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Methionine sources in turkeys – an update*

Methionine sources in turkeys – an update*

Dr. Juliano Cesar de Paula Dorigam, Dr. Andreas Lemme, and Holly Malins; Evonik Operations GmbH, Hanau, Germany

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Key information

• A compilation of scientific information was done based on 15 available data sets from studies conducted since 1981 resulting in average relative efficacy value (RBV) of 76 % (weight gain), and
68 % (feed conversion) for liquid MHA-FA compared to DL-Met in turkeys.
 
• Despite the compilation indicate slightly higher values, a RBV of 65 % on a weight-to-weight basis is recommended for liquid MHA-FA in turkey nutrition. The recommendation was validated in several feeding trials where DL-Met and liquid MHA-FA were supplemented at 65:100 ratio without significant change in performance.
 
• At marginal dietary Met+Cys levels, which represent more sensitive test conditions, liquid MHA-FA could be replaced by DL-Met in turkey diets without compromising performance and DL-Met even provided extra benefits regarding improved breast meat yield, antioxidant capacity of liver and foot pad health.
 
*Presented at the 15th Turkey Science and Production Conference, Chester, UK, March 22nd – 24th, 2023

Introduction

Commercial poultry diets are routinely supplemented with methionine or its precursors to precisely meet their sulphur amino acid (SAA) specifications. Methionine (Met) is an essential amino acid for poultry and serves as a building block for protein synthesis, being also a precursor for cysteine (Cys) and important methyl donor (Selle et al., 2020). Most of the supplemental Met commercially available is supplied as crystalline DL-methionine (DL-Met, 99 % content) or as liquid DL-2-hydroxy-4-methylthio butanoic acid (methionine hydroxy analogue-free acid, MHA-FA; 88 % content) (Willke, 2014). Both products provide Met activity to poultry, but chemically MHA-FA is not an amino acid due to the replacement of the characteristic amino group by a hydroxy group (Yang et al., 2020). In addition to the chemical differences, there are studies indicating slower and less efficient absorption of liquid MHA-FA due to differences in transport systems in intestinal brush border membrane and potential catabolism by enteric bacteria (Maenz and Engele-Schaan 1996, Drew et al., 2003). Therefore, understanding the nutritional value indicated by the relative bioefficacy value (RBV) of liquid MHA-FA compared to DL-Met is an important precondition to cost-effective purchasing, feed formulation, and optimum animal production (Sauer et al., 2008).

A relative bioefficacy of 65% for liquid MHA-FA is recommended for turkeys

A proper evaluation of potential differences in the bioefficacy of two products is strongly dependent on an adequate trial design. In such experiments, the treatments comprise a basal diet deficient in Met+Cys and a set of diets with incremental Met+Cys levels. The effect of the incremental Met supplementation on performance criteria describes the dose-response relationship which is not linear but follows the law of diminishing returns. The first unit of supplemental Met results in a relatively strong effect, i. e. significantly increases the performance. This effect diminishes progressively with each additional amount of Met supplementation. Eventually an optimum supply status is reached where no further increase in performance can be achieved. This curve can accurately be described by an exponential regression equation (Rodehutscord and Pack, 1999). The more data points describe the non-linear first section of the response curve, the more robust is the conclusion. The methodology of simultaneous exponential regression as proposed by Littell et al. (1997) assumes that both nutrient sources under test allow for the same asymptote of the response curves. Although the experimental setup to determine the RBV has been a matter of discussion and different asymptotes have been proposed by Gonzales-Esquerra et al. (2007), the meta-analysis performed by Sauer et al. (2008) including 46 dose-response experiments with broilers, as well as a more recent broiler trial (Lemme et al., 2020), provided evidence that the asymptotes are the same for liquid MHA-FA and DL-Met.

