Rice straw for ruminants: processing technologies, nutritive value, feeding responses and environmental trade-offs

1 Introduction

Rice cultivation generates a very large seasonal biomass stream; a recent synthesis estimates annual global rice-straw production at more than 700 million tonnes, with management still constrained by collection windows, low bulk density and open-field burning in several rice-growing regions (Singh et al., 2024). For ruminant nutrition, the attraction of rice straw is not high intrinsic nutritive value but the opportunity to convert a human-inedible crop residue into roughage and animal-source food. This can reduce pressure on conventional forage supply and aligns conceptually with SDG 2 (Zero Hunger), SDG 12 (Responsible Consumption and Production) and SDG 13 (Climate Action), provided that feed use does not increase methane, nitrogen loss, contaminant transfer or avoidable processing burdens (McAllister et al., 2025; Mottet et al., 2017; Van Zanten et al., 2019). Reviews consistently identify low crude protein, high structural fibre, silica-rich tissues, slow ruminal degradation and low voluntary intake as the dominant feed-value constraints (Sarnklong et al., 2010; Van Soest, 2006; Aquino et al., 2020).

Earlier rice-straw reviews established the value of physical, chemical and biological upgrading but were written before the current emphasis on complete-diet validation, standardized methane denominators, traceable feed-safety gates and reproducible reporting of microbial/enzyme treatments (Sarnklong et al., 2010; Van Soest, 2006; Aquino et al., 2020). More recent sustainable-feed reviews increasingly integrate nutritional adequacy, processing feasibility, animal performance and environmental implications rather than treating residue inclusion as an end point (Prasannakumaran et al., 2026). The unresolved gap is therefore translational: which processing effects survive complete-diet formulation and animal validation, and which claims remain unsupported at farm and food-system scales?

Therefore, the central feed-science question transcends the mere feasibility of feeding rice straw: it asks how a defined batch, processed by a specific method and included at a defined level in a complete diet, affects dry-matter intake, rumen fermentation, digestibility, microbial protein synthesis, animal performance, methane, nitrogen use and health. This formulation shifts the unit of evaluation from raw straw or laboratory digestibility to the complete feeding response. Host adaptation matters, but it does not remove diet constraints: buffaloes and some cattle populations can differ in fibre degradation and rumen bacterial structure under high-roughage feeding (Pu et al., 2023), yet productive animals still require adequate fermentable energy, rumen-degradable nitrogen and safety control.

The present review therefore focuses on rice straw as a ruminant feedstuff and uses six linked questions to organize the evidence: (i) which feedstock, species and harvest constraints govern feed value; (ii) which processing and conservation technologies have credible evidence; (iii) how should treated straw be evaluated within complete ruminant diets; (iv) what do recent in vivo studies show about intake, digestibility and production; (v) how should methane, nitrogen use and feed safety be interpreted; and (vi) what reporting and implementation standards are required before a rice-straw treatment can be recommended. Bioenergy, biochar and materials are considered only as controlled non-feed destinations for batches that fail quality or safety gates.

2 Review approach, evidence mapping and grading

2.1 Search strategy and study selection criteria

This article is a critical Review with a transparent targeted-search and evidence-mapping protocol; it is not presented as a Systematic Review or meta-analysis. Searches were updated through 18 June 2026 in Web of Science Core Collection, Scopus, PubMed and ScienceDirect, with backward and forward citation tracking. Search terms coupled “rice straw” or “paddy straw” with ruminant species, processing/conservation terms, digestibility and production outcomes, methane, nitrogen utilization, feed safety and implementation. The final synthesis used 54 verifiable scholarly or regulatory sources. Because the original review was not prospectively designed as a record-level systematic review, PRISMA-style identified/screened counts were not retrospectively invented; instead, the recent in vivo evidence subset was bounded a priori and is reported transparently below.

Electronic retrieval was supplemented by backward and forward citation tracking of high-impact reviews and foundational studies. Bibliographic metadata and DOI strings were cross-checked against publisher or PubMed records where available. Grey literature and non-peer-reviewed conference abstracts were excluded from the scientific synthesis, except that formal feed standards were retained solely for jurisdiction-specific compliance statements.

2.2 Inclusion/exclusion criteria and data extraction

Studies were eligible for detailed evaluation if they fulfilled the following targeted criteria: (i) peer-reviewed technical reviews defining mechanistic constraints; (ii) controlled, randomized in vivo feeding trials evaluating processed rice straw; and (iii) mechanistic in vitro or in situ characterizations directly elucidating cell-wall deconstruction, microbial kinetics, or gastrointestinal safety gates. Conversely, studies focusing strictly on bioenergy extraction, direct combustion, industrial material synthesis, or agronomic soil amendments were excluded, unless they specifically addressed the diversion and management pathways of toxicologically or biochemically feed-disqualified straw batches. Due to the high heterogeneity in animal species, baseline physiological stages, roughage-to-concentrate ratios, and chemical/biological additive doses across the global literature, a quantitative meta-analysis with pooled effect sizes was deemed statistically inappropriate; instead, a critical narrative and semi-quantitative synthesis was executed.

2.3 Methodological evidence mapping and grading matrix

To move beyond purely qualitative description, a methodological evidence matrix was constructed for 10 representative recent in vivo target studies evaluating processed or ensiled rice-straw-based diets. Each trial was coded across seven reporting domains using a binary reported/not-reported rule: nutritional or ruminal responses; productive output; animal-level enteric methane; feed-safety testing; socio-economic feasibility; manure nutrient/contaminant fate; and whole-chain life-cycle performance.

