Determining the Brushing Capacity of Mulberry Gardens

Every successful silkworm rearing season starts with one critical question: how many disease-free layings (DFLs) can your mulberry garden actually support? Overestimate it and your silkworms run out of food at their hungriest stage. Underestimate it and you leave productive capacity – and profit – on the table. This is precisely what brushing capacity helps you determine, and getting it right is one of the most important pre-rearing calculations a sericulturist can make.

What is brushing capacity? 🔗

Brushing capacity is the maximum number of disease-free layings (DFLs) that can be brushed and reared from a given mulberry plot during a single rearing season, based on the available leaf supply. The term “brushing” itself refers to the process of transferring freshly hatched silkworm larvae – known as “ants” – from egg cards onto rearing trays using a soft feather or brush. Brushing capacity, therefore, is essentially a measure of how many such layings your garden can sustain through to cocoon formation.

This is not a fixed number. It changes from season to season, plot to plot, and even between silkworm breeds. Estimating it accurately requires understanding the leaf production potential of your mulberry garden and matching it against the specific leaf demands of the breed you intend to rear.

Why leaf availability is the starting point 🔗

The mulberry leaf is the sole nutritional source for the silkworm (Bombyx mori), and as the FAO notes, the quality and quantity of leaf produced per unit area have a direct bearing on cocoon harvest. A garden that produces high volumes of nutritious leaves will support more DFLs; a thin or poorly managed plot will limit your rearing right from the start.

Leaf yield varies significantly depending on the mulberry variety, plant age, irrigation availability, and agronomic management. According to FAO data on Indian sericulture, mulberry grown for silkworm rearing can yield 30-35 tonnes of leaf per hectare annually under suitable conditions. High-yielding irrigated varieties like V-1 (Victoria-1) can reach up to 70 tonnes per hectare per year, while varieties like S-36 and S-54 typically deliver 38,000-55,000 kg per hectare annually. Rainfed plots produce considerably less, and this directly constrains how many DFLs can be supported.

The estimation of brushing capacity therefore begins with a reliable assessment of how much harvestable leaf your plot will yield in the upcoming season. This involves sampling 10-15 representative plants across the garden, weighing their harvestable leaves, calculating an average yield per plant, and then multiplying that figure by the total number of plants in the plot. The resulting number gives your estimated leaf supply – the foundation from which brushing capacity is then derived.

Leaf consumption varies by silkworm breed 🔗

Once you know how much leaf your garden will produce, you need to know how much leaf each DFL will consume. This is where silkworm breed selection becomes directly relevant to brushing capacity calculations.

Different silkworm races have significantly different food requirements. Multivoltine breeds – such as Pure Mysore or Nistari, which are reared throughout the year in tropical India – tend to be smaller, faster-maturing worms with lower per-DFL leaf consumption. Their leaf-to-cocoon ratio is higher (meaning they are less efficient converters), but the total quantity of leaf they consume per laying is generally lower, typically around 8-10 kg per DFL across all instars.

Bivoltine breeds – including popular hybrids like CSR2 × CSR4, which are reared during the cooler months of August to February – are larger-bodied worms with a longer larval duration and much higher leaf demands. Bivoltine silkworms can consume up to 15 kg of leaves per DFL, making the leaf budget for a bivoltine crop substantially larger. Research from the Central Sericultural Research and Training Institute (CSRTI), Mysore confirms that food ingestion and assimilation in bivoltine hybrids are significantly influenced by season and temperature, which in turn affects how efficiently consumed leaf is converted into cocoon mass.

The stage-wise breakdown of leaf consumption also matters. Guidelines from the Central Silk Board show that during early instar rearing, just 5 kg of leaf is needed per 100 DFLs in the first stage and 18 kg per 100 DFLs in the second. The real leaf burden comes in the fourth and fifth instars, when silkworms are growing rapidly and building silk glands. In fact, published research in PMC confirms that 80-85% of total leaf consumption occurs during the fifth instar alone, making late-age leaf supply the most critical factor in any brushing capacity calculation.

Stage-wise leaf planning in practice 🔗

For a sericulturist planning a rearing batch, this means allocating leaf across the full larval period – from first instar through fifth – rather than simply dividing total leaf yield by a per-DFL average. Early-stage leaf is typically harvested from the upper canopy (tender, high-moisture leaves suitable for young worms), while late-stage feeding demands the larger, more mature leaves from the middle and lower branches. Both quality and quantity must be available at the right time. A garden that produces plenty of early-season tender growth but runs short in mid-season will fail the silkworms precisely when they need the most food.

The role of fecundity in brushing capacity estimation 🔗

Fecundity – the number of eggs laid per female moth – is another variable that feeds into brushing capacity calculations, though its influence is sometimes overlooked. A DFL is defined by a standard number of eggs, but the actual number of eggs per laying varies by breed and season. Multivoltine races are characterised by lower fecundity, with egg counts per laying ranging from roughly 400 to 500 eggs, while bivoltine races lay more eggs per card. Seasonal conditions – particularly temperature and humidity – affect fecundity further, with unfavourable conditions during moth emergence and egg-laying reducing the number of viable eggs per DFL.