Table 1: Relative bioefficacy of liquid MHA-FA compared to DL-met (%) in turkeys from fifteen available data sets1

1 updated table from Lemme et al. (2012)

The European Food Safety Authority (EFSA), which examines and assesses dossiers for product registration in the European Union, released a scientific opinion on liquid MHA-FA and its calcium salt in 2018 (Rychen et al., 2018). This opinion concludes a lower RBV for MHA-products for non-ruminant animals and fish and suggests an RBV of 75 % for MHA-products compared to DL-Met on equimolar basis which is equivalent to 66 % on product-to-product basis. This is in line with a recent meta-analysis by Lemme et al. (2020) concluding that RBV of MHA-FA was 62 %. In contrast to broilers, less scientific dose-response data on turkeys is available. While a couple of studies were published back in the 1980’s, Hoehler et al. (2005), Gonzales-Esquerra et al. (2007) and Batonon-Alavo et al. (2022) conducted trials to determine the RBV of MHA-FA relative to DL-Met in turkeys using the simultaneous dose response approach more recently (Table 1). If only these studies would be considered, average RBV for liquid MHA-FA would be 75 % and 66 % for body weight gain and feed conversion ratio, respectively, and are therefore slightly lower than the overall average. Indeed, particularly the RBV for feed conversion ratio would be in line with the conclusion by EFSA (2018) indicating a RBV of 66 % for monogastric animals on product basis. Therefore, like recommendations for broilers, laying hens, swine, and aqua species (Lemme et al., 2012; Htoo and Rademacher, 2012; Lemme, 2010) an RBV of 65 % for liquid MHA-FA compared to DL-Met is recommended for turkey nutrition.

The following example is given to demonstrate an appropriate experimental design, as well as the proper mathematical model which should be used for data analysis and proper interpretation. In a feeding trial from day 7 to 50 with female Hybrid Converter turkeys, the effects of DL-met (99 % pure), a diluted DL-Met (65 %) which was made by blending DL-Met (99 %) with glucose, and liquid MHA-FA (88 %) on performance were compared (Hoehler et al., 2005). Corn-soy diets were supplemented with graded levels of the products (0.07, 0.14, 0.21, 0.28 %). In this trial, a RBV of 65 % was assumed a priori for diluted DL-Met (65 %) relative to pure DL-Met (99 %). Thus, these treatments could be regarded as an internal standard to check the validity of simultaneous regression analysis as suggested and validated by Lemme et al. (2020). With 3 out of 4 inclusion levels within the curve-linear part of the curve, the weight gain response of turkeys to graded levels of DL-Met, diluted DL-Met (65 %), and liquid MHA-FA described a nonlinear trend and, therefore, data were suitable for simultaneous exponential regression according to procedure proposed by Littell et al. (1997) (Figure 1). The determined RBV for diluted DL-Met (65 %) confirmed expectations although it was slightly lower than expected 65 % which is due to biological variation of data. RBV of both diluted DL-Met (65 %) and liquid MHA-FA were significantly lower than 88 %.

Figure 1: Weight gain of female Hybrid turkeys 7 to 50 days of age fed incremental levels of DL-Met, diluted DL-Met (65 %), or the liquid hydroxy analogue of Met (MHA-FA; 88 %). Values in brackets indicate the 95 % confidence interval.

A recent publication by Batonon-Alavo et al. (2022) proposed Single and Two Slope broken line regression to determine Met+Cys requirements of turkeys with DL-Met and liquid MHA-FA from 0 to 28 days. The authors concluded there is no difference in requirement regarding Met sources. While data confirm that both products can be used to meet the Met+Cys requirement of turkeys, the required supplementation to achieve the maximum response is different between the sources. The data by Batonon-Alavo et al. (2022) were re-analyzed by simultaneous exponential regression. The supplemental levels of DL-Met were calculated as the difference between the dietary SAA in the supplemented feeds and the SAA in the basal diet (0.700 % Met+Cys). For liquid MHA-FA, equimolar inclusion levels were assumed as suggested by the authors (88 %). The results of the re-evaluation can be obtained from Figure 2.

Figure 2: Re-evaluation of data reported by Batonon-Alavo et al. (2022) using simultaneous exponential regression on weight gain and feed conversion of Hybrid Converter poults fed increasing doses of either DL-Met (99 %) or MHA-FA (88 %) from 0 to 28 days of age. Values in brackets indicate the 95 % confidence interval.