The target cases were bounded a priori to characterize current reporting practice rather than to create a pooled global dataset. They primarily cover 2024–2026; one immediately adjacent 2023 methane study was retained because it directly quantified enteric methane and energetic efficiency. Foundational pre-2024 feeding studies remain in the narrative synthesis but were not used to score current reporting completeness. Eligible cases required direct ruminant in vivo feeding, rice straw as a treated/ensiled or ration-level roughage component, a defined comparator or basal ration, at least one animal-level outcome, and verifiable peer-reviewed metadata. Tables 1, 2 therefore constitute a recent reporting-gap map, not a meta-analysis.

Table 1. Representative recent in vivo target cases for rice-straw-based feeding systems.

These studies were selected according to the bounded criteria in Section 2.3 to expose reporting gaps and should not be interpreted as a pooled global meta-analysis.

Table 2. Evidence coverage and reporting gaps in 10 representative recent in vivo rice-straw feeding studies.

Coverage: High ≥75%, Moderate 50–74%, Low 25–49%, Very low 1–24%, Absent 0%.

Nutritional and ruminal kinetics: encompassing volatile fatty acids (VFAs), ammonia-nitrogen (NH₃-N), pH, and apparent total-tract nutrient digestibility.

Productive output: restricted to objective animal performance metrics, including average daily gain (ADG), feed conversion ratio (FCR), or standardized milk yield.

Enteric methane kinetics: explicitly requiring direct animal-level quantification (e.g., respiration chambers, SF6 tracer technique, or automated head-box systems), while omitting trials relying solely on in vitro gas production extrapolations.

Feed safety and toxicological compliance: verified by explicit screening panels for heavy metals (e.g., cadmium), mycotoxins, or agrochemical residues.

Socio-economic feasibility: including structural evaluations of labour inputs, processing costs, or net farm economic returns.

Manure nutrient fate: tracking the excretion and environmental recycling pathways of nitrogen or co-transferred contaminants.

Whole-chain life-cycle performance: requiring comprehensive boundary analysis or carbon footprint assessments against alternative roughage baselines.

Coverage labels in Table 2 were defined a priori as High (≥75% of target studies reporting the domain), Moderate (50–74%), Low (25–49%), Very low (1–24%) and Absent (0%). No additive weights were assigned because the seven domains are not commensurable: feed safety functions as a veto condition rather than a benefit score, whereas nutrition, production and environmental outcomes answer different questions. The evidence ladder is therefore ordinal and claim-specific rather than a weighted ranking.

This evidence-grading framework was specifically designed to separate biological plausibility established in laboratory settings from true farm-scale efficacy and whole-chain sustainability claims.

2.4 Recent research-focus shifts

Within the recent target cases, research attention has shifted toward co-ensiling, microbial/additive approaches, rumen microbiome responses and methane reporting, while farm economics, manure fate and whole-chain LCA remain unreported in the mapped studies. This is a descriptive trend within the bounded evidence set, not a formal bibliometric analysis. The resulting priority is to connect increasingly detailed biological measurements with standardized implementation, safety and system-level outcomes.

2.5 Review limitations and risk of bias acknowledgment

This targeted critical-review design enables multidisciplinary integration but does not provide exhaustive record-level coverage or formal risk-of-bias scoring. Accordingly, “no reliable evidence” means that no directly applicable, verifiable peer-reviewed study was identified within the stated search boundaries; it does not prove absolute empirical absence. This limitation is made explicit to avoid overstating the completeness of the evidence map.

3 Feed value constraints of rice straw

3.1 Cell-wall recalcitrance, silica and structural heterogeneity

Rice straw contains potentially fermentable cellulose and hemicellulose, but microbial access is restricted by interacting lignin–carbohydrate associations, phenolic esterification, crystallinity and a silicified epidermal/cuticular layer. Microscopy and FTIR work on rice-straw fractions showed that the leaf sheath combined high hydrogen bonding, crystallinity and esterification, while alkali treatment disrupted the silicified cuticle and improved ruminal degradability (Ghasemi et al., 2013). More recent rice germplasm phenotyping likewise found strong negative associations between silica and in vitro digestibility (Jajoriya et al., 2026). These data support a mechanistic distinction: mechanical size reduction increases accessible surface area, whereas alkali, ammoniation or biological deconstruction targets chemical/structural barriers.

3.2 Nitrogen–energy synchrony and species-specific tolerance

A second limitation is nutritional rather than structural: rice straw supplies little nitrogen and little rapidly fermentable carbohydrate. Here, “nutrient imbalance” refers specifically to asynchronous availability of rumen-degradable nitrogen and fermentable organic matter, not to a generic nutrient deficiency. When rapidly released ammonia is not matched by fermentable energy, nitrogen absorption and urinary loss increase; when fermentable carbohydrate is supplied without adequate degradable nitrogen, fibrolytic microbial growth can remain constrained. A recent systematic review of nitrogen–carbohydrate synchronization supports this causal interpretation and links synchrony with microbial protein synthesis and nitrogen-use efficiency (Bezerra et al., 2026).

