It is the deer’s breeding time and dogs might pose a threat to wildlife, especially young animals.
In the cantons of Aargau and Lucerne dogs must be kept on a leash at all times and are never to be left unattended to prevent them from disturbing the wildlife, including barking at wildlife.
This is in effect till July 31st in and close to forests.
Also the dogs are not allowed to play on agricultural land till October 31st.
For the cantons of Zurich and Zug please pay attention to signposting.