SPECIAL STORY 70 FEED & ADDITIVE MAGAZINE September 2023 tration may not be an appropriate method for estimating the availability of HMBi and its potential to supply Met (Ardalan et al., 2021; NRC, 2021), although the linear/nonlinear plasma responses of Met to HMBi supplementation have been considered the best approach. CONCLUSION Altogether, the cumulative scientific data based on isotope enrichments (Lapierre et al., 2011), nonlinear area under curve responses (Kihal et al., 2021), linear plasma-free AA responses (Ardalan et al., 2021), in vitro tissue batches (Breves et al., 2010), indirect milk responses (European Commission, 2003), and ruminal microbiota assays (Li et al., 2023) indicate that HMBi is a reasonable additive to meet Met requirements of ruminants and to balance individual dietary amino acids for sustainable production. However, science-driven sophisticated approaches and specific expertise are needed to enhance the maximum performance of the Met analogs in ruminant nutrition due to complexity of biological fate of the HMBi. References 1. Ardalan, M., Vargas-Rodriguez, C.F., Zanton, G.I., Vázquez-Añón, M., Bradford, B.J., Titgemeyer, E.C., 2021. Relative availability of metabolizable methionine from 2 ruminally protected sources of methionine fed to lactating dairy cattle. J. Dairy Sci. 104, 1811–1822. https://doi.org/10.3168/jds.202019042 2. Breves, G., Schröder, B., Heimbeck, W., Patton, R.A., 2010. Short communication: Transport of 2-hydroxy-4-methyl-thio-butanoic isopropyl ester by rumen epithelium in vitro. J. Dairy Sci. 93, 260–264. https://doi.org/10.3168/jds.2009-2200 3. European Commission, 2003. Report of the scientific committee on animal nutrition on the use of a HMBi (isopropyl ester of the hydroxylated analogue of methionine). https://ec.europa.eu/food/fs/sc/scan/ out129_en.pdf 4. Graulet, B., Richard, C., Robert, J., 2004. The isopropyl ester of methionine hydroxy-analogue is absorbed through the rumen wall in the cow. J. Anim. Feed Sci. 13, 269–272. https://doi.org/10.22358/ jafs/73884/2004
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