The invention relates in particular to a method for determining the amount of feed that causes a reduction in enteric methane, a feed contained in a feed ration consumed by a "ruminant to be assessed", comprising, from a database, the following steps: determining, for a biological material, a calibration equation that makes it possible to quantify a tracer from absorption spectra of said database； subjecting a biological material of a new "ruminant to be assessed" to radiation in order to obtain a corresponding measured absorption spectrum, and obtaining the amount of said tracer from said corresponding spectrum and said equation of the previous step； depending on the amount of said tracer, calculating the intake amount that causes a methane reduction and displaying it on an interface.
