Branched-Chain Amino Acids: Potential Antagonisms in Practical Formulation

Interactions among the branched-chain amino acids (BCAA) have long been observed in poultry (D’Mello and Lewis, 1970; Tuttle and Balloun, 1976; Mendonca and Jensen, 1989), but their effects on broiler performance have been contested with various researchers reporting no effects of BCAA interactions on broiler performance in practical diets (Burnham et al., 1992; Barbour and Latshaw, 1992; Waldroup et al., 2002). Early studies concerning the BCAA have attributed the adverse effects of BCAA interactions to excess Leu which decreases appetite resulting in decreased bodyweight (BW) gain (D’Mello and Lewis, 1970). Expanding on these findings, Allen and Baker (1972) found that when Leu was in excess, the efficacy of dietary Ile and Val decreased and could exacerbate reductions in performance due to either Ile or Val being 4^th limiting. This hypothesis was supported by the findings of Jackson and Potter (1984), who found that in the event of a Leu excess, additions of Ile could further reduce performance when Val was 4^th limiting due to a reciprocal antagonism. This antagonism was again identified in 1989 when Mendonca and Jensen observed reductions in performance when Ile was added to a diet with marginal Val but, when Ile and Val were added in concert, no adverse effects were observed. Due to the location of Ile and Val as the 4^th limiting amino acids in maize-soybean meal diets with or without an animal protein meal, respectively (Kidd and Hackenhaar, 2005), it is of importance to not only understand the antagonism between these two amino acids but also their connection with Leu as crude protein (CP) is reduced in commercial diets and these amino acids are supplemented as feed grade amino acids to allow for crude protein reduction.

Recent research at the Arkansas Agricultural Experiment Station has attempted to characterize interactions among the BCAA and evaluate Val impact on live performance and carcass traits. Four studies were conducted to determine: the 4^thlimiting amino acid in maize-soybean meal diets, Val requirements for live performance and carcass traits, and assess potential interactions or antagonisms among the BCAA. These studies served to provide new data points concerning BCAA manipulation at levels currently used in the poultry industry.

Cobb 500 male broilers were used in an amino acid deletion assay to determine the 4^th limiting amino acid for broilers during a 15 to 35 d grower period. A positive control diet was formulated to 186.5 g/kg of CP but was fortified with crystalline amino acids in such a way that its amino acid profile was similar to that of a 202.9 g/kg CP diet. A negative control diet was created by removing all supplemental amino acids. Eight subsequent deletion diets (Val, Ile, Leu, Trp, Arg, His, Phe, and Gly+Pro) were then made by refortifying the negative control with all crystalline amino acids except the one being tested. Reductions (P = 0.05) in BW gain were observed for birds fed diets devoid in Val compared with birds fed the positive control. Feed conversion ratio was decreased (P = 0.01) for birds fed diets devoid of Ile compared with birds fed the positive control. No differences were observed for any carcass traits as a result of deletion of any amino acid.

Cobb 500 male broilers were also used to access BCAA interactions using Box-Behnken Design (BBD) in two experiments. For both experiments, Val and Ile ratios to Lys were maintained at 65, 75, and 85 and 58, 66, and 74, respectively. The first BBD experiment included Leu as the third factor and it was included at ratios to Lys of 110, 130, and 150 for a 15 to 34 d grower period. The second BBD study included Gly as the third factor and it was included at ratios of total Gly+Ser to Lys of 131, 151, and 171. Both experiments evaluated live performance and carcass traits. In Experiment 1, interactions between Ile and Val were found for feed conversion ratio (P < 0.01, Figure 1) and breast meat yield (P < 0.05, Figure 2). Interactions were also observed between Leu and Val for breast meat yield (P = 0.03). In Experiment 2, no interactions were observed between Ile and Val, but a significant interaction between Gly+Ser and Val was observed for leg quarter yield (P < 0.05).

Figure 1 - Feed conversion ratio (FCR, g:g) of Cobb MV × 500 male broilers fed gradient levels of Isoleucine and Valine. Dietary Leucine held constant at Leucine/Lysine ratio of 130. P< 0.01.

Figure 2 - Breast meat yield (TBY, g/kg) of Cobb MV × 500 male broilers fed gradient levels of Isoleucine and Valine. Dietary Leucine held constant at Leucine/Lysine ratio of 130. P< 0.05.

In the final study, Cobb 500 male and female broilers were used to determine the Val requirement during a 15 to 35 d grower period. A Val deficient diet was formulated to titrate Val to Lys ratios of 52, 64, 71, 78, 85, and 92. Additionally, Ile was increased from 66 to 70 in an additional 78 Val diet to observe any potential interactions between Ile and Val. Cubic responses were observed for feed conversion ratio in both male (P = 0.01) and female broilers (P = 0.02), and a linear response (P = 0.05) of increasing body weight gain to increasing dietary Val was observed in females. Valine requirement for feed conversion ratio was estimated for males and females using 95% of the quadratic response and was determined to be Val to Lys ratios of 73 and 72, respectively. No linear, quadratic, or cubic response was observed for any processing yield in male or female broilers. Breast meat yield was reduced for male broilers fed diets with increased Ile compared to those fed the lower level of Ile. No other response was observed as a result of increased Ile.

The aforementioned responses align with those currently seen in the literature as a response to BCAA interactions, although to a lessened degree. The identification of Val as the 4^th limiting amino acid agrees with previous literature (Fernandez et al., 1994; Kidd and Hackenhaar, 2005) and the observation of the reduction of Ile improving feed conversion agrees with the findings of Mendonca and Jensen (1981) that there exists a potential antagonism between Ile and Val when either is the 4^th limiting amino acid.

Combined results of the BBD studies give insight into the interconnected nature of the BCAA. In both studies, no linear or quadratic effects were observed for any measured live performance or processing measurement; but when considering potential interactions, effects were observed for both live performance and processing measurements. Also, the fact that the interactions between Val and Ile were not observed in both experiments suggests that Leu levels may have been in excess and, therefore, caused interactions to occur between Ile and Val. Classically, Leu has been shown to have detrimental effects when included at levels far beyond the bird’s requirement (D’Mello and Lewis, 1970) but when high levels of Leu were used in practical diets, concerns about the detrimental effects of high Leu seem unfounded (Waldroup et al., 2002). Indeed no direct effects of Leu were observed in the first BBD study but the competitive nature of Ile and Val observed in the first study were not duplicated in the second, aligning with the theory of Allen and Baker (1974) that excess Leu impairs the efficacy of Val and Ile.

In the final study, only the feed conversion was affected enough to allow for the estimation of a requirement which agrees with previous Val titrations for this growout period (Campos et al., 2009; Morais et al., 2010; Naseimento et al., 2016). The estimated requirements reported herein for males and females are reflective of values established by Farran and Thomas (1990). The reduction in breast meat yield observed in male broilers as a response to increasing dietary Ile was similar to that observed in the previous BBD studies, but similar responses have not been reported in the literature (Dozier et al., 2012).

In conclusion, there do appear to be interactions among the BCAA in practical diets fed to Cobb MV × 500 broilers, but the extent of their effect may be more subtle than the drastic changes in body weight seen in the literature. The Val titration study found no effects on processing yields, which agrees with previous Val titration studies, whereas they were observed in the BBD study and a similar response surface study by Ospina-Rojas et al. (2017). Therefore, the lack of significant linear or quadratic effects in the BBD studies, as well as in the Val titration, highlights the importance of evaluating the BCAA as a whole, instead of as individual amino acids.