Amino Acid Imbalances

I. INTRODUCTION

The term ‘amino acid imbalances’ was probably originated by Elvehjem and Krehl (1955) and the topic was addressed by Harper and Rogers (1965). Their conclusion was that imbalances retard growth by altering the normal pathways of amino acid metabolism. Thus, while the relevance of amino acid imbalances to efficient chicken-meat production is recognised, a precise definition has yet to be developed (Kurpad, 2018). Antagonisms between arginine and lysine (Austic and Scott, 1975) and the branched-chain amino acids, isoleucine, leucine and valine (Calvert et al., 1982) have been reported to depress feed intake in poultry and could be seen as amino acid imbalances. However, this paper considers amino acid imbalances generated by inclusions of non-bound (synthetic, crystalline) amino acids (NBAA) as opposed to protein-bound amino acids in diets for broiler chickens. These imbalances are increasingly declared in birds offered reduced-crude protein (CP) diets because of high NBAA inclusions to meet specifications. The lack of bioequivalence between non-bound versus protein-bound amino acids is fundamental to the genesis of amino acid imbalances in this context (Selle et al., 2022a).

Initially, synthetic d,l-methionine was made available for animals in 1953; however, intestinal uptakes of synthetic or non-bound methionine were subsequently shown to be more rapid than protein-bound methionine by Canolty and Nasset (1975). Cumulative plasma methionine concentrations in rats offered synthetic methionine at 15, 30, 60 and 120 minutes post-administration were 2.75 time higher (858 versus 312 μmol/L) than in rats receiving methionine only from intact protein. Moreover, that non-bound lysine and methionine are absorbed more rapidly than their protein-bound counterparts in broiler chickens was reported by Liu et al. (2013). It may be deduced from this study that the average digestion rate constants of non-bound lysine and methionine were approximately 3.7 times higher (8.64 versus 2.35 10⁻²min⁻¹) than protein-bound amino acids.

II. AMINO ACID IMBALANCES IN BROILERS OFFERED REDUCED-CP DIETS

Instructively, Baker (2009) suggested that there are limits to the extent that intact protein can be replaced by NBAA in terms of achieving maximal weight gain and feed efficiency. This was illustrated by Macelline et al. (2022) in an equilateral triangle response surface design with diets formulated to 203 g/kg true protein but the three apical diets contained 6.75, 19.4 and 66.9 g/kg NBAA. The diet containing 13.1 g/kg NBAA supported maximum weight gain and minimum FCR observed and higher NBAA inclusions penalised growth performance.

Broiler chickens are intermittent, rather than continuous feeders (Aydin and Berckmans, 2016) and the likelihood is that this contributes to post-enteral imbalances between non-bound and protein-bound amino acids at sites of protein synthesis, stemming from differences in amino acid intestinal uptake rates. Amino acids may be captured by catabolic pathways as they transit enterocytes of the small intestinal mucosa and are denied entry into the portal circulation (Wu, 2008). However, the possibility is that non-bound amino acids are less likely to be catabolised given their proximal sites of absorption where starch/glucose is readily available as an alternative energy substrate (Fleming et al., 1997), which is supported by data in Moss et al. (2018). If so, this would exacerbate differences in amino acid intestinal uptake rates. Nevertheless, dietary amino acids that exceed requirements for protein synthesis are rapidly catabolized (Brosnan, 2003), which had been defined as post-prandial amino acid oxidation (Schreurs et al., 1997). Subsequently, Nolles et al. (2009) compared postprandial oxidation of egg white protein as the sole amino acid source with a corresponding blend of non-bound amino acids via [13CO2] breath tests in rats. Postprandial oxidative losses of non-bound leucine were significantly higher than protein-bound leucine by approximate factors of 1.52 (24.8 versus 16.3%) after a short adaptation period. It appears that NBAA are more likely to be lost to post-prandial oxidation because of their more rapid intestinal uptakes.

The moderation of amino acid catabolism would decrease amino acid requirements (Klasing, 2009); potentially, this holds importance and would be partially achieved if amino acid imbalances were to be diminished. Moreover, amino acid catabolism attracts metabolic costs in terms of both protein and energy. The catabolism of amino acids axiomatically generates a protein cost; however, the resultant synthesis and excretion of uric acid to void N in urine generates a minimal energy cost of 64.7 kJ/g N excreted as uric acid (Van Milgen, 2021). Uric acid concentrations in broiler excreta were determined in Selle et al. (2021) and the proportion of dietary energy intakes partitioned to uric acid synthesis and excretion was up to 2.26% of gross energy (17.21 MJ/kg GE) or 2.98% of metabolisable energy (13.06 MJ/kg AME) over the total excreta collection period to determine AME. Excreta uric acid concentrations were also determined in Brink et al. (2022) in a study involving wheat-based, grower and finisher diets with three CP levels. Mean excreta uric acid concentration was 66.4 mg/g (range: 47.9 to 80.5 mg/g), which represented 47.1% (range: 40.0 to 53.1%) of total N excreted. Thus, 47.1% of total N in excreta was derived from uric acid in urine and the balance of 52.9% was derived from undigested and microbial N in faeces. Interestingly, Brink et al. (2022) suggested that N derived from uric acid in litter is more readily volatilised into atmospheric NH3 than other N forms in excreta.