Why does this matter for brushing capacity? Because a DFL with fewer viable eggs produces fewer larvae, which in turn consumes less leaf. When fecundity drops due to heat stress or poor grainage management, the effective leaf demand per DFL decreases – but so does cocoon output. Sericulturists who monitor fecundity data across seasons can refine their brushing capacity estimates, adjusting planned DFL numbers based on anticipated hatching percentages and larval survival rates, rather than relying on fixed averages alone.

Research published in the journal Psyche demonstrates clearly that seasonal differences in environmental conditions – temperature and relative humidity in particular – significantly affect the phenotypic performance of silkworm crops, including cocoon weight, shell weight, and silk ratio. These seasonal effects ripple back to influence brushing capacity planning, since a rearing season known for adverse conditions warrants more conservative DFL estimates to avoid feed shortages.

Putting it all together: the brushing capacity formula 🔗

In practical terms, estimating brushing capacity comes down to a straightforward relationship:

Brushing Capacity (DFLs) = Total estimated leaf yield (kg) ÷ Leaf requirement per DFL (kg)

For example, if a well-managed irrigated mulberry plot is expected to yield 1,500 kg of harvestable leaf in a given season and the farmer intends to rear a bivoltine breed requiring 15 kg of leaf per DFL, the brushing capacity for that season would be 100 DFLs. If the same plot were used for a multivoltine crop consuming 10 kg per DFL, it could support up to 150 DFLs. The garden is the same; the breed choice changes the calculation entirely.

A practical buffer of 10-15% is typically recommended to account for leaf lost to weather events, pest damage, harvesting inefficiency, or unexpected quality drops. Leaf quality shortfalls – wilted, diseased, or nutrient-deficient leaves – reduce effective feeding efficiency and may require larger quantities to meet larval needs, effectively lowering the real-world brushing capacity below the calculated figure.

Seasonal adjustments to brushing capacity 🔗

No mulberry garden produces the same leaf yield in every season. In tropical sericulture states like Karnataka, Tamil Nadu, Andhra Pradesh, and West Bengal – which together account for the majority of India’s silk output – leaf flush, growth rates, and moisture content all shift across the rearing cycles of the year. According to FAO, mulberry thrives at temperatures of 24-28°C with atmospheric humidity in the 65-80% range; deviations from these conditions during hot summers or cold winters reduce both leaf yield and leaf quality.

In India’s tropical conditions, sericulturists can typically harvest four to six crops per year from a well-maintained mulberry garden. Each of those crops corresponds to a rearing cycle, and the brushing capacity must be recalculated for each one – not carried over from a previous season’s estimate. Summer crops, for instance, often see reduced leaf moisture content and lower protein levels, which can reduce the effective nutritional value of the leaf even when dry weight quantities seem adequate.

Bivoltine rearing is deliberately concentrated in the cooler months (August-February in Indian conditions) because bivoltine hybrids like CSR2 × CSR4 are sensitive to high temperatures and humidity fluctuations – and because mulberry leaf quality during those months is better suited to their demanding nutritional requirements. Planning brushing capacity around the seasonal calendar, not just raw leaf quantities, is therefore a hallmark of skilled sericulture management.

Common mistakes in brushing capacity estimation 🔗

Several planning errors consistently lead to mismatched brushing capacity in the field. Relying on last season’s actual yield as a fixed benchmark, without accounting for pruning regrowth cycles, fertiliser application, or rainfall variation, is one of the most frequent. Another is using average breed-level leaf consumption figures without adjusting for the specific hybrid being reared – a multivoltine × bivoltine F1 hybrid, for instance, will have different leaf demands than a pure multivoltine.

Failing to account for the uneven distribution of leaf demand across instars is also problematic. A farmer who calculates total leaf adequacy but doesn’t verify that sufficient mature leaf will be available during the fifth instar may face a critical shortage at the worst possible time – just as worms are preparing to spin cocoons. As a guide, the Central Silk Board recommends providing 2,500 kg of mulberry shoots per 100 DFLs during late-age shoot rearing, which underscores just how leaf-intensive the final instar period is.

Finally, brushing capacity is not just a one-time pre-season figure. It should be reassessed after each pruning cycle and at key intervals during the growing period, since unexpected events – disease outbreaks in the mulberry garden, drought, or pest pressure – can reduce available leaf and require the farmer to scale back planned DFL numbers before brushing begins.

What do you think? Given that bivoltine silkworms consume significantly more leaf per DFL than multivoltine breeds but produce higher-quality silk, how should a small-scale sericulturist with a limited mulberry plot weigh the trade-off between DFL numbers and cocoon quality? And with leaf yield varying so considerably by season, what steps do you think sericulturists should take to protect their brushing capacity estimates against unexpected crop shortfalls?

References

1. https://www.fao.org/4/x9895e/x9895e04.htm
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC3596942/
3. https://silks.csb.gov.in/bagalkote/rearing-of-mulberry-silkworm/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC3465930/
5. https://hbmahesh.weebly.com/uploads/3/4/2/2/3422804/2._classification_of_silkworms.pdf
6. https://onlinelibrary.wiley.com/doi/10.1155/2012/121234