Livestock Research

1. Introduction

Calcium is, in one sense, a deceptively familiar mineral — ubiquitous in the geological record, easy to measure in a blood sample, and routinely supplemented in livestock diets the world over. But for a dairy cow in the hours surrounding calving, the body’s capacity to mobilize calcium becomes one of the most consequential physiological tests in all of domestic animal production. At the moment of parturition, calcium demand surges three- to four-fold almost overnight as colostrum synthesis begins (Goff, 2008; Horst et al., 1997). The extracellular fluid holds only about 4–5 g of calcium at any given moment — barely enough to sustain colostrum production for thirty minutes — and everything beyond that instant must be drawn from intestinal absorption and skeletal resorption, mobilized by a tightly coordinated hormonal response involving parathyroid hormone, calcitriol, and the upregulation of gut calcium transporters (Goff & Horst, 1997; DeGaris & Lean, 2008; NASEM, 2021). When that response is delayed or overwhelmed, the result is hypocalcaemia: a spectrum running from barely detectable subclinical blood-calcium depression to the classic recumbent “milk fever” presentation that most practicing veterinarians have encountered at least once.

The global burden of this disorder is, frankly, larger than casual observation of clinical cases might suggest. Subclinical hypocalcaemia — defined as blood calcium below 8.0 mg/dL in the absence of clinical signs — is far more prevalent than overt milk fever, and arguably more costly in aggregate. Studies of multiparous dairy cows in North American and European herds have found that 41–54% are subclinically hypocalcaemic in the immediate post-calving period (Reinhardt et al., 2011). Even without progressing to recumbency, these animals sustain measurable losses in milk yield, reproductive efficiency, and immune competence: Martinez et al. (2016) demonstrated experimentally that induced subclinical hypocalcaemia impairs neutrophil oxidative burst capacity — directly compromising the immune defences that protect against early-lactation mastitis. Compounded across thousands of cows in large herds, or across millions of animals in a national dairy sector, these subclinical costs are substantial (Lean et al., 2006).

What makes the Bangladeshi context particularly worth examining is that it layers two additional, structural constraints on top of this universal physiological challenge. The first is dietary. The great majority of Bangladesh’s estimated 1.4 million dairy farms — most keeping one to three cross-bred Holstein-Friesian × indigenous Zebu cows — feed their animals predominantly on rice straw and crop residues as the primary roughage (Roy et al., 2012; Khan et al., 2009). Rice straw supplies calcium at roughly 0.3% of dry matter (Sath et al., 2013), against a dietary requirement for a lactating cow that easily exceeds 0.6% of dry-matter intake, and can approach 1.0% at peak yield (NASEM, 2021). A national survey of 220 lactating cross-bred dairy cows in Bangladesh documented a subclinical hypocalcaemia prevalence of 30.0% — roughly ten times the clinical milk fever rate in the same sample — with rates highest in higher-parity, higher-yielding cows (Samad, 2020). As genetic improvement programs continue raising average milk yield, this gap between what straw-based rations supply and what the lactating cow physiologically requires will only widen.

The second constraint is structural import dependence. Where Bangladeshi dairy farmers do supplement calcium, the conventional sources — limestone, oyster shell, and dicalcium phosphate — are overwhelmingly imported, leaving smallholder producers exposed to global commodity-price movements, shipping costs, and customs-valuation rulings (LightCastle Partners, 2024b; Roy et al., 2012). A dispute over limestone import valuation that temporarily disrupted trade through several land ports illustrated, perhaps more vividly than any economic model could, the vulnerability of a smallholder nutrition strategy that depends on an unbroken international supply chain.

Against this backdrop, it is difficult not to notice what is happening, largely unobserved and unleveraged, in another wing of the same national livestock economy. Bangladesh’s poultry-hatchery sector — supplying an estimated 20 million broiler day-old chicks, 1.4–1.5 million layer chicks, and 3.0–3.5 million Sonali chicks per week (The Business Standard, 2025) — generates eggshell waste at a scale that the calculation developed in this review estimates at 8,000–9,500 tonnes per year. Eggshell is, at its chemical core, calcium carbonate: effectively the same active ingredient as mined limestone, reinforced by an organic protein matrix containing collagen-like proteins and glycosaminoglycans that may, if anything, improve palatability and intestinal calcium transport relative to the mineral alone (Hincke et al., 2012; Nakano et al., 2003). That waste stream is currently dumped, composted, or incinerated; almost none of it re-enters the agricultural input chain in a form that Bangladeshi dairy farmers could use (Rahman et al., 2022).

This review asks, with what we hope is appropriate caution, whether it should. Three objectives guide the inquiry: (i) to quantify, as far as available data permit, the scale and current management of hatchery eggshell waste in Bangladesh; (ii) to synthesize international and regional evidence on the nutritional value, bioavailability, processing requirements, and safety of eggshell-derived calcium; and (iii) to translate that evidence into a structured, honest assessment of the prospects, constraints, and most urgent research priorities for using hatchery eggshell waste as a calcium supplement in Bangladeshi dairy cattle farming. We are not arguing that the case is already proven — it is not. We are arguing that it is worth taking seriously, and that the research needed to resolve it is achievable.

2. Review Design and Literature Search Strategy

2.1 Study Design

This is a narrative evidence-synthesis review. It was not registered prospectively with PROSPERO or any equivalent registry, which is acknowledged as a limitation. A narrative rather than systematic design was chosen deliberately: the primary literature directly relevant to ruminant use of hatchery eggshell calcium in South Asia is currently too sparse and heterogeneous to permit a systematic review with meaningful statistical pooling. A narrative framework allows integration of eggshell-composition studies, controlled feeding trials across multiple livestock species, valorization and processing literature, bioavailability data from human and monogastric research (where ruminant data are absent), and Bangladesh-specific sector analyses within a single coherent assessment. Where systematic reviews or meta-analyses exist for specific sub-questions — notably, the relationship between dietary cation-anion difference and milk fever incidence (Lean et al., 2006) — these are cited in preference to their constituent primary studies. Reporting follows applicable narrative-review guidance (Green et al., 2006).

