The present study aimed to determine the optimal moment of commercial slaughter (OMCS) of guinea pigs (Cavia porcellus) reared in a semi-intensive system under three feeding systems. The study was conducted in the department of Cusco-Peru (Lat: 13 ° 33'06.9 "South, Lon: 71 ° 53'01.2" West). 96 male guinea pigs of Peru genetic line were used, all weaned at 14 days in which they were randomly distributed to each feeding system (FS). FS1 used 100% alfalfa (Medicago sativa), FS2 used alfalfa

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... go sativa) (50%) and commercial feed (50%), finally the FS3 used 100% commercial feed in its diet. Diet consumption (DC), weight gain (WG), feed conversion (FC) were analised. SA2 showed higher values ? ? (p <0.05) for DC and WG, whereas the FS3 FC was higher (p <0.05) than the others FS. The OMCS for FS1 was at 9, FS2 at 10 and FS3 at 13 weeks, with live weights of 784.91±13.66; 1192.63±13.66 and 1028.59±13.66, where or FS2 showed superiority (p <0.05). The economic merit for FS2 determined that, in addition to promoting better financial return, can generate more production cycles per year.