Species and physiological state modify, but do not remove, these constraints. Swamp buffaloes and cattle can differ in in situ straw-fibre disappearance and fibre-associated bacterial communities under high-roughage diets (Pu et al., 2023). Maintenance animals generally tolerate lower diet digestibility than rapidly growing or high-yielding animals because their energy demand and gut-fill pressure are lower. Consequently, species response should be interpreted through complete-diet nutrient density, intake capacity and production level rather than by assuming a fixed species-specific preference for rice straw.

3.3 Harvest moisture, deterioration and processing choice

Rice-straw management begins at harvest, and processing should be matched to moisture, drying opportunity, logistics and intended feeding use rather than to a universal technology ranking. Wet or monsoonal harvests favour rapid collection and ensiling/co-ensiling because delayed drying increases mould, aerobic heating and loss of water-soluble carbohydrate; reliable dry windows favour baling and urea-based treatment when airtight curing is feasible (Aquino et al., 2020; Wanapat et al., 2009; Polyorach et al., 2015; Nguyen et al., 2025; Oladosu et al., 2016; Sarwono et al., 2022; Sun et al., 2023; Abo-Donia et al., 2024; Lu et al., 2024; Ma et al., 2024; Feng et al., 2026; Deng et al., 2025). Table 3 converts these literature-supported principles into operational archetypes. Here, a “standard operating procedure” means a documented sequence for dose, moisture adjustment, mixing, compaction/sealing, curing time, opening/aeration and quality checks. “Curing verification” means confirming that the planned sealed period and treatment conditions were achieved and checking post-treatment odour, residual ammonia where relevant, mould, pH/fermentation quality and representative chemistry before feeding.

Table 3. Harvest and storage conditions affecting rice-straw feed conservation and processing choice, synthesized from feeding, ensiling and implementation literature (Aquino et al., 2020; Wanapat et al., 2009; Polyorach et al., 2015; Nguyen et al., 2025; Oladosu et al., 2016; Sarwono et al., 2022; Sun et al., 2023; Abo-Donia et al., 2024; Lu et al., 2024; Ma et al., 2024; Feng et al., 2026; Deng et al., 2025; Baba et al., 2019; Balehegn et al., 2022; Pawera et al., 2024).

These scenarios are decision archetypes, not rigid geographic labels. Southern double-cropping systems may often resemble the wet-harvest archetype, whereas single-cropping systems with a dependable dry period may resemble the baling-and-ammoniation archetype, but farm-level weather, machinery, storage and market access remain decisive. Feed suitability also declines after deterioration. Treatment should not be used to legitimise mouldy or contaminated material. Moisture, odour, visible mould, temperature history and representative chemical sampling should be assessed before a batch enters storage or processing.

4 Processing and conservation technologies targeting feed value

4.1 Physical processing: necessary but rarely sufficient

Chopping, shredding, grinding, baling, pelleting and total mixed ration preparation improve collection, dosing, mixing and reduction of selective feeding. Their main contribution is logistical and managerial. Very fine grinding can reduce physically effective fibre and increases energy demand; densification is justified only when transport or market benefits exceed processing cost. For reproducibility, physical studies should report theoretical length of cut or particle-size distribution, bulk density before and after processing, electrical or fuel energy use and storage loss. Physical processing should therefore be reported as an enabling step, not as evidence that the fibre barrier has been overcome.

4.2 Urea, ammonia and alkaline treatments

Urea treatment remains one of the most transferable methods for dry straw in smallholder and cooperative systems. Hydrolysis releases ammonia, raises nitrogen concentration and weakens ester linkages in the cell wall. Controlled cattle studies reported higher intake, digestibility, rumen fermentation and milk output after urea treatment, with further improvements when calcium hydroxide was combined with urea (Wanapat et al., 2009; Polyorach et al., 2015). A recent study in swamp buffaloes confirmed that urea-lime treatment can improve nutrient digestibility, rumen fermentation and microbial nitrogen synthesis and can lower measured methane under the tested diet (Nguyen et al., 2025). Operationally, urea ammoniation should not be reported simply as “urea-treated straw.” A practical baseline is 40–50 g urea/kg straw DM, adjusted to a target moisture of about 400–500 g/kg fresh material, followed by uniform spraying, compaction, airtight sealing and sufficient curing before aeration and feeding. For urea-Ca(OH)2 combinations, the evidence base supports reporting the exact urea and Ca(OH)2 doses separately; commonly tested starting points are approximately 20–30 g urea plus 20–30 g Ca(OH)2/kg straw DM, with the final ratio adjusted to straw moisture, treatment temperature, residual ammonia and palatability rather than assumed to be universally optimal. Treatment temperature and season should also be disclosed. In hot, humid subtropical conditions, urease hydrolysis and ammoniation reactions can proceed rapidly, but the same climate increases the risk of secondary aerobic heating, localized moulding and ammonia loss when sealing is imperfect. Low-temperature curing requires a longer sealed period, whereas high-temperature curing requires stricter compaction, plastic integrity, opening-time aeration and residual-ammonia/palatability checks before feeding.

These benefits are conditional on accurate dosing, adequate moisture, uniform application, airtight storage and sufficient curing. Incomplete sealing wastes nitrogen and creates variable treatment; premature feeding or excessive residual ammonia introduces health risk. Lime and other alkalis add worker-safety, water and residue-management burdens. For sustainable production, the upstream footprint of urea and lime, storage materials and treatment loss must be counted alongside the avoided burden of burning or replacement feed. A digestibility gain alone is not a life-cycle conclusion.

