méda et al. (2017) conducted a life-cycle assessment to estimate environmental impacts of producing 1 kg of broiler meat fed different dietary CP levels. On average, impacts were reduced by 8%, 7% and 5% for climate change, eutrophication and acidification, respectively, when broilers were fed 16% of dietary CP from 21 to 35 days of age compared to 19%.

When feeding low CP diets to broilers, uric acid synthesis, a high water consuming pathway, is reduced due to a lower amount of AA excesses. On the other hand, SBM content is reduced, gradually replaced by cereals, leading to a lower amount of dietary K. Those two effects combined lead to a reduction of water intake and therefore an improvement of litter humidity. Based on 29 trials, Cirot et al., (2017) showed that water consumption was decreased by 1.4% per CP percentage point, leading to a reduction of 2.2% of litter humidity per CP percentage point (Figure 4). It was also shown in recently published experiments that reducing dietary CP indeed led to lower water:feed ratio, lower litter humidity and lower incidence of foot pad lesions (van Harn et al., 2017; Sacranie et al., 2017; Belloir et al., 2017). In addition, reducing dietary CP is known to have antibacterial effects indicated by lower pH content (Namroud et al., 2008) and lower E. coli or C. perfringens counts (Dahiya et al., 2006).

Figure 4 - a) left, Daily water consumption (% of the positive control) and b) right, litter humidity (% of the positive control) according to dietary Crude Protein (CP) level.

Dietary CP reduction also showed improvement in meat quality traits. First of all, breast meat yield (BMY) was improved in recent experiments, showing that reducing excess of dietary AA improved BMY, although a breaking point could be found at 16% CP. In addition, the ultimate pH and drip loss of the breast meat were increased and decreased, respectively, by dietary CP reduction (Belloir et al., 2017). It is speculated that, as dietary CP is reduced, available nutrients for glycogen production are also decreased.

V. CONCLUSION

In conclusion, dietary CP can be reduced in broiler chickens when controlling dietary IAA levels. Many interactions between AA arise in low CP broiler diets, in particular Val x Leu, with growth depression at low Val content exaggerated by an excess of Leu and Thr x Gly, with Thr being able to spare Gly requirement, unique to poultry species. Based on scientific findings, successful practical implementations of low CP diets can be undertaken, enjoying environmental and health benefits.

Presented at the 29th Annual Australian Poultry Science Symposium 2018. For information on the latest and future editions, click here.