# Why Camera-Based and Scale-Based Measurements Differ: A Physiological Model of Diurnal Weight Variation in Finishing Pigs

## TL;DR

This study explains why camera-based and scale-based weight measurements of pigs differ by modeling natural daily weight fluctuations due to normal physiological processes.

## Contribution

A stochastic simulation model quantifies diurnal weight variation in finishing pigs, providing a physiological basis for measurement discrepancies.

## Key findings

- Short-term weight fluctuations of ±3–5 kg occur daily in finishing pigs due to normal physiological processes.
- Camera-based and scale-based measurements differ because they capture different aspects of pig weight dynamics.
- Physiological variation often exceeds typical accuracy claims of camera weighing systems, suggesting caution in interpretation.

## Abstract

Live weight is widely used to assess pig growth and to evaluate the performance of weighing technologies. However, pig body weight is not constant throughout the day. Normal activities such as drinking, eating, urinating, and defecating naturally change the amount of water and digesta within the body, leading to short-term weight fluctuations. A key issue addressed in this study is that these normal physiological variations are often overlooked when discrepancies are observed between camera-based weight estimates, which infer weight from body shape, and floor-scale measurements. The aim of this study was to quantify the extent to which pig body weight can vary within a single day solely due to normal physiological processes. Using published data, we developed a simulation model to reproduce diurnal weight changes in finishing pigs weighing approximately 100 kg. These findings may help explain why camera-based measurements often differ from scale readings by several kilograms. Such differences do not necessarily indicate measurement errors but instead reflect normal biological variation. Recognizing body weight as a dynamic physiological variable is essential for fair evaluation of weighing technologies and improved decision-making in modern pig production.

Live weight is widely used as a reference indicator for growth performance and for evaluating the accuracy of weight measurement technologies in pig production. However, live weight is not a fixed physiological quantity, and finishing pigs naturally experience substantial short-term mass fluctuations due to normal behaviors such as drinking, feeding, urination, and defecation. In this study, we integrated published physiological and behavioral parameters into a stochastic simulation model to quantify within-day live-weight dynamics in finishing pigs weighing approximately 100 kg. The simulation was conducted with 1-min temporal resolution over a 24-h period. The model demonstrated that short-term weight fluctuations of approximately ±3–5 kg can occur within a single day, even when measurement error is minimal. Across 1000 simulated pigs, the mean daily fluctuation range was 4.2 kg, confirming that kilogram-scale variation is physiologically expected under normal conditions. These results provide a plausible physiological basis for understanding the frequently reported discrepancies between camera-based weight estimates and instantaneous floor-scale measurements. Camera systems primarily reflect body mass derived from external morphology, whereas floor scales measure instantaneous total mass that includes transient contributions from gastrointestinal contents, ingested water, and retained waste. Consequently, direct comparisons based on instantaneous scale readings can be misleading when used as ground truth. Our findings indicate that commonly cited accuracy claims of ±2–3 kg for camera weighing systems should be interpreted with caution, as normal physiological weight variation often exceeds this range. Recognizing live weight as a dynamic physiological variable is essential for developing biologically meaningful evaluation frameworks and for the appropriate interpretation and comparison of weight measurement technologies in precision livestock farming.