Measuring shade use of dairy cattle at pasture with an on-cow light sensor: a case study

• On-cow light sensors can monitor shade use by cows at pasture. • A light threshold accurately distinguishes cows in shade from cows in sun. • Specificity and sensitivity were very good for thresholds from 15,688 to 40556 lx. • An efficient light threshold can be determined with only a few direct observations. • This method can be applied to other grazing species (e.g. sheep). Grazing cows preferentially access shade to shield against the sun. However, the conditions that provide cows with optimal shade access and use (e.g. no competition for access to shade) are still unknown. Continuous monitoring of shade use by grazing cattle could help to understand how and when cows use shade resources. The aim of this study was to validate a method based on a light sensor (HOBO Pendant MX2202) attached to the back (on the transverse processes of the lumbar vertebrae) of 7 dairy cows at pasture to continuously record their use of natural shade for research purposes. Live behavioral observations of shade use and cow posture were recorded in summer (June to September, between 9 am and 6 pm). Based on the behavioral observation data, we determined thresholds in lux to discriminate between cows in shade and cows in sun on a randomly-generated training dataset representing 15 % of the initial dataset. This process was repeated 100 times, generating 100 thresholds and threshold performances. Data loss due to sensor loss or battery discharge was 9 %, which is acceptable. The thresholds ranged from 15,688 to 40556 lx: sensitivity ranged from 92.0 % to 99.8 % and specificity ranged from 88.7 % to 99.9 %, showing that the performances were robust to threshold variation within this range. This study demonstrates that an efficient threshold to discriminate cows in shade from cows in the sun can be determined via a relatively short (about 12 h) series of live observations. As performances seem to be slightly lower for lying cows than for standing cows (mean false-positive rate is 7.4 % for lying cows versus 1.8 % for standing cows), future studies should consider the posture (which can also be monitored continuously with other sensors such as accelerometer installed on the legs or on the neck collar of the cows).