There are indications that NH3 is more toxic in poultry than mammalian species (Wilson et al., 1968). Under normal conditions, broiler chickens detoxify NH3 via a reaction catalysed by glutamine synthetase in which NH3 and glutamic acid are condensed into glutamine. Glutamine then enters the Krebs uric acid cycle which generates uric acid, which is voided in urine (Stern and Mozdziak, 2019). However, if NH3 detoxification is inadequate, plasma NH3 concentrations will be elevated and this has been associated with depressed growth performance in three broiler studies (Namroud et al., 2008; Ospina-Rojas et al., 2013, 2014). Adequate NH3 detoxification could be challenged by excessive amino acid catabolism triggered by high dietary NBAA inclusions. Also, as glycine is a prerequisite for the Krebs uric acid cycle (Salway, 2018), it follows that any deficiency of glycine (and serine) would result in inadequate NH3 detoxification. Selle et al. (2021) estimated that between 25.0% and 80.9% of dietary glycine entered the Krebs cycle for uric acid synthesis in the Chrystal et al. (2021) study, which does not account for endogenous glycine synthesis.

The concept of inadequate NH3 detoxification or ‘ammonia overload’ is supported by the outcomes reported in Greenhalgh et al. (2022). The inclusion of 75 mg/kg L-carnitine in 160 g/kg CP, sorghum-based diets, containing 51.02 g/kg NBAA, improved weight gain by 15.0% (1580 versus 1374 g/bird) and FCR by 8.82% (1.521 versus 1.615) from 7 to 33 days post-hatch. However, it is recognised that L-carnitine is protective against NH3 toxicity (Kloiber et al., 1988). This raises the distinct possibility that the L-carnitine responses observed stemmed from its capacity to counteract the negative effects of ammonia overload. This is because L-carnitine inclusions in 220 and 190 g/kg CP diets, containing 15.19 and 29.26 g/kg NBAA, respectively, failed to generate growth performance responses. Also increasing NBAA inclusions were found to linearly (r = 0.546, P = 0.019) related to plasma NH3 concentrations in an unpublished study. Both outcomes support the contention that high NBAA inclusions in reduced-CP diets could trigger ‘ammonia overload’.

III. WHEAT-BASED, REDUCED-CP DIETS

Wheat is the dominant feed grain in Australian chicken-meat production. However, the capacity of broilers to accommodate CP reductions in wheat-based diets is highly variable as evidenced by several local studies. For example, 30 g/kg CP reductions in grower and finisher diets numerically depressed FCR by 2.19% (1.542 versus 1.509) from 10 to 35 days post-hatch in Hilliar et al. (2020). Alternatively, similar CP reductions significantly compromised FCR by 7.24% (1.452 versus 1.354) in broilers in Hilliar et al. (2019) and by 9.38% (1.609 versus 1.471) from 7 to 35 days post-hatch in Dao et al. (2021). In Yin et al. (2020), CP reductions from 215 to 190 g/kg CP depressed FCR by 1.42% (1.497 versus 1.476) and from 215 to 165 g/kg by 4.74% (1.497 versus 1.476) from 14 to 35 days post-hatch. In contrast, CP reductions from 197.5 to 180 g/kg CP compromised FCR by 11.5% (1.878 versus 1.684) and from 197.5 to 162.5 g/kg CP by 44.1% (2.426 versus 1.684) from 14 to 35 days post-hatch in Greenhalgh et al. (2020). Also, the CP reduction from 222 to 165 g/kg CP compromised FCR by 26.6% (1.840 versus 1.453) in Chrystal et al. (2021).

Maize was superior to wheat as the basis of reduced-CP diets in Chrystal et al. (2021). Moreover, there is a quadratic relationship (r = 0.962; P = 0.0004) between NBAA inclusions, which ranged from 7.23 to 49.39 g/kg, with mean FCR observed in birds offered nine dietary treatments. It may be deduced from the quadratic equation that the minimum FCR of 1.403 from 7 to 35 days post-hatch was realised with NBAA inclusions of 17.49 g/kg and FCR deteriorated in a quadratic manner when this inclusion level was exceeded. In a European study (Brink et al., 2022), 30.0 g/kg CP reductions in wheat-based grower and finisher diets numerically depressed FCR by 2.03% (1.50 versus 1.48) from 1 to 39 days post-hatch when fed as pellets. This promising outcome was probably facilitated by the relatively low average NBAA inclusions of 16.6 g/kg in the reduced-CP grower and finished diets. Curiously, when fed as mash, a significant improvement in FCR of 5.33% (1.60 versus 1.69) was observed. Presumably, mash diets had higher protein solubility and digestibility and it is probable that birds offered mash diets consumed feed more frequently (Fujita, 1974). Both factors may have reduced the magnitude of post-enteral amino acid imbalances to the benefit of FCR in birds offered reduced-CP, mash diets. Finally, the likelihood is that wheat-based, reduced-CP diets will be advantaged by limited NBAA inclusions, which can be facilitated by incorporating feedstuffs with lower protein contents than soybean meal into their formulations. This strategy should constrain the deleterious impacts of amino acid imbalances in birds offered reduced-CP, wheat-based diets, but the likelihood that there are additional inherent factors in wheat that will need to be addressed including soluble NSP, rapid starch digestion rates and possibly amylase-trypsin inhibitors and gluten (Selle 2022b).

Presented at the 34th Annual Australian Poultry Science Symposium 2023. For information on the next edition, click here.