2.2 Information Sources and Search Dates

Peer-reviewed literature was retrieved through systematic searching of five electronic databases: PubMed/PMC (U.S. National Library of Medicine), ScienceDirect (Elsevier), Scopus (Elsevier), Wiley Online Library, and Frontiers in Life Sciences (Frontiers Media). Searches were conducted between January 2025 and June 2025 with no formal lower date limit applied, given that foundational eggshell-composition studies (e.g., Nakano et al., 2003; Hincke et al., 2012) and seminal dairy-calcium physiology papers (e.g., Goff & Horst, 1997; Goff, 2008) pre-date any plausible lower cutoff. Reference lists of all retrieved articles were hand-searched for additional relevant citations. Forward citation tracking was conducted for key papers (Goff, 2008; Reinhardt et al., 2011; Ako et al., 2024; Jahan et al., 2024) using Google Scholar.

2.3 Search Strategy and Terms

Search queries combined the following terms using Boolean AND/OR operators across title, abstract, and keyword fields:

Population/intervention: “eggshell waste” OR “eggshell meal” OR “eggshell powder” OR “eggshell calcium” OR “hatchery by-product”

Target species: “dairy cattle” OR “dairy cow” OR “ruminant” OR “bovine” OR “laying hen” OR “broiler” OR “turkey”

Outcome/comparator: “calcium supplement” OR “calcium carbonate” OR “mineral supplement” OR “calcium bioavailability” OR “calcium absorption”

Condition: “milk fever” OR “hypocalcaemia” OR “subclinical hypocalcaemia” OR “periparturient”

Context: “Bangladesh livestock” OR “smallholder dairy” OR “eggshell processing” OR “circular economy” OR “waste valorization”

2.4 Inclusion and Exclusion Criteria

Articles were considered eligible if they reported on: (a) the chemical composition, mineral content, or structural characteristics of avian eggshell; (b) controlled feeding trials comparing eggshell-derived calcium against conventional mineral sources in any livestock species; (c) calcium bioavailability from carbonate sources in ruminants or monogastrics relevant by analogy; (d) eggshell processing methods and associated microbiological or chemical safety data; (e) the pathophysiology, epidemiology, or economic consequences of hypocalcaemia in dairy cattle; (f) dietary calcium requirements in dairy cattle across the production cycle; or (g) the structure, scale, or waste-management practices of Bangladesh’s poultry or dairy sectors.

Articles were excluded if they: were available only as conference abstracts without a corresponding peer-reviewed full paper; reported compositional data from egg products other than eggshell (e.g., egg white, yolk, or whole egg); were restricted to human nutraceutical applications of eggshell membrane with no livestock nutritional relevance; or were not retrievable in full text through institutional or open-access channels.

2.5 Grey Literature Handling

Grey-literature sources — including sector reports from LightCastle Partners, government statistical yearbooks (BBS, 2023; DLS, 2022), international organizational publications (FAO, 2022; FAO, 2023; World Bank, 2022), and poultry-industry news sources (The Business Standard, 2025) — were used exclusively to characterize the scale and structure of the Bangladeshi poultry and dairy industries, where peer-reviewed national statistics were unavailable. All grey-literature citations are flagged explicitly in the text. Where a grey-literature claim is central to the argument, corroboration from at least one additional source is provided.

2.6 Hatchery Waste-Tonnage Estimation

No nationally measured estimate of Bangladeshi hatchery eggshell-waste tonnage was identified in the literature — a gap explicitly noted by the only Bangladeshi study to process hatchery eggshell for feed use (Jahan et al., 2024). An order-of-magnitude estimate was constructed using a bottom-up calculation from reported day-old chick production volumes. All assumptions, input values, and the full lower-to-upper-bound sensitivity range of the estimate are reported in Section 3.2. This figure should be treated as a planning estimate pending primary survey data collected directly from commercial hatcheries.

2.7 Data Extraction and Evidence Synthesis

For each controlled feeding trial, the following data were extracted where reported: species; experimental design (sample size, number of treatment groups, group size, trial duration); calcium sources compared; inclusion levels; and quantitative outcomes. Extracted data were summarized in Table 2. No formal risk-of-bias assessment tool was applied — the feeding-trial literature in this area is small enough that each eligible study is described individually — but limitations of individual studies (particularly sample size and generalizability) are noted explicitly in the text. No meta-analysis was performed given the heterogeneity in species, designs, and outcome measures across trials.

3. Results and Discussion

3.1 Bangladesh’s Poultry-Hatchery Sector and the Scale of Eggshell Waste Generation

3.1.1 Structure and Recent Trajectory of the Hatchery Sub-Sector

Bangladesh’s poultry industry has undergone a striking structural transformation over the past two decades, evolving from a predominantly backyard enterprise into a vertically integrated commercial sector organized around approximately 100 licensed commercial hatcheries, roughly 200 breeder farms, and tens of thousands of downstream grow-out operations supplying broiler, layer, and Sonali (cross-bred) products to domestic markets (One Health Poultry Hub, 2023; LightCastle Partners, 2024a). National egg production is estimated at approximately 23 billion units per year, with poultry-meat output near 1.46 million tonnes and the sector supporting approximately 8 million workers across the value chain (LightCastle Partners, 2024a; FAO, 2022). These figures are impressive by regional standards, and they translate directly into a waste-generation profile of proportionate scale.

Within this system, hatcheries occupy a distinct position. They receive fertile hatching eggs from breeder farms, incubate them for approximately 21 days, and deliver the resulting day-old chicks to grow-out operators — leaving behind a residual waste stream of hatched eggshells, infertile eggs detected during candling, dead-in-shell embryos, and culled chicks (King’ori, 2011; Rahman et al., 2022). It is worth pausing on this list: the waste stream is not simply empty shells. Dead-in-shell embryos and infertile eggs contribute biological material — protein, lipids, and microbial load — that makes the raw waste stream microbiologically more complex than the clean hatched shell fraction alone (Rahman et al., 2022; Ahmed et al., 2021). Efficient source segregation of the shell fraction from this broader waste matrix is therefore not merely a logistical convenience but a safety prerequisite, as discussed further in Section 3.6.

