fit_dual_logistic: Fit dual-pool logistic model

In rumenGP: Rumen Gas Production Modeling, Comparison, and Visualization

Fit dual-pool logistic model

Description

Fits a dual-pool logistic model representing rapidly and slowly degradable fractions.

Usage

fit_dual_logistic(data, start = NULL)

Arguments

data	A rumen_gp object.
start	Optional list of starting values. May contain any of: V1F V2F k1 k2 lambda

Details

Equation

V(t)= \frac{V1F} { 1+\exp\left[2-4k1(t-\lambda)\right] } + \frac{V2F} { 1+\exp\left[2-4k2(t-\lambda)\right] }

where:

• V(t) is cumulative gas production at time t

• V1F is the final gas volume from the rapidly fermentable fraction

• V2F is the final gas volume from the slowly fermentable fraction

• k1 is the fractional rate constant of the rapid fraction

• k2 is the fractional rate constant of the slow fraction

• \lambda is lag time

Interpretation

The Dual Logistic model assumes that gas production originates from two independent fermentation pools:

• A rapidly degradable fraction (V1F)

• A slowly degradable fraction (V2F)

Each fraction follows a logistic fermentation pattern with its own rate constant.

Advantages

• Represents complex fermentation dynamics

• Separates rapid and slow fermentation pools

• Biologically meaningful parameterization

• Useful for heterogeneous substrates

Limitations

• Requires estimation of five parameters

• More computationally demanding

• Greater risk of parameter correlation

• May require careful starting values

Value

A dual_logistic_fit object containing:

• Parameter estimates

• Model diagnostics

• Predicted values

• Residuals

• Rapid and slow pool estimates

Examples

files <- example_data()

raw_data <- read_ankom(
  files$ankom
)

metadata <- read_metadata(
  files$metadata
)

gp <- process_ankom(
  raw_data,
  metadata,
  headspace_ml = 210,
  temperature_c = 39
)

# Fit using package default starting values
fit_default <- fit_dual_logistic(
  gp
)

summary(fit_default)

# Fit using custom starting values
fit_custom_start <- fit_dual_logistic(
  gp,
  start = list(
    V1F = 30,
    V2F = 70,
    k1 = 0.20,
    k2 = 0.05,
    lambda = 0.50
  )
)

summary(fit_custom_start)

rumenGP documentation built on Oct. 2, 2026, 5:09 p.m.