Dog attacks on livestock cost farmers £165,000 last year in Wales, according to figures from NFU Mutual.
It says the cost of the attacks on farm animals last year dropped by a third compared to 2019, but incidents continue to 'cause suffering to livestock and anxiety for farmers.'
NFU Mutual says sheep farmers will be under stress as they enter the lambing period when pregnant ewes and new born lambs are vulnerable to attack.
A survey of dog owners commissioned by NFU Mutual has also revealed that 64 per cent of dog owners are letting their pets roam free in the countryside, despite half of owners surveyed admitting their dog doesn't always come back when called.
The research also found that 42 per cent of dog owners have been walking their pets more often in the countryside during the pandemic, and that 81 per cent of those surveyed have noticed more people exercising their pets in rural areas.
Rebecca Davidson, Rural Affairs Specialist at NFU Mutual said: "With more people walking in the countryside as COVID restrictions continue and an increase in dog ownership, we have seen many more horrific attacks resulting in large numbers of sheep being killed and a trail of horrific injuries.
"It's a critical time in the farming calendar and there is widespread concern as we enter the peak lambing season, that there will be a surge in new visitors who are simply unaware of the countryside code or how their dog will behave around farm animals.
"We want people to enjoy the countryside as it's so important for people's wellbeing. It's vital that dog owners act responsibly and keep dogs on a lead and under control whenever there is a possibility livestock are nearby."
NFU Mutual said 40 per cent of dog owners surveyed accepted that their pet could cause the injury or death of a farm animal.
Rebecca Davidson said: "Even if a dog doesn't make physical contact, the distress and exhaustion of the chase can cause sheep to die or miscarry their lambs.
"It's important that owners realise that all dog breeds, not just the big, fierce looking ones, are capable of chasing livestock, or attacking them."
With lambing season approaching and many more people planning to walk in the countryside, NFU Mutual is calling for dog owners to keep their pets under control at all times.
Advice for dog owners from NFU Mutual:
Always keep dogs on the lead when walking in rural areas where livestock are kept
Be aware that even small lap dogs can attack and kill farm animals
Take special care to keep close control of dogs unused to farm animals
Report attacks by dogs and sightings of dogs roaming the countryside to local farmers or the police
Don’t let dogs loose in gardens adjoining livestock fields – many attacks are caused by dogs which escape and attack sheep grazing nearby