As of mid-2025, industry sources reported weekly broiler day-old chick production of approximately 20 million birds against estimated demand of 15–16 million — a chronic oversupply that has periodically depressed chick prices to below the cost of production, threatening smaller hatchery operators (The Business Standard, 2025). Weekly layer chick supply stood at approximately 1.4–1.5 million birds and Sonali chick output at 3.0–3.5 million (The Business Standard, 2025). The Department of Livestock Services (DLS, 2022) has confirmed continuous growth in registered poultry enterprises over the preceding decade, and Bangladesh Bureau of Statistics data (BBS, 2023) place the livestock sub-sector’s contribution at approximately 1.6% of national GDP, with poultry representing the fastest-growing component.

3.1.2 Estimating Annual Hatchery Eggshell-Waste Generation: A Transparent Calculation

No measured, nationally representative estimate of hatchery eggshell-waste tonnage from Bangladesh exists in the peer-reviewed literature, as Jahan et al. (2024) explicitly noted in the only Bangladeshi study to have processed hatchery eggshell for feed use. In the absence of primary survey data, an order-of-magnitude estimate was constructed as follows.

Summing the reported weekly day-old chick outputs — approximately 20 million broiler, 1.45 million layer, and 3.25 million Sonali chicks (The Business Standard, 2025) — yields a weekly national hatch of approximately 24.7 million chicks, or roughly 1.285 billion chicks per year. Applying a representative hatchery hatchability of 80–85% (King’ori, 2011) — meaning that for every chick successfully hatched, an additional 0.18–0.25 eggs are removed as infertile or dead-in-shell — places total eggs set annually at approximately 1.5–1.6 billion. With each hatching egg yielding approximately 5–6 g of dry-weight eggshell (eggshell constituting 8–10% of total egg weight; Aditya et al., 2021; Nakano et al., 2003), the resulting national hatchery eggshell stream amounts to approximately 8,000–9,500 tonnes per year (central estimate). A conservative lower-bound scenario (75% hatchability; 5 g shell weight) reduces this to approximately 5,700 tonnes; the upper bound (85% hatchability; 6 g shell weight) reaches approximately 9,600 tonnes. These figures exclude eggshell arising from table-egg processing, food service, or domestic kitchens, and should be read as a first-order planning estimate pending primary survey data from commercial hatcheries.

3.1.3 Current Management Practices: The Status Quo for Hatchery Waste

Survey-based evidence paints a familiar and somewhat dispiriting picture of poultry-waste management across Bangladesh. In a study of farms in Jhenaidah district, a majority of farm owners reported having received no formal waste-management training, and roadside dumping or open-pit disposal was the dominant practice, with only a minority composting waste or diverting it to fish ponds (Rahman et al., 2022). A national review reached similar conclusions: hatchery by-product, including eggshell, is typically applied directly to agricultural land or aquaculture ponds without pre-treatment, generating ammonia emissions, greenhouse-gas production, and documented risk of microbial and heavy-metal contamination of receiving soils and water bodies (Rahman et al., 2022). Biogas generation and centralized composting remain frequently proposed but persistently under-adopted alternatives.

The largest integrated operators are beginning to move. The 2024 commissioning of what was reported as Bangladesh’s first dedicated hatchery by-product processing line — oriented primarily toward composting and organic fertilizer production rather than feed-grade calcium recovery — represents an important structural shift, signaling that at least the logistical infrastructure for by-product aggregation is starting to take shape (LightCastle Partners, 2024c). What it does not yet represent is a valorization pathway oriented toward the calcium-deficiency gap in the dairy sector — which is precisely the gap this review is concerned with.

3.2 Composition and Nutritional Value of Eggshell as a Calcium Source

3.2.1 Structural and Chemical Characteristics

The avian eggshell is a biomineralised composite of calcite (calcium carbonate, CaCO3) deposited on an organic protein matrix and lined internally by a fibrous membrane containing collagen-like proteins, glycosaminoglycans, and antimicrobial peptides including lysozyme and ovotransferrin (Hincke et al., 2012; Gautron et al., 2001). The shell is organised into three structurally distinct layers: the inner mammillary layer, the palisade layer where the bulk of calcium carbonate deposition occurs over a 16–18-hour calcification period at a rate of approximately 2 g of CaCO3 per hour, and the outer cuticle, each with distinct protein and mineral compositions (Hincke et al., 2012; Gautron et al., 2001; Mann et al., 2006). The organic matrix constitutes approximately 2–4% of shell mass by weight but is physiologically disproportionate in its influence, directing crystal nucleation and growth during mineralisation (Mann et al., 2006).

On a dry-matter basis, whole eggshell (shell plus adherent membrane) typically comprises approximately 95% calcium carbonate, around 1% magnesium carbonate, roughly 1% calcium phosphate, and 3–4% organic matter, predominantly protein (Nakano et al., 2003; Jahan et al., 2024; Ahmed et al., 2021). This makes eggshell chemically very similar to mined limestone (ground calcite) and oyster shell (largely aragonite/calcite) — the two calcium sources it would most directly substitute in a dairy ration — while contributing a small additional fraction of crude protein and bioactive peptides that mined mineral sources do not supply (Aditya et al., 2021; Ahmed et al., 2021). Whether that protein fraction provides any meaningful nutritional benefit in a cattle ration, where it would represent a negligible contribution to overall protein intake, is debatable; its more plausible contribution may be to palatability, discussed in Section 3.2.3.

