INTRODUCTION

Dairy production systems are increasingly expected to deliver high-quality animal-source foods while reducing their dependence on human-edible feed resources and lowering environmental burdens throughout the supply chain [1]. Within the broader circular bioeconomy framework, livestock, particularly ruminants, are often positioned as biological upcyclers that convert heterogeneous, low-opportunity-cost biomass into nutrient-dense foods while returning nutrients to agroecosystems through manure management [2]. In parallel, growing interest in agro-industrial co-products and food system residues has shifted the discussion from general sustainability narratives toward practical questions: which residue streams are nutritionally suitable, how variable are they, what hazards accompany them, and what processing and quality control measures are required for safe and consistent use in dairy production.

Among regionally abundant residue streams in tropical and subtropical agri-food systems, cassava (Manihot esculenta) processing residues have attracted renewed interest as potential energy sources for ruminant diets. Cassava production remains concentrated in tropical regions, and the scale of residue generation is substantial. In 2024, Africa and Asia produced approximately 222.84 and 90.8 million tons of cassava, respectively, while review-based estimates indicate that solid cassava wastes may reach about 50 million tons annually in Africa alone. These figures highlight cassava residues as a regionally abundant biomass resource with potential value for livestock feeding [3]. Recent syntheses highlight cassava’s relevance to ruminant feeding in many low- and middle-income regions due to its agronomic adaptability and high carbohydrate content, alongside growing emphasis on value-chain approaches that enable broader utilization in livestock systems [4]. Importantly, cassava waste is not a single material; it encompasses distinct streams generated along processing pathways (e.g., cassava pulp/pomace from starch extraction, peels, and other residues), each with different nutritional profiles and risk characteristics [5–7]. The dairy sector has a particular incentive to evaluate these streams because concentrate costs strongly influence the economics of milk production and because localized residue availability may buffer feed supply volatility [8]. This review focuses primarily on tropical and subtropical dairy systems, especially smallholder and cooperative settings where cassava residues are regionally abundant and wet handling is common, while also noting implications for more commercial, high-producing herds when residue streams are adequately standardized and stabilized [9, 10]. However, the practical integration of cassava processing residues into dairy rations remains constrained because variability, safety hazards, and rapid spoilage are often discussed separately rather than integrated into an operational framework for procurement, stabilization, and risk management.

A first constraint is compositional and physical variability, which is amplified when residues are produced as high-moisture materials that deteriorate rapidly under warm climates. In practice, cassava pulp may range from relatively starch-rich to more fibrous fractions depending on processing efficiency, dilution, and contamination with soil or fibrous materials; such variability complicates diet formulation and increases the risk of inconsistent rumen fermentation outcomes if not managed through specification-based procurement and blending strategies [11–13]. The circular feed literature emphasizes that variability is intrinsic to many residue streams, and that successful valorization depends on aligning processing technologies, quality assurance, and end use specifications rather than treating residues as nutritionally equivalent substitutes for conventional cereals [14]. For dairy systems, where rumen health and milk components are sensitive to carbohydrate fermentability and fiber effectiveness, variability management is not a secondary issue but a central determinant of feasibility.

A second, and distinctive, constraint is safety associated with cyanogenic glycosides in cassava-derived materials. Cyanogenic glycosides can release hydrogen cyanide (HCN) under specific conditions, posing toxicity risks that limit cassava utilization without appropriate processing and monitoring [15]. Recent work has increasingly shifted from simply describing the presence of cyanide toward evaluating practical mitigation methods and their effectiveness under farm-relevant conditions. For example, ensiling of cassava materials has been investigated not only as a preservation strategy but also as a detoxification approach, with evidence that fermentation conditions and microbial interventions can influence residual cyanide levels and associated rumen fermentation responses [16–19]. This creates a critical opportunity for dairy nutrition: cassava residues may be rendered safer and more stable through targeted processing, but only if the control points (e.g., chopping, moisture management, fermentation dynamics, aerobic stability) are systematically integrated into handling and feeding protocols.

