fit_orskov: Fit Orskov and McDonald model

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

Fit Orskov and McDonald model

Description

Fits the Orskov and McDonald gas-production model to each bottle in a rumen_gp dataset.

Usage

fit_orskov(data, start = NULL)

Arguments

data	A rumen_gp object.
start	Optional list of starting values. May contain any of: VF b k

Details

Equation

V(t) = VF + b \left( 1-e^{-kt} \right)

where:

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

• VF is the intercept (initial gas volume)

• b is the fermentable fraction

• k is the fractional rate constant

Interpretation

The Orskov and McDonald model partitions gas production into:

• An intercept term (VF)

• A fermentable fraction (b)

The asymptotic gas production is:

VF + b

The parameter k controls the rate at which the asymptote is approached.

Advantages

• Widely used in ruminant nutrition research

• Parameters have straightforward biological interpretation

• Stable convergence

• Useful benchmark model for comparison

Limitations

• No explicit lag parameter

• Limited flexibility for highly sigmoidal fermentation profiles

• Less adaptable than Gompertz, Groot, or Dual Logistic models

Value

An orskov_fit object containing:

• Parameter estimates

• Model diagnostics

• Predicted values

• Residuals

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_orskov(
  gp
)

summary(fit_default)

# Fit using custom starting values
fit_custom_start <- fit_orskov(
  gp,
  start = list(
    VF = 5,
    b = 120,
    k = 0.05
  )
)

summary(fit_custom_start)

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