3.2.2 Comparative Calcium Content Across Conventional and Eggshell-Derived Sources

A controlled comparative analysis of calcium sources used in Bangladeshi poultry research measured eggshell collected from two points in the value chain alongside limestone and oyster shell using consistent analytical methods (Islam & Nishibori, 2021). Hatchery-extruded eggshell contained approximately 25.5% calcium on a dry-matter basis, and kitchen-extruded eggshell approximately 29.8%, compared with 37.1% for limestone and 35.2% for oyster shell from the same study (Islam & Nishibori, 2021). Feed-grade dicalcium phosphate typically supplies around 22–23% calcium alongside roughly 18% phosphorus (EFSA, 2012; Coker Feed Mill, n.d.). Eggshell therefore supplies somewhat less calcium per unit weight than limestone or oyster shell — a difference in the range of 7–12 percentage points, depending on source — but this is well within the range correctable by a modest upward adjustment of inclusion rate, which any nutritionist formulating a ration would routinely calculate (Oikeh et al., 2019; Dos Santos et al., 2019).

3.2.3 Calcium Bioavailability: Mechanisms, Evidence, and the Ruminant Knowledge Gap

Bioavailability from carbonate sources depends primarily on dissolution rate in the gastrointestinal tract — governed by gastric acid secretion, particle size, and the nature of any surrounding organic matrix (Schaafsma et al., 2002; González-Muñoz et al., 2012). In monogastrics, eggshell calcium has been shown in human supplementation trials to be equivalent or superior to synthetic calcium carbonate on markers of fractional absorption and urinary calcium excretion; this is attributed to the eggshell’s microcrystalline calcite structure and the co-presence of organic matrix peptides that may facilitate intestinal calcium transport (Schaafsma et al., 2002; González-Muñoz et al., 2012). Poultry feeding trials consistently support nutritional equivalence to limestone, a finding replicated across laying hens, broilers, and turkeys (Islam & Nishibori, 2021; Jahan et al., 2024; Swiątkiewicz et al., 2015).

In ruminants, the picture is more nuanced — and the evidence thinner. The rumen operates at a pH of approximately 5.5–7.0 under normal fermentation conditions, which substantially affects CaCO3 solubility before absorption occurs predominantly in the small intestine. At rumen pH 6.5 (typical of a roughage-fed cow), carbonate solubility is orders of magnitude lower than at the abomasal pH of 2.0–3.5, meaning that much of the calcium released from CaCO3 within the rumen is subsequently exposed to the highly acidic abomasal environment that drives final ionization and absorption (Goff & Horst, 1997; Prasad et al., 2020; Braun et al., 2020). Particle size modifies this pathway substantially: finer CaCO3 particles present greater surface area per unit mass and dissolve more rapidly under acidic conditions, leading to higher fractional absorption in high-demand physiological states (Ahmed et al., 2021; Nada et al., 2025). Ball-milling of eggshell powder to micron or nano-scale particles has been shown in vitro to increase calcium release rate dramatically relative to coarsely ground material (Baláž et al., 2021; Ahmed et al., 2021), though whether this translates into meaningfully improved ruminant bioavailability — where rumen retention time and microbial processing provide extensive mechanical comminution already — remains to be established in controlled trials.

What is perhaps most telling is the complete absence of isotope-dilution or mineral-balance studies for eggshell calcium in cattle or other ruminants (Prasad et al., 2020). We are, in effect, extrapolating from human supplementation data, poultry feeding trials, and a single Indonesian dairy-cattle experiment to make inferences about ruminant calcium bioavailability from a source that has not been mechanistically characterized in that species. This is not an argument against using the available evidence — it is the best we have — but it is a clear signal that ruminant-specific bioavailability data should be among the first measurements taken in any future Bangladesh-specific feeding trial.

3.3 Calcium Nutrition and the Current Situation in Bangladesh’s Dairy Cattle Sector

3.3.1 Status and Structure of the Dairy Sector

Bangladesh’s dairy sector has expanded considerably over the past decade. Recent estimates place national milk production at approximately 10.8 million tonnes in 2024, positioning the country fifth in Asia and nineteenth globally by volume (IFCN Dairy, 2025). Yet this aggregate output is generated by an overwhelmingly smallholder sector: an estimated 1.4 million dairy farms, most with one to three cows per farm (Roy et al., 2012; World Bank, 2022). The dominant genotype is a cross of Holstein-Friesian or Sahiwal bulls with indigenous Zebu cows — cross-bred animals that achieve substantially higher milk yields than pure indigenous cattle while also having proportionately higher mineral demands (Bhuiyan et al., 2005; BBS, 2023). As the genetic and management transition toward more productive cross-breds continues, the calcium-supply challenge this review addresses will only become more pressing.

3.3.2 Dietary Calcium Requirements Across the Production Cycle

Dietary calcium requirements for dairy cattle vary substantially with physiological stage, increasing markedly from maintenance through gestation and into peak lactation (NASEM, 2021; Horst et al., 1997). Table 1 summarises indicative requirement ranges compiled from the major reference sources; the key observation is that the gap between what rice-straw roughage provides (approximately 0.3% of dry matter; Sath et al., 2013; Khan et al., 2009) and what the lactating cow requires (0.6–1.0% of dry matter; NASEM, 2021) is substantial and physiologically consequential.

A recent meta-regression of 57 published mineral-balance trials proposed dietary calcium requirements somewhat higher than earlier NASEM (2021) values once realistic mineral absorption coefficients are incorporated (Oliveira & Soares, 2024) — a reminder that requirement estimates continue to be revised upward as methodology improves. From a practical standpoint in Bangladesh, where baseline forage calcium is reliably deficient and purchased supplement access is limited, the relevant target is not the theoretical NRC optimum but a meaningful improvement over an unimplemented zero baseline.