4.3 Ensiling and co-ensiling

Ensiling is particularly compatible with wet harvests, but rice straw is intrinsically difficult to preserve because it contains little water-soluble carbohydrate and often carries insufficient epiphytic lactic acid bacteria. Moisture correction and a functional additive are usually required. Molasses supplies fermentable sugar; lactic acid bacteria accelerate acidification when substrate is available; cellulase can release sugars from structural carbohydrate; and urea supplies nitrogen but also increases buffering if poorly synchronized (Oladosu et al., 2016; Sarwono et al., 2022; Sun et al., 2023). For low-WSC straw, homofermentative LAB are primarily used to drive rapid lactic acid production and pH decline, whereas obligate or facultative heterofermentative LAB produce acetic acid and other antifungal products that may improve aerobic stability but can be slower to acidify. Because straw has low soluble substrate and relatively high buffering constraints when urea or high-ash residues are present, molasses or another soluble carbohydrate source should be specified; 30–50 g molasses/kg DM is a practical supplementation range for improving fermentation reliability. The additive should be chosen for the limiting process, rather than treated as a universal package.

Co-ensiling can solve complementary residue problems. Abo-Donia et al. co-ensiled rice straw with whole sugar beet and reported improved fibre digestibility, silage intake, milk production and feed conversion in lactating cows under their formulation (Abo-Donia et al., 2024). Lu et al. showed that Chinese cabbage waste–rice straw mixed silage complied with the tested Chinese feed-safety limits and did not impair growth or nutrient digestibility in Hu sheep (Lu et al., 2024). Importantly, the latter study did not demonstrate that ensiling destroys metals; it demonstrated batch-specific compliance after testing. Wet vegetable and processing residues may add sugars, moisture or protein, but they can also add nitrate, pesticide or microbial risk. Complementarity and traceability must therefore be assessed together.

Recent mixed-silage studies support a similar conditional interpretation. Additives altered the chemical composition, aerobic stability and in vitro digestibility of Pennisetum giganteum–rice straw silage (Ma et al., 2024), and a subsequent lamb study found that the mixed silage could replace maize silage under the tested ration, but performance depended on inclusion and diet balance (Feng et al., 2026). Haylage additives also changed fermentation, rumen parameters and measured methane in Hu sheep (Deng et al., 2025). These experiments expand the evidence base, yet they do not define a universal optimal additive or inclusion rate. No reliable literature supports a single rice-straw inclusion ceiling across species, physiological stages and ration formulations.

4.4 Fermented, biological and enzyme-assisted upgrading

A 2026 controlled feeding study compared rice straw ensiled with the rumen-derived yeast Pichia kudriavzevii KKU20 with rice straw ensiled with Saccharomyces cerevisiae in Thai native bulls. Intake, digestibility, energy and nitrogen efficiency, and growth were broadly comparable between inoculants (Cherdthong et al., 2026). This supports P. kudriavzevii KKU20 as a potential substitute for a commercial yeast under the tested conditions. However, biological ensiling studies should disclose viable inoculation load, carrier, application volume, ensiling temperature and time; for yeast or LAB inoculants, 10⁵–10⁶ CFU/g fresh material is a commonly used practical order of magnitude, but strain- and substrate-specific viability should be measured rather than assumed. The potential advantage of P. kudriavzevii in hot, humid climates should be framed around its thermotolerance and survival during fermentation, not as a generic probiotic effect. The absence of an untreated or conventionally ensiled control means that the experiment does not establish the net benefit of yeast ensiling itself.

For enzyme-based additives, activity should not be reported merely as U/g. Cellulase should be anchored to a named reference assay, preferably IUPAC filter-paper activity (FPA) and/or carboxymethylcellulase (CMCase), with substrate, buffer, pH, incubation temperature, incubation time, reducing-sugar calibration and the definition of one activity unit specified. For example, FPA is commonly benchmarked with filter paper in sodium-citrate buffer around pH 4.8 at 50 °C for 60 min, whereas CMCase and xylanase values require the exact CMC or xylan substrate and assay pH/temperature to be stated (Ghose, 1987; Bailey et al., 1992).

Fermentation with microbial consortia, ligninolytic fungi, fibrolytic enzymes and accessory proteins aims to increase cell-wall accessibility under milder conditions. A diet based on rice straw co-fermented with probiotics and enzymes altered the rumen bacterial community and metabolites of beef cattle compared with a fresh-corn-stover diet, while dry-matter intake remained similar (Hu et al., 2020). The operational description of that treatment is important: Hu et al. used Bacillus subtilis and Enterococcus faecalis with declared viable counts of 1.0 × 10¹¹ CFU/g, cellulase activity of 1.0 × 10⁴ U/g and xylanase activity of 5.0 × 10⁴ U/g in a 1:1:10:30 starter ratio, followed by 14 d fermentation (Hu et al., 2020). This level of disclosure anchors biological upgrading in microbial dose and enzyme kinetics rather than in a vague claim of “probiotics plus enzymes”. The study supports biological functionality but did not directly measure enteric methane; methane reduction should therefore not be inferred as an observed outcome.

