The number of stolen animals increased nearly 15% year over year in April, figures from NFU Mutual showed.
The company, which insures around three-quarters of UK farmers, puts the cost of cattle rustling at £ 3million.
Rebecca Davidson of NFU Mutual said: “Ten years ago, we rarely had more than a dozen sheep stolen from each other at one time.
“There is concern that this meat will enter the black market, so it has a huge impact on our farmers who work to very high welfare standards.
“The animals they raised are gone, it has an impact on the breeding programs, the welfare of the animals themselves.
“We are also concerned about the threat to human health as these animals could be slaughtered in unsanitary conditions. ”
The increase is part of a wider spike in rural crime in every region and country across the UK last year, as criminal gangs targeted expensive tractors, quads and large numbers of cattle, according to company data.
The NFU Mutual report found that the total cost of rural crime had increased by almost 9% in 12 months to reach £ 54million last year – its highest level in eight years.
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The surge was in part due to the theft of tractor GPS equipment typically costing between £ 8,000 and £ 10,000, “a highly prized item on rural thieves’ shopping lists,” the company said.
The largest percentage increase was seen in Scotland (44%), although its cost of rural crime remains below the UK average.
Northern Ireland experienced the second-highest regional increase of 18%, followed by eastern England (16.9%).
The rise in crime is “of huge concern” to farmers, said Davidson, because there is a human impact too.
“There is a disruption in business, but also enormous anxiety for the rest of the family, because farms are not only places of work, they are also family homes.”
And theft is not the only problem farmers face as the lockdown has seen an “influx” of walkers to their land, and not all have respected the countryside.
“Dogs have been left off leash in fields full of cattle. This comes at a huge cost to farmers because dogs can attack and kill sheep, ”said Ms. Davidson.