3.3.3 Pathophysiology of Hypocalcaemia and Its Clinical and Economic Consequences

Clinical milk fever (blood calcium < 5.0 mg/dL) and subclinical hypocalcaemia (5.0–8.0 mg/dL) share a common mechanism: the periparturient cow must mobilise 20–30 g of calcium per day into colostrum and milk from the first day of lactation, a demand that overwhelms the approximately 4–5 g of calcium present in the extracellular fluid at any moment (Goff, 2008). In cows whose homeostatic response — activation of calcitriol synthesis, upregulation of intestinal calcium transporters, and parathyroid-hormone-mediated skeletal resorption — is slow or insufficient, the resulting hypocalcaemia impairs smooth-muscle contractility (producing rumen atony and risk of displaced abomasum), neuromuscular function (causing recumbency in severe cases), and immune-cell activity (predisposing to mastitis and retained placenta) (Goff, 2008; DeGaris & Lean, 2008; Horst et al., 1997).

Subclinical hypocalcaemia, by virtue of its high prevalence relative to clinical disease, carries the greater aggregate economic burden. Martinez et al. (2016) demonstrated experimentally that induced subclinical hypocalcaemia in Holstein cows reduced neutrophil oxidative burst capacity — a direct impairment of the immune mechanisms that defend against mastitis in the early post-calving period. A meta-analysis of dietary cation-anion difference (DCAD) interventions confirmed that pre-partum DCAD manipulation significantly reduces clinical milk-fever incidence (Lean et al., 2006), though such approaches require precision feed formulation largely inaccessible to Bangladeshi smallholder farmers, reinforcing the importance of simpler post-partum calcium-supplementation strategies — strategies into which

Table 1. Indicative dietary calcium requirement ranges for dairy cattle across the production cycle. Values are representative ranges compiled from NASEM (2021), Merck Veterinary Manual (2025), and Horst et al. (1997).

Table 2. Summary of controlled feeding trials comparing eggshell-derived calcium with conventional calcium sources. Limitations of individual studies are noted explicitly to support appropriate evidence weighting.

Table 3. SWOT analysis of hatchery eggshell waste utilization as a calcium supplement in Bangladeshi dairy cattle farming, synthesizing evidence reviewed in Sections 3.1–3.8.

eggshell-derived calcium could, in principle, be integrated.

3.3.4 The Forage Calcium Gap and Prevalence of Hypocalcaemia in Bangladesh

Rice straw — the dominant roughage in Bangladeshi cattle diets — is characteristically low in both calcium and phosphorus. Regional studies from comparable South and Southeast Asian feeding systems report rice-straw calcium concentrations of approximately 0.3% of dry matter (Sath et al., 2013; Khan et al., 2009), against lactating-cow requirements of 0.6–1.0% (NASEM, 2021). A systematic survey of feeds and forages offered to dairy cows in eastern India — a feeding environment broadly analogous to rural Bangladesh — found that all locally available feed ingredients, including wheat straw, crushed maize, and wheat bran, supplied calcium below the critical 0.30% threshold (Bhanderi et al., 2016). No equivalent nationally representative forage-mineral survey has been published specifically for Bangladesh — that is a notable evidential gap, identified as a research priority in Section 3.10. But the clinical consequences of this structural calcium deficiency are directly documented in the Bangladeshi literature: Samad (2020) recorded a subclinical hypocalcaemia prevalence of 30.0% in a sample of 220 lactating cross-bred dairy cows, with concurrent hypophosphataemia and hypermagnesaemia, indicating that the underlying problem is one of broad mineral-supply inadequacy rather than a calcium deficit in isolation.

3.3.5 Current Calcium-Supplementation Practice and Import Dependence

Where Bangladeshi dairy farmers do supplement calcium, the conventional sources — limestone, oyster shell, and dicalcium phosphate — are overwhelmingly imported (LightCastle Partners, 2024b). Feed-grade limestone trades internationally at approximately USD 30–60 per tonne and dicalcium phosphate at approximately USD 300–600 per tonne depending on grade and volume (EFSA, 2012; Coker Feed Mill, n.d.), with both prices additionally subject to shipping costs and exchange-rate depreciation in the Bangladeshi import context. EFSA (2012) has reviewed and confirmed the technical acceptability of calcium compounds, including calcium carbonate and calcium phosphates, as feed additives for all animal species — a regulatory framework that Bangladesh has yet to formally adapt to domestic eggshell-derived feed-ingredient standards. The current mismatch between the domestic availability of a chemically near-equivalent calcium source and the near-total import dependence of conventional supplementation is precisely what motivates the remainder of this review.

3.4 Evidence Base: Eggshell-Derived Calcium in Livestock and Dairy Cattle Diets

The body of controlled experimental evidence on eggshell as a calcium source is considerably larger for poultry than for ruminants, but is nonetheless directly informative for the present question. The underlying chemistry of calcium dissolution and absorption from a carbonate matrix is broadly similar across species, even if the ruminant gastrointestinal environment introduces additional complexity (discussed in Section 3.2.3). Table 2 summarises the principal feeding trials identified in this review that compared eggshell-derived calcium against conventional sources under controlled conditions.

The single dairy-cattle trial (Ako et al., 2024) is, unavoidably, the pivot around which this review’s ruminant argument turns — and it deserves careful scrutiny. With n = 20 cows allocated across five treatment groups, each group comprised only four animals. That sample size provides insufficient statistical power to detect most plausible effect sizes in milk-yield or blood-calcium outcomes with the reliability expected in controlled veterinary trials (Martinez et al., 2016; Lean et al., 2006). Additionally, the study was conducted in Indonesia rather than Bangladesh, and used kitchen-sourced eggshell rather than hatchery eggshell — the former having a somewhat higher calcium content (approximately 29.8% versus 25.5% for hatchery eggshell; Islam & Nishibori, 2021). These are not reasons to dismiss the study’s findings; they are reasons to treat them as an encouraging signal that motivates hypothesis-testing rather than as sufficient evidence to establish practice. The principal finding — that total dry-matter intake was unaffected by complete eggshell-meal substitution while milk yield and milk calcium content rose progressively — is biologically plausible and internally consistent with the mechanistic picture of eggshell calcium as bioavailable and palatable. But it needs replication, in Bangladesh, at adequate scale.