Mechanistic studies illustrate both promise and scale limitations. A lysing polysaccharide monooxygenase from Aspergillus niger increased in vitro digestibility and promoted fibre-associated rumen fermentation, but the improvement was modest and remains to be validated in long-duration feeding systems (Li et al., 2024). Rumen-derived expansin-like proteins enhanced enzymatic deconstruction of rice straw in biochemical assays, but farm-scale efficacy, production cost and storage stability remain unresolved (Dai et al., 2025). LPMO-assisted upgrading requires an additional level of biochemical disclosure because these copper-dependent oxidative enzymes are not simple hydrolases: activity depends on oxygen or hydrogen peroxide control and an appropriate electron donor/reductant, commonly ascorbate in assay systems, and uncontrolled peroxide can inactivate enzymes or damage substrates (Vaaje-Kolstad et al., 2010; Bissaro et al., 2017). Expansin-like proteins should likewise be reported as non-catalytic accessory proteins with dose, binding substrate, partner enzyme cocktail and synergy criterion rather than as stand-alone fibrolytic enzymes. Biological upgrading should thus be described as an emerging adjunct, not a substitute for reliable harvest, storage and ration management (Table 4).

Table 4. Critical comparison of rice-straw processing and conservation strategies for ruminant feeding.

4.5 Implementation evidence and reproducibility

Technical efficacy and implementation are not equivalent. The mapped in vivo studies rarely report labour, processing failure, storage loss or farm economics (Table 2), while broader crop-residue adoption research identifies labour demand, storage access, equipment, skills and stable markets as recurrent constraints (Baba et al., 2019; Balehegn et al., 2022; Pawera et al., 2024). Recent feeding evidence also shows why implementation must retain diet-level benchmarks: in Hu lambs, fermented rice straw at 5% of diet DM (50% replacement of a 10% peanut-vine component) maintained intake and digestibility and improved selected rumen fermentation traits, whereas 10% diet DM (100% replacement) reduced digestibility and worsened feed conversion (Wang et al., 2025). Thus, post-implementation benefit should be reported as both process reliability and animal response, not as treatment adoption alone.

5 Processed rice straw in complete ruminant diets

5.1 Complete-diet formulation determines feeding response and usable inclusion

Processed rice straw is not a complete feed. Even after ammoniation or ensiling, metabolizable energy, true protein, minerals and vitamins may remain inadequate for growing, finishing or lactating animals. Rice straw can provide physically effective fibre and part of maintenance energy, but productive performance depends on synchronization of fermentable energy, rumen-degradable protein (RDP), rumen-undegradable protein (RUP), physically effective fibre and supplemental true protein. Therefore, processed straw should be evaluated within a complete diet, not as an isolated treated material.

Recent studies reinforce this ration-level view. Indigofera leaf protein concentrate improved fermentation and in vitro digestibility of rice-straw basal diets but still required in vivo validation (Puastuti et al., 2024). Mixed grass–legume silages improved growth and feed conversion when supplementing a urea-treated rice-straw basal diet in Vietnamese Yellow calves (Nguyen et al., 2024), and black cumin meal altered performance and digestibility in lambs fed rice-straw-based diets (Ramdani et al., 2024). Most directly, a 2025 Hu-lamb trial tested fermented rice straw at 5 and 10% of diet DM: 5% maintained growth and digestibility, whereas 10% reduced digestibility and worsened feed conversion (Wang et al., 2025). These data illustrate why the relevant question is not a universal straw percentage but the response of a defined animal to a defined complete ration.

Ration-level nitrogen synchronization also affects the environmental meaning of straw feeding. In an 11-week trial with growing Hu sheep fed pelleted total mixed rations containing rice and wheat straw, reducing rumen-degradable protein lowered urinary nitrogen output, but rumen fermentation, fibre digestibility and nitrogen utilization were optimized near 10% dietary rumen-degradable protein (Zang et al., 2026). This result reinforces that circularity cannot be assessed by maximizing residue inclusion or minimizing protein input in isolation. When straw inclusion is increased, RDP must remain sufficient for fibrolytic microbial growth, while RUP and true protein must support small-intestinal amino acid supply; otherwise, excess ruminal ammonia is shifted to urinary nitrogen or protein becomes limiting for production. Nutrient balance, animal response and excreted nitrogen must therefore be evaluated together.

5.2 Animal category, inclusion ceiling and evidence ladder

The appropriate inclusion rate is animal- and ration-specific. Mature maintenance animals generally tolerate lower diet digestibility better than fast-growing calves or high-yielding dairy cows, but the literature does not support a universal rice-straw ceiling across species and production stages. Practical recommendations should therefore be derived from the tested complete diet and reported against standardized fibre metrics, including ash-corrected amylase-treated NDF (aNDFom) and, where physically effective fibre is discussed, a validated particle-size method (Mertens, 1997; National Academies of Sciences, Engineering, and Medicine, 2021; Mertens, 2002; Lammers et al., 1996). The objective is to identify an inclusion level that maintains DMI, rumen function, health and productive output while meeting energy, RDP/RUP, mineral and physically effective fibre requirements.

Evidence should be separated by scale. In vitro gas or digestibility studies are useful for screening, but they cannot measure voluntary intake, sorting, adaptation, morbidity, carcass traits, milk yield or farm reliability. In situ degradation and short-term animal trials strengthen biological plausibility. Long-duration controlled trials support performance claims, and multi-farm implementation is needed for adoption and economic claims. This hierarchy is summarized in Table 5.

