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

Like humans, cows suffer from the heat. When exposed to the sun, especially in summer, their comfort decreases: they eat less, produce less milk and their health can deteriorate. To protect themselves, they naturally seek shade from trees, hedges or other possible shelters. However, the amount of time spent and the conditions under which cows actually use these shaded areas remain poorly understood. A better understanding of these behaviours is essential for designing grazing methods that are better adapted to global warming.

Measuring the light received by cows and validating the method

UMRH researchers attached a small light sensor directly to the backs of seven lactating Holstein dairy cows. These cows were taken to pasture with the rest of the herd, spending time indoors during the two daily milkings. Each pasture plot included an area shaded by hedges and/or trees (grouped together or isolated), located directly within the plot. The sensor was placed on the transverse processes of the lumbar vertebrae, behind the iliac bone. The sensor automatically recorded the amount of light received (in lux) once per minute. 

To test the reliability of the method, the scientists combined technology with human observation. During the summer, they directly observed cows in the pasture: were they in the sun or in the shade? Standing or lying down? These observations were then compared with the data recorded by the sensors.

The researchers thus identified a threshold of light intensity that reliably distinguishes a cow in the sun from a cow in the shade. In practice, below approximately 25,000 lux, the cow is considered to be in the shade; above that, it is considered to be exposed to the sun. This simple rule proved to be very reliable, with more than 95% of classifications matching visual observations.

Robust results, with some nuances

Once the system has been calibrated, the sensors enable continuous, low-cost monitoring, even for large herds. The method works particularly well when the cows are standing. When they are lying down, the sensor may be slightly tilted, which can sometimes distort the measurement, much like a poorly oriented solar panel. Despite this, the error remains limited.

This study shows that it is possible to automatically and effectively measure the use of shade by grazing cows. The method developed will ultimately enable better design of pasture layout: number of trees required, location of shaded areas, reduction of inequalities in access between animals, and this for different species. The main objective is to improve animal welfare by taking their needs into account and thus adapting livestock farming to climate change through a better understanding of the behaviour of animals exposed to heat.