The Bangladeshi turkey trial (Jahan et al., 2024) provides the most locally relevant non-ruminant evidence, and is notable for two reasons beyond its production findings. First, it is, to the authors’ knowledge, the first published Bangladeshi study to process hatchery-adjacent eggshell waste specifically for re-use as a feed-grade calcium source. Second, and practically important, it directly addressed the microbiological safety question by testing the finished eggshell powder for Salmonella spp. following a washing, boiling, and oven-drying protocol — obtaining a negative result (Jahan et al., 2024). The broader poultry literature — spanning laying hens, broilers, and turkeys across multiple countries — consistently supports nutritional equivalence of eggshell calcium to limestone, with equivalent or superior egg production, eggshell quality, and bone mineralisation outcomes at matched calcium inclusion levels (Hudson et al., 1988; Swiątkiewicz et al., 2015; Oikeh et al., 2019; Dos Santos et al., 2019).

3.5 From Waste to Feed: Processing Technologies for Eggshell-Derived Calcium Supplements

Converting raw hatchery eggshell into a feed-grade calcium supplement involves a processing sequence that is technically simple relative to manufactured mineral supplements, but that must be performed consistently to deliver a safe, stable, and nutritionally predictable product. The pathway synthesized from the valorization literature (Aditya et al., 2021; Ahmed et al., 2021; Baláž et al., 2021; Jahan et al., 2024) comprises five stages, each with practical implications for the Bangladeshi context.

Stage 1 — Source segregation: Separating the relatively clean hatched-eggshell fraction from dead-in-shell embryos, culled chicks, and litter at the point of generation. This is the logistically most demanding step in the Bangladeshi context, where co-mingled disposal is currently the norm (Rahman et al., 2022). It is also, from a safety standpoint, the most important: dead-in-shell embryonic material carries higher intrinsic microbial load — including not only Salmonella but also Campylobacter and potentially residual avian influenza or Newcastle disease virus antigen in periods of active circulation — than clean hatched shell. Segregating the shell fraction at source markedly reduces the pathogen challenge that subsequent heat treatment must overcome.

Stage 2 — Washing: Rinsing in clean water removes adherent membrane debris, visible soil, and the majority of surface microbial contamination. The Bangladeshi turkey study and the broader literature converge on thorough washing as a prerequisite for effective subsequent heat treatment (Jahan et al., 2024; Aditya et al., 2021; King’ori, 2011).

Stage 3 — Heat sterilization: Boiling for one to two hours, or autoclaving, achieves pathogen inactivation, followed by oven-drying at approximately 90–100°C until a stable, low-moisture product is obtained (Jahan et al., 2024; Aditya et al., 2021). Industrial alternatives include pasteurization tunnels or continuous rotary dryers operated at validated temperature-time combinations. EFSA (2012) emphasises that time-temperature combinations must be validated against actual pathogen load rather than assumed from general cooking guidance — a standard that any commercial-scale Bangladeshi eggshell-calcium product would need to meet. The WOAH (2021) Terrestrial Animal Health Code provides relevant biosafety guidance for the re-use of animal-origin by-products in feed, requiring processing conditions to be validated against the pathogen-risk profile of the source material.

Stage 4 — Size reduction: Dried shell is reduced to a powder using a hammer mill, ball mill, or basic grinding equipment. For practical dairy-cattle supplementation, a particle size equivalent to fine limestone (< 2 mm) is likely adequate and achievable with low-cost equipment already present at feed-mill or cooperative scale (Aditya et al., 2021). Ball milling to micron or sub-micron particles may improve in vitro calcium solubility (Baláž et al., 2021; Ahmed et al., 2021) but its practical benefit in a ruminant ration has not yet been established.

Stage 5 — Quality testing and packaging: Minimum batch-level quality-assurance requirements for a commercially marketed product should include microbiological testing (Salmonella at minimum; Listeria and coliform enumeration for commercial scale), heavy-metal analysis (cadmium, lead, arsenic, mercury), calcium assay, and moisture content determination. Internationally applicable reference limits are available from Codex Alimentarius (2019) and EFSA (2012).

3.6 Safety, Quality Assurance, and Regulatory Considerations

Because hatchery eggshell originates from a biologically active, potentially pathogen-bearing matrix, any proposal to redirect it into the livestock feed chain must address microbiological and chemical safety explicitly. This section identifies three distinct safety dimensions: microbial pathogens, dead-in-shell biological contamination, and chemical contaminants.

Microbial safety. Eggshell and its adherent membrane can carry Salmonella and other enteric organisms, with hatchery-derived material carrying a higher burden than kitchen eggshell owing to the incubation environment and the inclusion of infertile and dead-in-shell eggs in the raw waste stream (Ahmed et al., 2021; Jahan et al., 2024). The single directly relevant safety result — Salmonella-negative culture following a two-hour boiling plus 24-hour oven-drying protocol for Bangladeshi turkey-feed eggshell (Jahan et al., 2024) — is encouraging. It should not, however, be over-generalised: one negative culture from one processing batch does not establish the general safety of hatchery eggshell across the variety of Bangladeshi hatchery conditions, pathogen burdens, and seasonal variations. Routine, batch-level microbiological testing is the appropriate quality-assurance standard, consistent with EFSA (2012) guidance on calcium compound feed additives.

Dead-in-shell embryo contamination. The raw hatchery waste stream contains not just empty shells but dead embryos at various stages of development, particularly from infertile eggs detected during mid-incubation candling. This material presents a broader pathogen profile than clean shells alone: beyond Salmonella, it carries risk of Campylobacter spp., and in periods of avian influenza or Newcastle disease virus circulation, residual viral antigen may be present in embryonic tissue. Heat-treatment protocols validated for Salmonella may not provide equivalent inactivation against all relevant pathogens in heavily contaminated embryonic material. Efficient pre-processing source segregation — separating clean hatched shells from the broader organic waste fraction — is therefore not simply good practice but a biosafety requirement that should be specified in any eventual national feed-ingredient standard.

