Don’t let stray voltage affect cow production and cell counts

Dairy farmers could improve milk production and quality by keeping a close check on stray electricity voltage in their milking parlours and collecting yards.

And, according to New Zealand service technician Tim McDonald, mistakes as simple as using tape to connect an electric fence to collecting yard steel can generate enough voltage to upset cows when they are about to be milked.

During a recent visit to the UK, Mr McDonald tested for stray voltage at 23 dairy farms. “I didn’t find one that was completely clear so, when you interpret those numbers on a national level, you could say most farms in the UK have a problem,” he says.

Stray electricity occurs when voltage is unable to make its way to the main earthing system and a natural path for voltage to travel back to earth in a milking parlour is the steelwork. Water, which is used in large volumes in the parlour, exacerbates the problem.

“When five standings are affected by stray voltage and you add water to the mix, it will double the affected area,” says Mr McDonald, Corkhill Systems.

Research shows cows are sensitive to just 0.5V and this in turn impacts on production and somatic cell counts. Stray voltage results in cows receiving a tingle or a mild shock that disturbs their normal behaviour.

“When a cow is nervous she won’t let go of her milk so it stays in the udder until the next milking. By the time the cow releases that milk it has been in the udder for several hours and is not of the best quality, so cell count levels will be affected.”

Herdsman Simon Musson of Vaynor Farm, Narberth, Pembrokeshire milks a spring-calving herd and suspected there was a problem with stray voltage when cows were mucking more than usual. “They were nervous before they came into the parlour and, after having the equipment tested, we now know this was caused by voltage from an electric fence running through the steel barriers in the collecting yard.”

He hadn’t picked up on the problems because, although cows and humans can be similarly sensitive to electric current, cows are more susceptible to stray voltage because they have much lower body impedances.

Mr Musson followed the advice he was given by Mr McDonald and replaced the tape securing the fence with timber fencing. The problem has now been eliminated.


Mr McDonald says farmers should avoid having an electric fence active while milking and instead should install a relay system which switches the power off at milking times. An electric fence controller should never be installed in or near a milking parlour and electric fences should always have a separate earth at least 20m from the mains earth.

The biggest problems Mr McDonald has come across are in rotary parlours, due to the many variable-speed drives associated with this type of system and the positioning of the milk pump on the platform. “The drives are not suitable for use in a parlour unless they are correctly fitted,” he says.

The worst case he has come across is a reading of 14V in an 80-point rotary parlour which had an incorrect drive. The herd’s cell count reading was a phenomenal 650,000 cells/100ml. “We fitted the correct drive and the cell counts plummeted,” says Mr McDonald.

The usual source of stray voltage is faulty wiring. This could be due to cracked insulation, incorrect grounding or overloaded circuits. But new wiring incorrectly installed could be a possible source too.

Problems can also arise from milk float stems that protrude into the milk vessel. Each time the pump starts up it creates a spike of electricity. “It may be a case of replacing it with a variable speed pump which runs all the time, slowing down or speeding up according to how much milk is going through the system so there isn’t a spike,” says Mr McDonald.

In some cases, stray voltage can also be brought in through power lines.

Testing the system to identify the sources means the problem can easily be rectified, says Mr McDonald. “The solutions to dealing with stray voltage are so often simple and easily done.”

Symptoms of stray voltage

• Excessive or unusual nervousness and excessive movement

• Reluctance to enter or eagerness to leave the milking parlour

• Increased frequency of defecation and/or urination in the milking parlour

• Reluctance to consume water

• Poor milk let-down

• Increased milking time

• Lowered milk production

• Increased somatic cell counts and incidence of clinical mastitis

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