Quinoa (Chenopodium quinoa Willd.) is native to the Andean region and belongs to the group of crops known as pseudocereals. It has great potential for improving food for humans and animals due to its interesting nutritional value. Quinoa was studied to determine the fatty acid (FA) composition, chemical composition, gross energy, in vitro dry matter (DM) digestibility (IVDMD) and neutral detergent fibre digestibility (IVNDFD) of the seeds and plant during growth. Herbage samples were collected six times at progressive morphological stages from the early vegetative to the grain fill stage. Effects of plant ageing was analysed by polynomial contrasts. The chemical composition of quinoa is closely connected to development of the plant with the quality of crop decreased with increasing morphological stages. Dry matter, organic matter (OM), and neutral detergent fibre (NDFom) content increased linearly from the mid vegetative to the grain fill stage, while acid detergent fibre (ADFom) content increased linearly from the first to the last stage. The pattern of FA in the seed was characterised by: palmitic acid (PA, C16:0), oleic acid (OA, C18:1n-9) and linoleic acid (LA, C18:2n-6). Among main FA of the plant during growth, α-linolenic acid (ALA, C18:3n-3) was the most abundant FA (from 385 to 473 g/kg of total FA), while LA content, which ranged from 146 to 176 g/kg of total FA, decreased with increasing growth until the shoot stage and then increased, while PA, OA and stearidonic acid (C18:4n-3) did not show differences in their content during growth. IVDMD and IVNDFD decreased linearly with increasing growth stage. The first summer cut of quinoa, whose lipid fraction is rich in ALA and other polyunsaturated FA, should be before shooting, since its nutritional quality deteriorates when cutting is delayed.