Table 5. Evidence ladder for translating rice-straw upgrading into practice.

To capture current methodological reporting standards, Table 1 focuses primarily on recent 2024–2026 target cases; the 2023 methane study is retained because it provides direct animal-level methane and energy measurements. Foundational pre-2024 studies establish biological plausibility and are discussed elsewhere, but they were not used to dilute the assessment of current reporting gaps.

5.3 Evidence coverage and reporting gaps in recent in vivo studies

Across the 10 representative target in vivo studies summarized in Table 1, all reported at least one nutritional, digestibility or rumen outcome; seven reported growth, feed conversion or milk-related output; three measured methane; and one included an explicit contaminant-safety panel. None reported farm economics, manure nutrient fate or a whole-chain life-cycle comparison against a defined alternative roughage. The predominance of Absent-to-Low domains is therefore not a claim that these outcomes are unimportant; it shows that recent rice-straw feeding experiments still prioritise biological efficacy over implementation and whole-system reporting.

6 Methane, nitrogen use and productive performance must be assessed together

For sustainable food systems, the ecological value of rice-straw feeding depends on two linked substitutions: what happens to the straw if it is not fed, and which feed it replaces. Avoiding open burning can reduce a documented pollution burden, while using human-inedible residues can reduce feed–food competition (Singh et al., 2024; McAllister et al., 2025; Mottet et al., 2017; Van Zanten et al., 2019). These benefits can nevertheless be offset if a high-fibre ration depresses productivity, increases enteric methane intensity or transfers nitrogen to urine and manure. Current methane science therefore emphasizes both efficacy and practical feasibility of dietary mitigation (Roques et al., 2024). In Holstein crossbred bulls, increasing rice-straw proportion relative to cassava was associated with greater methane and methane-energy loss under the tested diet (Binsulong et al., 2023), demonstrating that residue use alone is not a climate claim.

Treatment can change methane and nitrogen-use responses, but denominator and time horizon matter. Urea-lime treatment lowered measured methane in swamp buffaloes (Nguyen et al., 2025), and a haylage additive treatment reduced methane per unit metabolic body weight in Hu sheep (Deng et al., 2025). Neither finding establishes a universal reduction per kilogram of milk or live-weight gain. Methane mitigation should be claimed only when lower methane output or yield is accompanied by maintained or improved product output within a comparable production period. Nitrogen responses likewise require diet-level interpretation. Nitrogen retention efficiency can be expressed from standard N-balance terms as NRE (%) = [(N intake − fecal N − urinary N)/N intake] × 100; this is equivalent to retained N divided by N intake and is consistent with contemporary nitrogen-use definitions (Bezerra et al., 2026; National Academies of Sciences, Engineering, and Medicine, 2021). Poor nitrogen–energy synchronization can transfer N to urine and manure, increasing ammonia and downstream N₂O risk rather than improving whole-chain nutrient use.

7 Feed safety and traceability are non-negotiable gates

Circularity is not sustainable if it recirculates toxicants into animals, manure, soil and food. Rice straw from mining-affected catchments, wastewater-irrigated fields or designated contaminated-land areas may contain cadmium and other elements; poor storage can introduce mould and mycotoxins. Cadmium exposure can impair ruminant health and productivity and accumulates particularly in kidney, while controlled dietary exposure alters gastrointestinal microbiota and transcriptomic responses in beef cattle (Lane et al., 2015; Xu et al., 2023). Dietary cadmium exposure therefore should not be treated only as a residue-compliance issue: it can chronically perturb ruminal bacterial taxonomic structure and gastrointestinal tight-junction-related transcriptomic profiles, potentially impairing long-term nutrient transport efficiency and barrier integrity (Xu et al., 2023).

A batch-level safety gate must precede processing. The analytical panel should be based on origin and jurisdiction and can include Cd, Pb, As, Hg, pesticide residues, nitrate/nitrite and major mycotoxins. Applicable limits must correspond to the destination jurisdiction, animal species and whether the material is an ingredient or complete feed; GB 13078-2017 and Directive 2002/32/EC illustrate this jurisdiction-specific approach (State Administration for Market Regulation of the People's Republic of China, 2017; European Parliament and Council, 2002). No universal raw-roughage screening threshold is proposed here beyond applicable legal or buyer limits. Ensiling, ammoniation and enzyme treatment cannot destroy elemental contaminants. For a mixed ration expressed on a DM basis, Cd_TMR(DM) = Σ(x_i × Cd_i, DM). To convert to an as-fed concentration at any moisture fraction M, use Cd_TMR(as-fed) = Cd_TMR(DM) × (1 − M); for a 12% moisture TMR, M = 0.12 and the multiplier is 0.88. A maximum inclusion calculation must then use the relevant legal limit and concentrations of all other ingredients. Nutritional or mineral strategies may modify bioavailability but cannot replace batch exclusion and legal compliance.

Traceability should connect field origin, harvest date, treatment batch, laboratory result, storage record and recipient farm. This creates a second circularity safeguard: contaminants excluded at the feed gate are also prevented from returning to rice fields through manure. Feed safety is therefore both an animal-product requirement and a nutrient-recycling requirement.