Chemical contaminants. Unlike mined limestone or oyster shell, whose contaminant profiles arise from the geological source material, eggshell calcium reflects the mineral content of the parent hen’s diet and environment. Heavy metals — lead, cadmium, arsenic, and mercury — that are present in the hen’s feed or drinking water at sub-toxic concentrations can accumulate in the calcium-depositing tissues of the oviduct and appear in the shell (Ahmed et al., 2021). Bangladeshi poultry feeds have not been systematically surveyed for heavy-metal content, and the transferability of international eggshell contaminant data to Bangladeshi hatchery conditions is uncertain. Codex Alimentarius (2019) specifies maximum levels for lead (≤ 10 mg/kg), cadmium (≤ 1 mg/kg), and arsenic (≤ 2 mg/kg) in feed ingredients — standards that a Bangladeshi framework could directly adapt as provisional targets.

Regulatory gap. At present, no Bangladesh-specific regulatory standard governs eggshell-derived material as a feed ingredient. The Department of Livestock Services (DLS, 2022) and Bangladesh Standards and Testing Institution would be the natural regulatory bodies to develop and enforce such a standard. The minimum useful standard would specify: an approved processing protocol (washing, heat-treatment time-temperature parameters, drying), permissible microbiological limits (Salmonella absent in 25 g; coliform count), heavy-metal ceilings, minimum calcium assay (e.g., ≥ 25% Ca on a dry-matter basis), and a moisture limit. Without such a standard, eggshell-derived calcium will remain an informal, variable-quality material that veterinary and feed-mill professionals are unable to confidently recommend.

3.7 Environmental and Circular-Economy Dimensions

Redirecting hatchery eggshell waste from disposal to dairy-cattle calcium supplementation would simultaneously advance several distinct sustainability objectives that align with circular-economy and waste-valorization frameworks increasingly applied to agri-food by-product use (Aditya et al., 2021; Singh et al., 2021; FAO, 2022). The most direct effect is substitution: displacing a corresponding quantity of imported mineral supplement reduces both the foreign-exchange cost borne by Bangladeshi dairy farmers and the upstream environmental footprint associated with limestone quarrying, processing, and long-distance maritime shipping. Life-cycle assessments of eggshell valorization conducted in other country contexts have demonstrated reductions in global warming potential, cumulative energy demand, and acidification potential per kilogram of calcium delivered relative to mining and processing equivalent quantities of limestone (Matilainen & Pajarre, 2021; Singh et al., 2021). While site-specific life-cycle data for Bangladesh are not available, the directional conclusion is unlikely to differ substantially.

A second effect is waste diversion. Hatchery eggshell waste is protein-rich and prone to microbial decomposition; at open-disposal sites it attracts pests, generates odour, and contributes to localized ammonia and greenhouse-gas emissions (Aditya et al., 2021; Rahman et al., 2022). The Bangladesh-specific survey evidence documents air, soil, and water-quality concerns associated with current open-disposal practices at poultry farms (Rahman et al., 2022). Diverting even a fraction of the estimated 8,000–9,500 tonnes of annual hatchery eggshell into a processed feed ingredient would reduce the environmental burden at disposal sites while simultaneously creating economic value.

More broadly, eggshell valorization would create a tangible material linkage between two of Bangladesh’s most important livestock sub-sectors — exactly the kind of circular-economy coupling that sustainable-livestock and agri-food-systems policy is increasingly designed to promote (FAO, 2022; FAO, 2023). A by-product that is a cost and a regulatory liability in the hatchery sub-sector becomes an input and an asset in the dairy sub-sector. That alignment of incentives is, in principle, exactly what is needed for a circular intervention to be adopted without mandated subsidies.

3.8 Economic Feasibility: A Necessarily Qualitative Assessment

A rigorous, Bangladesh-specific cost-benefit analysis of eggshell-derived calcium has not been published and cannot be constructed reliably from currently available data. The following discussion identifies the main cost and benefit categories that such an analysis would need to quantify, and offers qualitative reasoning about likely magnitudes.

On the benefit side, feed-grade limestone is internationally traded at approximately USD 30–60 per tonne and dicalcium phosphate at approximately USD 300–600 per tonne (Coker Feed Mill, n.d.; LightCastle Partners, 2024b). If hatchery eggshell-derived calcium can be collected, processed, and delivered to a feed mill or dairy farm at a total landed cost below that of imported limestone, substitution would be economically attractive on a per-unit-calcium basis even before accounting for avoided waste-disposal costs. Given that the raw material is, at present, a net disposal liability rather than a feedstock with positive value, that threshold is plausibly within reach.

On the cost side, the inputs to consider are: collection and logistics (aggregating dispersed small hatchery waste streams); processing (washing, heat treatment, drying, grinding, and quality testing); packaging and distribution to dairy farms, most plausibly via existing dairy cooperatives, feed dealers, or dairy hub networks. Roy et al. (2012) documented that Bangladeshi smallholder dairy farmers are highly price-sensitive with respect to purchased feed inputs, and that even modest cost reductions in mineral supplementation would meaningfully affect household profitability at the one-to-three-cow scale that characterises the sector. For larger, vertically integrated poultry companies already investing in hatchery by-product processing infrastructure (LightCastle Partners, 2024c), the incremental capital cost of orienting part of that infrastructure toward feed-grade calcium production may be relatively modest.

Whether this cost structure ultimately favours eggshell calcium over imported alternatives depends critically on collection logistics and processing scale — factors that can only be resolved through pilot-scale implementation and formal techno-economic assessment. The current absence of any such assessment is identified as a priority research gap in Section 3.9.

3.9 SWOT Analysis: A Structured Feasibility Assessment

Table 3 synthesises the evidence reviewed above into a structured strengths-weaknesses-opportunities-threats (SWOT) assessment of using eggshell hatchery waste as a calcium supplement for Bangladeshi dairy cattle.

