Synthetic methionine sources vary in effectiveness
Although supplementing methionine has advantages over the risk of overfeeding proteins to meet requirements, not all sources may be equally effective.
Methionine is one of the essential amino acids in swine and poultry nutrition. Requirements can be met either through amino acids in feedstuffs or via the addition of synthetic sources. Satisfying the requirement with supplemental synthetic methionine is more economical because it avoids excessive levels of unbalanced proteins in feed, which are not only costly but detrimental to animal performance.
A requirement of 0.1 percent of methionine in the diet can be met by utilizing either of the following: 16 percent soybean meal, 5.6 percent fish meal or 0.1 percent DL-Methionine (DLM).
Properties of synthetic methionine
DL-Methionine (DLM) and liquid Methionine Hydroxy Analogue Free Acid (MHA-FA) are commonly used synthetic methionine sources in animal feeds. DLM is a dry product composed of 99 percent methionine and 1 percent water. Liquid MHA-FA consists of 12 percent water and 88 percent methionine hydroxy analogue. Methionine hydroxy analogue chemically is not real methionine. Moreover, studies conducted indicate an inferior absorption and metabolization (Drew et al., 2005) and a poor nutritional value of the oligomer fraction of liquid MHA-FA (Koban and Koberstein, 1984), both resulting in a reduced effectiveness of the commercially available product in broilers (Lemme, 2001).
Perez (2005) evaluated the dosing and handling properties of liquid MHA-FA as compared to DL-Methionine and found it less favorable. This may have led some MHA-FA manufacturers to produce a dry version in the form of MHA-Calcium salt (MHA-Ca). This product is the result of a chemical reaction between liquid MHA-FA and Calcium di-hydroxyde. If manufactured properly, it contains a minimum of 84 percent Methionine hydroxy analogue, a minimum of 12 percent calcium and a maximum of 1 percent water.
Table 1: Average relative biological efficacy
Comparing biological effectiveness
From the results of several studies conducted to compare the biological efficacy of DLM and liquid MHA-FA in farm animals (Jansman et al., 2003, Lemme et al., 2002 and Lemme and Petri, 2003) it has been derived that the relative effectiveness of liquid MHA-FA is about 65 percent on average on weight-to-weight basis compared to that of DLM in chickens.
Similar to liquid MHA-FA, the relative biological efficacy of MHA-Ca is well-researched and covered in the scientific literature, especially for broilers. A total of 69 dose response studies in broilers and layers have been included in the accompanying plot. Regression analysis of all data sets across various performance criteria such as weight gain, daily egg mass and feed conversion resulted in an average relative effectiveness of about 64 percent for MHA-Ca compared to DLM on a weight-to-weight comparison basis (Table 1). This does not show significant differences from liquid MHA-FA in broilers. Moreover, the nutritional advantage of having only methionine hydroxy analogue monomers in MHA-Ca is obviously counterbalanced by the lower purity (i.e. Calcium, water and impurities content) of this product.
As with liquid MHA-FA, the slower and less efficient absorption of methionine hydroxy analogue monomers from the small intestine may be the main reason for the relatively lower biological effectiveness of MHA-Ca. Esteve-Garcia (1988) found in 4 experiments that DLM is almost completely absorbed at the end of the small intestine. However, the amount of unabsorbed MHA-CA was consistently and significantly higher (Figure 2).
Absorption in the intestine
These findings are in line with the results obtained by Esteve-Garcia and Austic (1993) who tracked the absorption of MHA-Ca and DL-Methionine throughout the small intestine (Figure 3). They observed in their trials that DL-Methionine is absorbed more rapidly and efficiently from the small intestine than MHA-Ca. At the end of the small intestine (section 4) about 12 percent of the originally fed MHA-Ca molecules were unabsorbed, whereas most of the DLM was already absorbed in the upper part of the small intestine (section 1) and only about 3 percent were still present in section 4. Any nutritional compound which is not absorbed by reaching section 4 will be lost for the metabolism and will be subject to bacterial degradation in the lower intestine and eventually be excreted.
Lingens and Molnar (1996) conducted a trial to determine the metabolism of DL-M and DL-MHA-Ca in broilers at the University of Göttingen, Germany. Sixty male broilers were fed corn-soybean diets with either 0.05 percent DLM or DL-MHA-Ca on an equimolar basis for 15 days. In order to measure the protein deposition, C14 of either DLM or MHA was radio-labeled. At the end of the trial the amount of labeled C14 was measured after 24, 48 and 72 hours post administration. Results showed that when feeding labeled DL-MHA-Ca a significantly higher portion of administered C14 is excreted (17.0 percent MHA-FA vs. 4.4 percent DL-Met) (Figure 4). In addition, a significantly smaller portion of administered C14 in MHA-Ca is converted to methionine and deposited in breast and leg muscle. These results support the findings of Maenz & Engele (1996) of incomplete absorption of liquid MHA-FA.
Similar study results
Based on the reviewed studies, there is a strong indication that methionine sources differ in their effectiveness. For MHA-Ca, these physiological studies strongly indicate an incomplete absorption from the gut contributing to its lower biological efficacy compared to DLM as reported in the desk studies cited above. Data derived from these studies result in a similar biological efficacy for both, liquid MHA-FA and MHA-Ca of about 65 percent on average compared to DLM in broilers.