8 Feed-technology implications: environment, safety and implementation

8.1 Functional unit and environmental interpretation

Environmental interpretation of rice-straw feeding is substitution-dependent. The strongest potential benefit occurs when safe straw that would otherwise be burned or unmanaged replaces a higher-burden roughage without reducing animal output. The benefit may shrink or reverse when displaced feed is locally low-impact, processing is intensive, transport is long, or productivity falls. Comparative rice-straw LCA shows that pathway rankings depend on avoided products and system assumptions (Soam et al., 2017), while recent circular and consequential LCA work shows that functional unit, marginal substitution and value-chain context can change conclusions (Sukuman et al., 2025; Ferdous et al., 2024; Ding et al., 2024; Firdaus et al., 2024). Table 6 summarizes the LCA evidence relevant to feed interpretation and, importantly, the variables that remain absent from direct rice-straw feeding assessments.

Table 6. Life-cycle evidence and boundary requirements relevant to evaluating rice-straw feeding.

For food-system evaluation, results should include at least one feed-service unit and one animal-product unit. One tonne of raw straw ignores digestibility, storage loss and animal output. More informative units include digestible dry matter or metabolizable energy alongside live-weight gain or fat- and protein-corrected milk. These paired units reveal burden shifting when a treatment improves feed chemistry but does not improve the service ultimately delivered by the animal.

8.2 Processing burdens and practical feed losses

The feed-processing inventory should include straw collection, particle-size reduction, water, urea or lime, inoculants or enzymes, plastic film, storage loss, spoiled batches, transport and on-farm handling. Chemical treatment can shift emissions upstream to fertilizer manufacture; ensiling can shift burden to plastic and loss management; densification can shift burden to electricity. Unprocessed rice straw typically has a very low bulk density, often about 40–60 kg/m³, so hauling untreated straw rapidly becomes transport-limited. Without secondary high-pressure densification, pelleting or compacted fermented TMR production that raises product density to more than 400 kg/m³, the economically viable collection radius in fragmented smallholder landscapes is commonly constrained to about 15–30 km. These inputs should be compared with the actual avoided roughage, not credited against an unspecified “waste” baseline. Reviews of rice-straw life-cycle studies show substantial methodological heterogeneity, especially in functional units, system boundaries, co-product treatment and transport assumptions (Firdaus et al., 2024).

No study identified in this search provides a complete comparison of processed rice straw versus a specific alternative roughage while simultaneously including treatment inputs, storage loss, animal performance, enteric methane, feed safety and manure fate. Accordingly, a universal whole-chain environmental advantage has no reliable direct literature support. Current evidence should be used to define minimum reporting requirements and experimental design rather than to claim a single global carbon-saving value.

8.3 Feed safety and contaminant control

Feed safety is central to rice-straw utilization because contaminants can affect both animal health and the acceptability of manure-derived nutrient recycling. Rice straw from mining-affected catchments, wastewater-irrigated fields or designated contaminated-land areas may contain cadmium and other elements; poor storage can introduce mould and mycotoxins. Cadmium exposure can impair ruminant health and productivity and accumulates particularly in kidney, while controlled dietary exposure alters gastrointestinal microbiota and transcriptomic responses in beef cattle (Lane et al., 2015; Xu et al., 2023). Regulatory interpretation should be explicit rather than implicit: GB 13078-2017 should be used for Chinese feed products, Directive 2002/32/EC for EU-bound feed chains, and any lower local or buyer-specific threshold should be specified (State Administration for Market Regulation of the People's Republic of China, 2017; European Parliament and Council, 2002). All screening and blending decisions should therefore be based on jurisdiction-specific legal limits and batch-tested concentrations rather than a universal raw-roughage trigger.

A batch-level safety gate should precede processing. The analytical panel should be based on origin and jurisdiction and may include Cd, Pb, As, Hg, pesticide residues, nitrate/nitrite and major mycotoxins. Applicable limits must correspond to the destination jurisdiction, intended animal species and whether the material is an ingredient or complete feed. Ensiling, ammoniation and enzyme treatment cannot destroy elemental contaminants. The mixed-silage study by Lu et al. (2024) is useful because it measured compliance; it should not be interpreted as proof that fermentation detoxifies metals. No reliable evidence supports a universal increase or decrease in Cd bioavailability across the complete straw-animal-manure-soil pathway. Studies should therefore report both total contaminant concentration and, where relevant, bioaccessible or bioavailable fractions, while recognizing that legal compliance is normally based on total concentration. For mixed rations, the lot-by-lot calculation described above should be retained in quality records together with the raw straw certificate, final TMR inclusion rate and final Cd_TMR (12% moisture) value.

8.4 Farm implementation and adoption constraints

Farm-level feasibility depends on collection radius, labour, storage, treatment reliability, feed price and the value of displaced roughage. Urea treatment can fit smallholder systems when dosing and airtight storage are reliable; commercial pellets or fermented total mixed rations require higher throughput, quality assurance and stable demand. Low bulk density, seasonal labour peaks and fragmented fields impose an economic collection radius before any biological treatment begins. Labour should be valued at seasonal opportunity cost, not treated as free family input. Evidence from crop-residue feeding systems indicates that higher labour requirements and lack of straw storage can reduce adoption (Baba et al., 2019), while forage-conservation literature identifies labour, equipment, skills, infrastructure and dry-season timing as persistent constraints (Balehegn et al., 2022).