A third constraint involves nutritional “fit” within high-producing dairy diets, where the margin for error is narrow. Cassava residues are typically considered energy contributors, yet their fermentability characteristics and low intrinsic protein concentration (particularly for pulp streams) mean that substitution decisions have to account for synchrony between rapidly fermentable carbohydrates and rumen-degradable nitrogen, as well as maintenance of physically effective fiber to mitigate subacute ruminal acidosis (SARA) risk [20, 21]. While these issues are widely recognized in dairy nutrition, they are rarely discussed in cassava-focused reviews in ways that link residue-specific characteristics, processing-induced changes, and ration level constraints to practical decision rules for inclusion and adaptation. Recent evidence from dairy cows also indicates that responses are context-specific rather than uniformly positive. In multiparous Holstein cows, residue from cassava starch extraction, evaluated at 0, 8, 16, 24, and 32% of the diet dry matter (DM), reduced milk yield by about 15% at the highest inclusion level, indicating that excessive inclusion can compromise productive performance. By contrast, in mid-lactation Thai crossbred cows, replacing soybean meal with fermented cassava pulp with yeast waste did not reduce milk yield or most milk composition traits under the tested conditions, although ruminal ammonia-N increased [22, 23]. These findings underscore why distinctions between high-producing, total mixed ration (TMR)-oriented herds and tropical smallholder or cooperative systems matter: in the former, tight control of fermentability and physically effective fiber is essential for maintaining output and rumen stability, whereas in the latter, feed handling, management capacity, storage conditions, and access to low-cost stabilization are often the binding constraints [9, 10].

Recent advances in bioprocessing research provide a timely foundation for reframing cassava waste utilization in dairy systems from a byproduct-substitution narrative to an upcycling-pathway narrative. Fermentation, ensiling, and other biological treatments can enhance nutritional value and safety, enabling energy and nutrients to be upcycled back into the human food supply via animal production [14]. Consistent with this framing, experimental work on cassava pulp silage demonstrates that combining lactic acid bacteria (LAB) with nitrogen and fermentable substrates (e.g., urea and molasses) can improve fermentation characteristics and aerobic stability, attributes that directly address key barriers to adoption in humid environments [24]. Complementary dairy-focused evidence indicates that fermented cassava pulp products (including fermentation with yeast-related substrates) can be evaluated within lactating cow diets for impacts on rumen fermentation, digestibility, and milk production, supporting the premise that processing is central to converting cassava residues into reliable dairy feed ingredients rather than opportunistic supplements [23]. Accordingly, the transition from simple byproduct substitution to an upcycling pathway should be understood as a sequence of controlled interventions that link stream identity, stabilization route, hazard mitigation, batch verification, and ration-level suitability.

Despite these developments, there remains a notable gap in the dairy nutrition literature: a focused synthesis that (i) defines cassava waste streams in a manner consistent with procurement and processing realities, (ii) consolidates the most recent evidence on cyanide risk mitigation and its integration into preservation strategies, and (iii) translates findings into a quality control and feeding framework tailored to lactating cows. More broadly, recent work on circular feed ingredients, such as former foodstuffs, has highlighted that safety and standardization are decisive for acceptance and scaling, with attention to quality assurance and hazard control increasingly central to the scientific and practical discourse [25]. An analogous, residue-specific framework is needed for cassava waste in dairy systems, where both toxicological (HCN-related) and nutritional (rumen stability and milk component) risks must be managed simultaneously. There is also a need to more clearly distinguish between evidence generated in tropical smallholder or cooperative settings and its applicability to high-producing commercial herds, particularly in confinement or TMR-based systems, where tolerance for fermentability errors and effective fiber deficits is lower.

Although cassava processing residues have been investigated as potential ruminant feed ingredients, most studies have examined isolated aspects such as basic nutritional profiles or simple preservation techniques without integrating stream-specific variability, comprehensive hazard control, and dairy-specific ration level outcomes. Safety constraints (cyanogenic glycosides, mycotoxins, microbial hazards) and preservation efficacy are frequently discussed separately rather than as interconnected process-controlled factors. There is also limited evidence linking stabilization methods to rumen function, milk components, and practical implementation in tropical smallholder and cooperative dairy systems, where wet handling and seasonal availability pose unique logistical challenges. This gap limits the translation of cassava residues from opportunistic by-products to reliable circular feed ingredients governed by specification-based procurement and risk management.

Therefore, the objective of this review was to critically evaluate cassava processing residues as circular energy feed ingredients for dairy cows, with particular emphasis on (1) classification and variability of cassava waste streams, (2) cyanogenic glycosides/HCN as a safety constraint and the effectiveness of mitigation strategies, especially ensiling and microbial/chemical interventions, (3) processing pathways that enhance preservation, aerobic stability, and nutritional usability, and (4) ration level considerations for lactating cows, including implications for rumen function and milk production outcomes. The review is oriented primarily toward tropical and subtropical dairy systems, with particular relevance to smallholder and cooperative implementation, while identifying where current evidence remains insufficient for broad generalization to high-producing temperate commercial herds.

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