The validation of the recommendation on relative bioefficacy

The recommended RBV of 65 % for liquid MHA-FA as compared with DL-Met (on product basis) was challenged and validated in feeding trials. Male 1-d-old B.U.T. Big 6 turkeys were evaluated in a 21 day feeding trial where dietary treatments comprised a basal diet, which was deficient in Met+Cys, and six diets with three graded levels of liquid MHA-FA (0.154, 0.308, and 0.462 %) or DL-Met (0.100, 0.200, and 0.300 %; Hoehler et al., 2005). The ratio between both products at each of the three corresponding inclusion levels was 65 %, assuming that 100 units of liquid MHA-FA can be replaced by 65 units DL-Met without compromising performance. The summary of the results is presented in Table 2.

Table 2: Effect of graded levels of DL-Met and the liquid hydroxy analogue of Met (MHA-FA; 88%) on weight gain and feed conversion in male B.U.T. Big 6 turkey poults from 1 to 21 d of age, trial 2 (Hoehler et al., 2005)

Accordingly, performance in corresponding treatments (2 vs 5, 3 vs 6, and 4 vs 7) did not differ, thus, confirming and validating the recommendation.
 
Lemme and Meyer (2009) conducted a trial with B.U.T. Big 6 turkey toms in a 6-phase feeding program. Also in this study, DL- Met or MHA-FA were supplied in a 65:100 ratio to either meet adequate Met+Cys levels in turkey diets or to be below requirement by supplementing half of the dose of both products. Dietary Met +Cys levels (normal vs half) resulted in a significant final body weight difference indicating that at half dosage Met+Cys supply was limiting performance (Table 3) making the 65:100 test more sensitive at lower supply.

Table 3: Growth and slaughter performance of male B.U.T. Big 6 turkeys toms fed with adequate and low Met+Cys levels and supplemented with either DL-Methionine (DL-Met) or liquid methionine hydroxy analogue free acid (MHA-FA) at a ratio of 65:100 after 21 weeks of life (Lemme and Meyer, 2009)

Supplementation of the products in a 65 (DL-Met) : 100 (MHA-FA) ratio revealed similar body weight, mortality and carcass traits. However, there was a trend for better feed conversion ratio which may confirm a basically lower RBV observed in the literature review (Table 1).
 
Agostini et al. (2017) conducted a study with similar design. Male B.U.T. Big 6 turkeys were used. Four dietary treatments comprised two products (DL-Met, liquid MHA-FA) and two dietary Met+Cys levels. Sub-optimal digestible Met+Cys levels were around 87 % of the optimal levels which were set at 9.9, 9.0, 8.5, 7.4, 6.6, and 5.8 g/kg feed in phases 1 to 6. The amount of DL-Met added in both Met dose groups was 65 % of that of added MHA-FA (on product basis). Again, growth rate of turkeys fed the sub-optimal Met+Cys diets was significantly lower confirming that sub-optimal Met+Cys levels limited performance although no difference in feed conversion was observed (Table 4). In corresponding treatments neither a difference in body weight nor in feed conversion ratio was observed. In contrast, especially at low Met+Cys supply, breast meat yield was higher with DL-Met than with liquid MHA-FA.

Table 4: Growth and slaughter performance of male B.U.T., Big 6 turkeys fed diets with adequate and marginal Met+Cys levels and supplemented with either DL-Methionine (DL-Met) or liquid methionine hydroxy analogue free acid (MHA-FA) at a ratio of 65:100 after 21 weeks of life (Agostini et al., 2017)

