Digestibility trials using total collection and indicator methods

Introduction

Digestibility trials are essential tools in animal nutrition for evaluating how efficiently animals utilize nutrients from feed. The digestibility coefficient reflects the proportion of a nutrient that is absorbed from the gastrointestinal tract, influencing both feed formulation and animal performance. Two widely used approaches in digestibility trials are:

• Total Collection Method
• Indicator (Marker) Method

1. Total Collection Method

Principle

This method involves direct measurement of the total feed intake and the total fecal output over a specific period. The difference between the nutrients ingested and those excreted gives the amount digested and absorbed.

Procedure

• Animals are adapted to the test diet (5–7 days).
• Over a collection period (usually 5–10 days), all feces are collected, weighed, and sampled.
• Feed offered and feed refused are recorded to calculate actual intake.
• Samples of both feed and feces are analyzed for dry matter (DM) and other nutrients.

Digestibility Calculation

Digestibility (%) =
[(Nutrient intake – Nutrient in feces) / Nutrient intake] × 100

Advantages

• Direct and accurate measurement.
• Reliable for research and nutrient evaluation.

Limitations

• Labor-intensive and time-consuming.
• Requires facilities to separate and collect feces completely.
• Stress on animals and potential inaccuracies from urine contamination.

2. Indicator (Marker) Method

Principle

This method uses an indigestible marker that passes through the gastrointestinal tract unchanged. By comparing the marker concentration in feed and feces, digestibility can be estimated without total feces collection.

Types of Markers:

• Internal markers: Naturally present in feed (e.g., lignin, acid-insoluble ash).
• External markers: Added to feed (e.g., chromic oxide, titanium dioxide, ytterbium).

Procedure

• Feed is supplemented with a known amount of the marker.
• A representative fecal sample is collected after adaptation.
• Feed and feces are analyzed for marker concentration and nutrients of interest.

Digestibility Calculation

Digestibility (%) =
100 – [(Marker in feed × Nutrient in feces) / (Marker in feces × Nutrient in feed)] × 100

Advantages

• Less labor and animal stress.
• Suitable for grazing animals or large-scale trials.
• Useful where total feces collection is impractical.

Limitations

• Requires precise laboratory analysis.
• Marker recovery must be near 100% to avoid error.
• Assumes even distribution and passage of marker, which may not always occur.

Comparison of Methods

Conclusion

Digestibility trials provide critical information about feed utilization in animals. While the total collection method remains the gold standard for accuracy, the indicator method offers flexibility and convenience, especially in field conditions. The choice of method depends on the species, resources, accuracy required, and the environment in which the study is conducted.