3.10 Future Research Priorities and Policy Recommendations

Translating the international evidence reviewed here into a validated practice for Bangladeshi dairy farming requires a focused set of research and policy actions, presented below in approximate order of scientific priority.

3.10.1 A Well-Powered Bangladeshi Dairy-Cattle Feeding Trial

No published trial has tested hatchery-sourced eggshell calcium in lactating or growing dairy cattle under Bangladeshi feeding conditions. This is the single most important gap. A randomized controlled trial modelled on the Indonesian design (Ako et al., 2024) but using hatchery-sourced Bangladeshi eggshell and local cross-bred cows fed rice-straw-based roughage, with milk yield, milk calcium, blood calcium, reproductive indicators, and dry-matter intake as primary outcomes, should be the first priority. Statistical power calculations based on expected effect sizes from Martinez et al. (2016) and Lean et al. (2006) suggest that a minimum of 30–40 cows per treatment group — substantially more than the 4 cows per group in the Indonesian trial — would be required to detect plausible treatment differences with adequate confidence (0.80 power at α = 0.05) in milk-yield outcomes of a magnitude consistent with the correction of subclinical hypocalcaemia.

3.10.2 National Quantification of Hatchery Eggshell-Waste Generation

The order-of-magnitude estimate developed in Section 3.1.2 should be replaced with primary data collected from a nationally representative sample of commercial hatcheries on hatching-egg throughput, hatchability rates, shell-waste mass, and current disposal practices. The DLS annual livestock census (DLS, 2022) provides a logical institutional home for this variable as a routine reporting item.

3.10.3 Systematic Microbiological and Heavy-Metal Safety Characterization

Systematic testing of processed eggshell powder from a representative sample of Bangladeshi hatcheries — covering Salmonella, Campylobacter, Listeria, coliforms, and heavy metals (Pb, Cd, As, Hg) across seasons and farm types — is needed to establish whether the favorable single-study safety result reported by Jahan et al. (2024) generalizes to the range of Bangladeshi hatchery conditions. EFSA (2012) and Codex Alimentarius (2019) provide applicable reference limits.

3.10.4 Development of a National Feed-Ingredient Standard

The Department of Livestock Services and Bangladesh Standards and Testing Institution could usefully define minimum processing requirements, microbiological limits, heavy-metal ceilings, minimum calcium assay, and moisture limits for eggshell-derived calcium supplements — analogous to existing standards for limestone and dicalcium phosphate and aligned with EFSA (2012) criteria and Codex Alimentarius (2019) contaminant standards.

3.10.5 Pilot-Scale Collection and Processing Models

Pilot projects testing alternative institutional aggregation models — centralized processing by large integrators already investing in by-product lines versus decentralized processing through dairy or poultry cooperatives — would clarify which arrangement is most cost-effective and scalable in the Bangladeshi context.

3.10.6 Formal Techno-Economic and Life-Cycle Assessment

A formal cost-benefit comparison of hatchery eggshell calcium against imported limestone and dicalcium phosphate, incorporating collection, processing, and distribution costs alongside avoided import and waste-disposal costs, together with a life-cycle assessment of environmental impacts (Singh et al., 2021; Matilainen & Pajarre, 2021), would provide the quantitative basis currently missing from the qualitative assessment in Section 3.8.

3.10.7 Systematic Forage Mineral Characterization for Bangladesh

A nationally representative survey of calcium and phosphorus content in Bangladeshi rice straw, crop residues, and concentrate ingredients — analogous to the Indian survey of Bhanderi et al. (2016) — would provide the primary data needed for quantitative ration modelling rather than cross-country extrapolation, enabling precise estimation of the calcium gap that eggshell supplementation would need to fill.

4. Conclusion

Bangladesh’s dairy and poultry-hatchery sectors share a structural complementarity that has, until now, gone largely unexploited. Evidence assembled in this review indicates that properly processed hatchery eggshell waste — approximately 95% calcium carbonate, yielding 25–30% elemental calcium on a dry-matter basis — is nutritionally comparable to imported limestone and oyster shell, is palatable in livestock diets, and can be rendered microbiologically safe through straightforward heat treatment. Against a backdrop of widespread subclinical hypocalcaemia in rice-straw-fed Bangladeshi dairy cows, near-total import dependence for conventional mineral supplements, and an estimated annual eggshell stream of 8,000–9,500 tonnes from national hatcheries, the case for valorization is compelling on nutritional, economic, and environmental grounds alike. What is needed now is not more analogical reasoning from overseas non-ruminant data, but a well-powered, Bangladesh-specific dairy-cattle feeding trial, systematic heavy-metal and microbiological profiling across hatchery sources, and a formal national feed-ingredient standard. These goals are specific, achievable, and — given the scale of the problem they address — well worth pursuing as a collaborative priority for researchers, industry, and national livestock-policy authorities.

Acknowledgements

The authors gratefully acknowledge the Department of Veterinary and Animal Sciences, University of Rajshahi, for institutional support and access to research library resources throughout the preparation of this manuscript. No external funding was received for this study.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Author Contributions

Conceptualization: M.S.I., S.S.J., Data curation (literature search and extraction): N.S., N.H., S.B.R., M.R.A., T.T., A.A.M., Formal analysis: N.S., N.H., Writing — original draft preparation: N.S., N.H., S.B.R., M.R.A., T.T., A.A.M., Writing — review and editing: M.S.I., S.S.J., M.A., Supervision: M.S.I., M.A., Project administration: M.S.I., All authors have read and agreed to the published version of the manuscript.

Competing Financial Interests

The authors M.S.I., S.S.J., N.S., N.H., S.B.R., M.R.A., T.T., A.A.M., N.S., N.H., S.B.R., M.R.A., T.T., A.A.M., M.S.I., S.S.J., M.A., M.S.I., M.A., M.S.I., declare no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

References