# No Effect of a Commercially Used Odor Repellent for Roe Deer (Capreolus capreolus) Protection During Meadow Harvest

## TL;DR

A commonly used odor repellent failed to protect roe deer fawns during meadow harvests in Central Europe, suggesting a need for alternative methods like thermal-imaging drones.

## Contribution

This study provides empirical evidence that a commercial odor repellent is ineffective for roe deer protection during meadow harvests.

## Key findings

- Odor repellent application did not reduce roe deer presence on treated fields compared to controls.
- Fawn numbers increased over time regardless of repellent use, aligning with the peak of parturition.
- Thermal-imaging drones are suggested as a more effective alternative for detecting and relocating fawns.

## Abstract

In the agricultural landscapes of Central Europe, hundreds of thousands of roe deer (Capreolus capreolus) fawns are killed annually during meadow harvests. To prevent this particularly unethical mortality of juveniles, wildlife managers employ various protection measures. One common method is to apply odor repellents in high-risk areas before harvest. Here, we evaluated the effect of the odor repellents on treated areas relative to controls without any measures. The abundance of fawns and adult roe deer was assessed before and after odor application using unmanned aerial vehicles equipped with thermal cameras. The results showed that roe deer numbers on treated fields did not differ from those on control meadows without repellents. The expected efficacy was not demonstrated either in the short term (i.e., in the days immediately following application) or over longer intervals of several weeks. The ineffectiveness of odor repellent for protecting roe deer during meadow harvest indicates shortcomings in current practice and underscores the need to adopt alternative methods, such as thermal-imaging drones.

In Central Europe, the fawning season of roe deer (Capreolus capreolus) directly overlaps with meadow and alfalfa harvest, typically from late May to early June. During these operations, tens or more likely hundreds of thousands of fawns are mutilated by agricultural machinery. To mitigate this unethical mortality, wildlife managers often deploy odor repellents to drive roe deer individuals from high-risk fields before mowing. Therefore, we evaluated repellent efficacy in a paired design. The abundance of roe deer was quantified by drones equipped with thermal cameras before and after repellent application and then compared with untreated control meadows. Results showed high adult abundance that did not differ significantly among treatments. The highest median was paradoxically observed on meadows “after application” (8.25 ind./10 ha), followed by “not treated” meadows (7.92 ind./10 ha), and “before application” (5.72 ind./10 ha). For fawns, differences between treated and untreated plots were likewise non-significant. Their numbers increased over time after application, consistent with the peak of parturition in the second half of May. Overall, the study confirms that the tested odor repellent, when applied according to the manufacturer’s protocol, did not reduce roe deer presence on meadows. This underscores the need to consider alternative approaches, such as the use of thermal-imaging drones combined with the subsequent translocation of detected fawns to safe locations.