Thistle control keeps orf at bay

UK - April 27, 2009

Orf causes pustular and scabby lesions to develop on the non-woolly areas of sheep's skin, mouth and throat. It costs the industry dearly in vets' bills, poor animal performance and lost days' work for staff unfortunate to pick up the virus when handling infected sheep.

It is highly contagious and infects animals through abrasions or wounds from brushing past sharp objects such as thistles. These weeds can act like dirty hypodermic needles, injecting and spreading orf throughout a flock.

If ewes scratch their udders and teats - infection can lead to acute mastitis, loss of udder function and in worst cases death. Affected ewes may also prevent their lambs from suckling because they feel so sore. 

Growing and finishing lambs can pick up orf infection while grazing thistle-ridden pasture which will affect their growth rates. A venereal form can affect rams making them reluctant to mate. Infected animals rarely die from orf alone, but from a secondary bacterial infection.

There is currently no effective treatment for orf and control strategies are limited. The main veterinary method is immunisation using a live vaccine. This is only recommended in flocks with an existing problem. It will reduce the severity of clinical outbreaks, but does not offer long term immunity.

Removing the thistle population, using a herbicide such as Thistlex, significantly reduces the spread of orf within a flock. Weed control removes the chance of skin injury - either when the sheep are grazing or when eating hay made from pasture contaminated with the prickly weed. On infected farms, it also reduces the need to vaccinate.

David Horman farms 106 ha near Llandovery in the Brecon Beacons National Park, with 300 Lleyn and crossbred ewes and 40 Limousin cattle. The lambs are finished on the farm.

Orf had been a problem for several years, and David had to vaccinate routinely. However, after using Thistlex to tackle creeping thistles on sheep pasture six years ago, the incidence of orf fell dramatically.

"Five years ago half our lambs were affected," says Mr Horman. "But by killing the thistles we removed the main way the virus spreads, and for five years did not have to treat at all. We saved money on the vaccine and the lambs were in much better shape at weaning.

"We had tried to get rid of the thistles before with MCPA - but by late summer they were shooting back," Mr Horman explains. "By the next spring it looked like they had never been treated. Thistlex gives almost complete control for two to three years, with just a bit of spot treating required in year four - that's all." 

Mr Horman carries out contract spraying for other farmers in the area and says the effect Thistlex has on reducing the incidence of orf is clear to see. "Where fields have been sprayed the lambs stay orf-free; where they have not been sprayed orf remains an ongoing problem."

While curbing the spread of orf is a key reason to choose Thistlex, the yield advantages should not be ignored, according to Scottish distributor David Cairns of MS&P Agriculture.
 
"A 10% infestation of thistles in a sward will reduce grass production by 1.1 tonnes of dry matter per hectare - wasting a valuable home-grown feed resource," says Mr Cairns.

"With many sheep farmers cutting back on fertiliser because of the price, grass yields will be down this year. So it will be very important to make the most of the fertiliser they do use, and the grass that grows.

"Controlling thistles with a long-lasting, grass-safe herbicide like Thistlex will not only increase yields for grazing, but also improve the quality of any forage that is made from the grass.

"For good control - treat actively growing thistles at the rosette stage; with little stem and four to ten leaves showing. If they have developed beyond this stage, it is better to top them and spray the regrowth six to eight weeks later."