Implementation should therefore be evaluated as part of feed technology. A process that relies on complex chemical handling without extension support may fail despite good laboratory performance. Stable local demand is required; otherwise, treatment converts a field residue into an unsold feed product. In the absence of long-term machinery, testing and processing subsidies or cooperative service platforms, capital expenditure (CapEx) transferred entirely to smallholders becomes a primary driver of adoption regression for technically valid biological or chemical treatments. Farmer participation should begin during technology design so that workload, storage practice, risk tolerance and willingness to pay shape the intervention (Pawera et al., 2024).

For feed-technology scale-up, shared testing services, extension-backed standard operating procedures, batch certification and stable procurement arrangements can lower transaction costs while preserving quality assurance. Subsidizing equipment without quality assurance or local offtake can create stranded assets and unsold treated straw (Table 7).

Table 7. Minimum reporting checklist for rice-straw feeding studies.

9 Feeding decision framework for processed rice straw

The evidence supports a sequential, gate-kept feeding decision framework (Figure 1) rather than a universal treatment ranking. Step 1 qualifies the batch by establishing origin, feed value, deterioration status and legal safety. Failure at this stage redirects the batch to a controlled non-feed destination. Step 2 matches processing to the dominant constraint and verifies post-treatment quality before the material is incorporated into a complete, animal-specific ration. Step 3 validates DMI, digestibility, rumen function, productive output, methane, nitrogen use and safety against a relevant benchmark. Failure of performance or environmental targets triggers reformulation or a change in processing rather than automatic scale-up. Compared with conventional treatment-by-treatment approaches, this framework makes the stopping rules explicit; its present limitation is the scarcity of comparable farm-economic and whole-chain LCA evidence.

10 Discussion and research priorities

First, rice-straw feeding trials should report the full treatment chain: field origin, raw composition, moisture, particle size, treatment dose, curing or fermentation profile, storage loss, post-treatment composition, ration composition, animal category and adaptation. Without these variables, transferability cannot be judged.

Second, new technologies should be tested against a relevant low-cost benchmark such as urea treatment or conventional ensiling. Novel enzymes, engineered enzymes, microbial consortia or fungal treatments require progression from in vitro screening to in situ degradation, controlled animal trials and multi-farm validation. They should be compared not only with untreated straw but also with a low-cost industry-standard urea-treated control so that any added biological sophistication is justified by measurable improvements in feed value, safety, animal response or economic return. A statistically significant laboratory effect is not sufficient evidence of a scalable feeding system.

Third, animal studies should measure both productivity and emissions. At minimum, long-duration trials should report intake, digestibility, average daily gain or milk yield, feed conversion, health and product quality. Climate studies should report methane in absolute, intake-based and product-based terms. This would resolve whether treatment reduces methane or merely changes the denominator.

Fourth, environmental studies should compare defined diets and relevant feed benchmarks using comparable functional units. Sensitivity analysis should include collection radius, treatment failure, storage loss, chemical and plastic inputs, displaced-feed price and footprint, animal response, methane and nitrogen excretion. Environmental interpretation should remain directly connected to feed inputs and animal responses.

Fifth, traceable supply chains and rapid testing are needed in humid or contaminant-risk production regions. Studies should distinguish total contaminant concentration from bioavailable fractions and avoid assuming that processing destroys elemental contaminants. Sixth, multi-omics can help link fibre-degrading taxa, enzymes, VFA and hydrogen fluxes to treatment response, but should be coupled to animal performance rather than used as a substitute for it. Seventh, implementation studies should quantify labour hours, seasonal opportunity cost, storage failure, quality assurance and market access. These priorities connect feed technology with agroecological resource use, climate impact and resilient crop–livestock integration, which is the food-system scale at which residue feeding should ultimately be judged.

11 Conclusion

Rice straw can be a functional partial roughage for ruminants, but its feed value is conditional. Physical processing improves handling and mixing; urea and alkaline treatments improve nitrogen supply and fibre accessibility; ensiling and co-ensiling match wet harvest conditions; and biological or enzyme-assisted technologies offer mechanistic promise but require stronger feeding validation. The strongest evidence supports complete diets in which processed straw supplies only part of the roughage and is balanced with fermentable energy, RDP/RUP supply, true protein, minerals and higher-quality forage. Accordingly, rice straw should be positioned as a certified, standardized and nutritionally synchronized roughage component, not as a simple volumetric replacement for higher-quality forage.

For sustainable food systems, the decisive safeguards are feed-quality characterization, complete-diet evaluation, animal-level validation, quantified processing conditions, logistics accounting and safety control. The descriptive evidence map (Table 2; Figure 2) is a reporting-coverage matrix of recent in vivo target cases, not an effect-size synthesis: it shows that nutrition and production are commonly measured, whereas economics, manure fate and whole-chain LCA remain largely disconnected from feeding trials. Accordingly, maximizing the volumetric substitution of high-quality forage with raw or poorly processed rice straw is not a defensible sustainability objective. The more robust paradigm is conditional use of certified, appropriately upgraded rice straw as a functional roughage component in nutritionally synchronized complete diets, with methane, nitrogen use, feed safety and implementation measured alongside animal performance.

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Footnotes

Edited by: Teresa Elizabeth Leslie, University of Vermont Extension, United States

Reviewed by: Xin Quan, Hainan University, China

Fawzy Mohamed Abo-Donia, Animal Production Research Institute (APRI), Egypt