More recently, Lingens et al. (2021) evaluated the effects of DL-Met and liquid MHA-FA in adequate and low protein diets on performance parameters, footpad health, liver health and oxidative stress. 63 day-old female turkeys (B.U.T. Big 6) were randomly assigned to four groups fed with diets differing in methionine source (DL-Met (65 %) vs. liquid MHA-FA (100 %) and crude protein content (15 % vs. 18 %) for 35 days. The results showed no interactions between the dietary crude protein and Met sources. Strong protein reduction significantly impaired water intake, feed intake, weight gain and feed conversion ratio, but improved footpad health. There was a trend for higher final body weight and weight gain, respectively with DL-Met, which was due to significantly higher feed consumption. However, feed conversion ratio was not affected by products at either dietary protein level. DL-Met resulted in a significant increase in the liver’s antioxidative capacity compared to liquid MHA-FA. Although the protein reduction resulted in impaired performance, the study showed that MHA-FA can be replaced by DL-Met in a 100:65 weight ratio without compromising performance and additional benefit of DL-met to improve the antioxidative capacity of the liver.

Table 5: Performance of female turkeys fed experimental diets with different dietary protein content and using either DL-Methionine (DL-Met) or liquid methionine hydroxyl analogue-free acid (MHA-FA) from day 63 to 98 days (Lingens et al., 2021)

Additionally, Lingens et al. (2021) observed a trend for lower footpad dermatitis (FPD) scores for DL-Met compared to MHA-FA fed turkeys at d 98. This result confirms earlier findings by Abd El-Wahab et al. (2014), who found that young turkeys fed higher levels of methionine (2 or 3g/kg diet) supplemented with DL-Met led to significantly lower FPD scores compared to those same levels in MHA-FA supplemented diets (4.54 vs. 5.04 and 4.12 vs. 5.19, P < 0.05). The level of dietary Met plays an important role for health of skin of foot pad. Already in 1974 Chavez and Kratzer (cited in Kamphues et al., 2011) reported higher frequency and score of FPD in turkeys fed liquid MHA-FA compared to turkeys fed DL-Met. Thus, it seems that Met has a structural function regarding foot pad health via protein synthesis and continuous production of keratin. Consequently, low availability of Met could affect protein synthesis negatively and affect skin of foot pad

Table 6: Comparison of responses to liquid MHA-FA and DL-Met1 on body weight gain or feed conversion ratio at adequate or marginal dietary Met+Cys

Table 6 summarizes the results of seven turkey feeding trials where DL-Met and liquid MHA-FA were supplemented at a 65:100 ratio at either adequate or marginal Met+Cys supply. These trials comprise experiments conducted in well controlled pen facilities as well as under commercial production conditions (Hoehler and Hooge, 2003). These commercial trials used a total of 54,906 turkeys and it should be noted that MHA-FA fed birds at adequate Met+Cys supply were slaughtered one day later which would explain the differences in final body weight. This compilation in Table 6 serves as strong validation for the recommended RBV of 65 % for liquid MHA-FA relative to DL-Met. Accordingly, it is applicable at any dietary Met+Cys level. This appears to be in contrast with Jankowski et al. (2017) who supplemented equimolar levels of liquid MHA-FA or DL-Met at Met+Cys at NRC (1994) recommendations, which were considered low, and at about 40 % higher dietary Met+Cys level. At both dietary Met+Cys levels, there were no performance differences between both products. There were indeed small but significant differences (10.8 vs 11.0 kg final body weight; 2.40 vs 2.37 kg/kg FCR) between dietary Met+Cys levels; however, respective recommendations by the breeder (Hendrix Genetic Company, 2014) suggested final body weights of 10.8 kg which indicates that Met+Cys supply was not strongly limiting performance. Indeed, such pairwise comparison is particularly sensitive at marginal Met+Cys supply and results in Table 6 are therefore strong evidence

Conclusions

A relative effectiveness of 65 % of liquid MHA-FA relative to DL-Methionine on a weight basis (1 kg liquid MHA-FA to 0.65 kg DL-Met) is recommended. A number of recently published validation studies not only provide the evidence that 100 units of MHA-FA can be substituted by 65 units of DL-Met at any dietary Met+Cys supply level or at different protein levels without affecting bird’s performance, but that DL-Met also provides additional benefits regarding improved breast meat yield and foot pad health.

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