Citrus Processing By-Products and Their Phytochemicals in Ruminant Nutrition: From Rumen Modulation to Animal Performance, Product Quality, and Environmental Sustainability

Citrus processing generates substantial quantities of peel, pulp, membranes, seeds, and extraction residues that present both a waste-management challenge and a potential resource for ruminant production. These by-products are characterized by a fermentable nutritional matrix rich in pectin and soluble sugars, together with diverse phytochemicals, including flavanone glycosides, polymethoxylated flavones, essential oils, phenolic acids, carotenoids, and limonoids. This review critically examines how interactions between these nutrients and phytochemicals affect rumen fermentation, microbial ecology, animal performance, metabolic health, product quality, and environmental sustainability. Citrus pulp can partially replace cereal-based concentrates and provide rapidly fermentable energy with relatively low lactate production. However, responses depend on inclusion level, basal diet, processing method, by-product composition, animal species, and production potential. Citrus flavonoids and essential oils may modulate rumen microorganisms, methanogenesis, nitrogen metabolism, oxidative balance, and inflammatory regulation, although evidence from high-dose in vitro studies cannot be directly extrapolated to whole by-products or commercial production. Citrus-derived interventions may also modify milk and meat fatty-acid profiles, antioxidant capacity, oxidative stability, sensory properties, and shelf life, but results remain heterogeneous. From a circular bioeconomy perspective, feeding citrus residues can divert organic material from disposal and reduce reliance on human-edible feeds. Nevertheless, drying, extraction, transportation, reduced animal productivity, and alternative residue uses may offset these benefits. Overall, citrus by-products should be considered integrated nutrient-phytochemical matrices rather than uniform feed ingredients. Standardized chemical characterization, long-term dose-response studies, mechanistic multi-omics research, and combined life-cycle and techno-economic assessments are required to support their safe, effective, and environmentally credible use in ruminant production systems.