The spring sunshine has brought fresh problems for Pembrokeshire's dairy and livestock farmers.
Last year the first cases of photosensitisation, a condition causing skin damage in cattle, were recorded in May by the National Animal Disease Information Service.
The condition, said NADIS vet Mike Howe, occurred when the presence of a chemical made skin become sensitive to sunlight.
"It is not very common in the UK, but it causes significant economic loss, particularly when it occurs in groups of animals," explained Mr. Howe, who is based in Pembrokeshire.
There three forms of photosensitisation - direct, secondary and local. Animals with a defect in the metabolism of red blood cells are susceptible to direct photosensitisation, but it can also occur from contact with a chemical in photosensitising plants such as St. John's wort and bogasphodel.
Liver damage is to blame for secondary photosensitisation, causing an interference with the complete breakdown of chlorophyll and an accumulation of a photosensitive chemical.
The third form of the condition, local photosenitisation, can result from a reaction to the sap of some plants.
According to a report by Richard Laven, manager of the Scottish Agricultural College's surveillance centre, the most commonly affected sites are those exposed to direct sunlight, including the udder.
Clinical signs include hair loss, ulceration of skin and a general reddening and peeling appearance to the skin.
"A veterinary diagnosis is important to rule out liver failure," suggested Mr. Laven.
Treatment includes removal of affected stock to cool, shaded housing, fly control and supportive therapy.
Mike Howe advised against breeding from animals with photosensitisation due to a genetic defect.
"Farmers should also take steps to identify and remove possible plant sources of photosensitising chemicals," he said.
• Research by the National Animal Disease Information Service, a network of veterinary practices and colleges monitoring diseases in the UK livestock industry, is sponsored by the Milk Development Council, the Meat and Livestock Commission, Elanco Animal Health and Intervet.