Feeding mid-lactation dairy ewes with a low in starch mixture of agro-industrial by-products: effects on performance, milk quality, and environmental impact

• Can by-products replace conventional ingredients in diets of mid-lactation ewes? • Milk yield and quality were maintained with the by-product based feeding strategy. • Oleic acid increased while de novo fatty acids decreased in by-product group milk. • By-product diet improved food conversion efficiency and net food production. • Environmental impact was reduced without affecting enteric methane concentrations. Small ruminants can exploit ecological niches unsuitable for other types of production and thrive on fiber-rich diets, especially during mid to late-lactation. Conventional concentrates, typically based on cereals and legumes, are used to supplement roughage and to meet nutritional requirements for optimal dairy performance. Through their ability to utilize fibrous, human-inedible feedstuffs, sheep can contribute to climate change mitigation by converting these resources into energy supporting productive functions such as milk production. However, the environmental impact of such systems may vary depending on the inclusion level of conventional concentrates. This study aimed to evaluate the dairy performance and environmental impact of an experimental ( EXP ) diet containing a concentrate composed solely of by-products, alongside alfalfa hay, and pasture, compared to a control ( CTRL ) diet containing conventional cereals and pulses as concentrate ingredients. The two dietary treatments were tested over six weeks in two homogeneous groups of 12 mid-lactation Comisana ewes, with the final four weeks considered the experimental period. Milk yield, composition, fatty acid ( FA ) profiles, milk clotting parameters, human-edible feed conversion efficiency ( heFCE ), and net food production ( NFP ) of gross energy ( GE ) and protein were assessed. Enteric methane concentration was measured using a handheld laser device, and the dietary global warming potential ( GWP ) was estimated through a life cycle assessment ( LCA ). Data were analyzed using a repeated measures mixed model, with diet, time (excluding CH 4 data), and their interaction as fixed effects, and animal as a random effect. Milk yield was similar between groups, with minor differences in gross composition. The milk from the EXP group showed slightly lower protein and casein contents but a higher proportion of oleic acid (mainly from olive pomace in the feed) and trans-FA, likely due to a reduced ruminal biohydrogenation of PUFA. In contrast, CTRL milk had a higher proportion of de novo FA due to greater dietary starch content. The EXP diet significantly improved heFCE and NFP for both energy and protein compared to the CTRL diet. Moreover, its lower GWP per kg of feed indicated a more environmentally sustainable profile. Individual enteric methane concentrations were unaffected by the diet. In conclusion, by-products demonstrated good potential to replace conventional feedstuffs in the diet of mid-lactating ewes, improving environmental performance without compromising productivity. Further research is needed to assess the long-term effects on animal performance and welfare and to optimize practical on-farm applications.