agri.seed-rate

def seed_rate(target_plants_per_square_metre: int, thousand_grain_weight_grams: float, establishment_basis_points: int) -> SeedRate

@dataclass(frozen=True)
class SeedRate:
    """What to set the drill to."""

    #: plants x TGW / establishment %, to 0.1 kg, half up
    kg_per_hectare: float
    #: seeds to sow per m², to 0.1, half up
    seeds_per_square_metre: float

from fune.agri.seed_rate import seed_rate  # agri.seed-rate@^1

import math

from .agri_seed_rate_types import SeedRate
from .math_round_div import round_div  ← from math.round-div ^1.0.0 · built alongside by fune
from .math_round_float import round_float  ← from math.round-float ^1.0.0 · built alongside by fune


def seed_rate(target_plants_per_square_metre: int, thousand_grain_weight_grams: float, establishment_basis_points: int) -> SeedRate:
    """Seed rate (kg/ha) = target plants/m2 x TGW (g) / expected establishment (%).

    The TGW is taken to the centigram and the establishment is in basis points,
    so kg/ha is exactly plants x centigrams / basis points and is rounded once.
    Doing it in floats gets 100 x 1.15 / 100 = 1.1499999999999999, which rounds
    to 1.1 kg/ha instead of 1.2.
    """
    plants = target_plants_per_square_metre
    if isinstance(plants, bool) or not isinstance(plants, int) or plants < 1 or plants > 100000:
        raise ValueError("targetPlantsPerSquareMetre must be a whole number from 1 to 100000, received %r" % (plants,))
    tgw = thousand_grain_weight_grams
    if isinstance(tgw, bool) or not isinstance(tgw, (int, float)) or not math.isfinite(tgw) or tgw <= 0 or tgw > 1000:
        raise ValueError("thousandGrainWeightGrams must be a finite number greater than 0 and at most 1000, received %r" % (tgw,))
    centigrams = int(round(round_float(float(tgw), 2) * 100))
    if centigrams < 1:
        raise ValueError("thousandGrainWeightGrams must be at least 0.01 grams, received %r" % (tgw,))
    bp = establishment_basis_points
    if isinstance(bp, bool) or not isinstance(bp, int) or bp < 1 or bp > 10000:
        raise ValueError("establishmentBasisPoints must be a whole number from 1 to 10000, received %r" % (bp,))
    return SeedRate(
        kg_per_hectare=round_div(plants * centigrams * 10, bp, "half-up") / 10,
        seeds_per_square_metre=round_div(plants * 100000, bp, "half-up") / 10,
    )

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add agri.seed-rate

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./agri.seed-rate-1.0.0-python.fune, or fetch it from a terminal with fune pull agri.seed-rate@1.0.0:python.

The whole function, every language, is one file too: agri.seed-rate-1.0.0.fune, 13,674 bytes, sha256 0cb3c657ec361c750a3bb5c33e092b800dbfa3b731ee943c781c2f8d4c3f0672. It installs into a project of any language.

# fune: before agri.seed-rate

# fune: after agri.seed-rate

# fune: replace math.round-div in agri.seed-rate
# fune: replace math.round-float in agri.seed-rate

# fune: step agri.seed-rate after <n|label>

More from the author

`seedRate(260, 48, 7000)` is 178.3 kg/ha and 371.4 seeds/m²: 260 plants at 70% establishment needs 371.4 seeds/m², and 1000 seeds weigh 48 g.

The formula follows from the units: seeds/m² = plants / establishment; one seed weighs TGW / 1000 g; and 1 g/m² is 10 kg/ha. It is the calculation behind AHDB's seed rate conversion tool (seeds/m² against TGW, read off as kg/ha) and the one UK agronomy guidance quotes.

## Establishment

`establishmentBasisPoints` is the share of seeds sown that become plants: germination and field losses (slugs, seedbed, sowing date) together. If you have a germination figure and a separate field-loss estimate, multiply them: 90% germination and 85% emergence is 76.5%, `7650`. AHDB's benchmarks are around 70% for well-timed winter wheat, falling as drilling gets later.

## Exactness

The thousand-grain weight is taken to the nearest 0.01 g (half away from zero, `math.round-float`), so the rate is exactly plants x centigrams / basis points and is rounded once, half up, to 0.1 kg/ha. A float calculation gets 1.15 g at 100 plants and 100% as 1.1499999999999999 and rounds it to 1.1; this gives 1.2.

It does not choose the target population, adjust for sowing date or soil, or round to what a drill can be set to.

## Limits

Plants 1 to 100,000 per m² (grass and small seeds are sown in thousands); TGW more than 0 and at most 1000 g, and at least 0.01 g after rounding; establishment 1 to 10,000 basis points.

## Sources

- AHDB, "Seed rate calculator – conversion tool", https://ahdb.org.uk/seed-rate-conversion-tool (seeds/m² to kg/ha by TGW). - AHDB, "Establishment in wheat: germination and emergence (GS0–GS2)", https://ahdb.org.uk/knowledge-library/establishment-in-wheat-germination-and-emergence-gs0-gs2 (the 70% establishment benchmark). - Foundation for Arable Research, "Seed rate calculation", https://www.far.org.nz/resources/seed-rate-calculation: sowing rate = target plants x TGW x 100 / (% germination x % emergence); its worked example (225 plants, TGW 40 g, 90% x 85%, about 118 kg/ha) is a vector.

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