Stray voltage is a low-level current in animal housing that most commonly affects dairy cows and swine. Behavioral changes, such as avoidance of the affected area, are typical clinical signs. Diagnosis is based on observing behavioral changes and performing electrical testing. To prevent the problem, an electrician should perform periodic inspection and testing of voltage/current levels, especially when changes to the farm's electrical system occur.
The term "stray voltage" describes a special case of voltage developed on the grounded-neutral system of a farm. According to the USDA publication Effects of Electrical Voltage/Current on Farm Animals, “stray voltage is a small voltage (less than 10 volts) measured between two points that can be contacted simultaneously by an animal" (1). For example, between a watering trough and the barn floor, a cow could become the "conduit" for the current to pass from the trough to the floor.
Because animals respond to the current produced by a voltage and not to voltage directly, the voltage must be high enough to produce current flows greater than the threshold current needed to pass through the animal and elicit a response. For this reason, stray voltage is also referred to as animal contact current.
Contact voltages < 10 V are not lethal to farm animals or humans in normal farm contact situations. Neutral-earth voltage levels > 10 V may be indicative of a fault in the farm electrical system and require careful evaluation by a competent electrician to identify and mitigate any potential safety hazards.
The grounding and neutral systems on a farm or in a home wiring system should be properly bonded into the earth to ensure electrical safety. As a result, some level of voltage between the grounded-neutral system and the earth (neutral-to-earth voltage) is always present as a normal consequence of the operation of properly installed electrical equipment.
Ohm's law describes the relationship between voltage exposure and current conducted through an animal: [current = voltage / resistance] or [Amps = Volts / Ohms]. This simple relationship has been a source of much confusion and resulting controversy (see Diagnosis). The current measurement most commonly used is the milliamp (mA), or 1/1,000 of an amp. Applying Ohm’s law, if a cow standing on a barn floor were exposed to 1 V, given that the typical level of resistance for a cow is 500 ohms, the current flow through the cow would be 2 mA (0.002 A).
The term stray voltage is often applied incorrectly to other electrical phenomena, such as electric fields, magnetic fields, electrical current flowing in the earth (earth currents), or electrical current flowing on a grounding conductor (ground currents).
Clinical Findings of Stray Voltage in Animal Housing
Over the past 70 years, a great deal of research has been conducted on the effects of stray voltage on dairy cows. A variety of behavioral responses have been reported in cows exposed to voltage and current. No one response is pathognomonic, however, because these behaviors can also be caused by other factors in the animal's environment. The ways to determine whether stray voltage is a potential cause of abnormal behaviors are to perform electrical testing and a thorough evaluation for other possible causes of aberrant animal behavior, such as the following:
hierarchical competition
housing density
ventilation
quality of housing and bedding
heat stress
fly control
feed management
At exposure to currents at levels that are perceptible only to animals, these animals may display behaviors (eg, flinches) indicative of perception of this current, with little change in their normal routines. At higher exposure levels, they may show avoidance behaviors. The severity of response depends on the amount of electrical current flowing through the animal’s body, the pathway it takes through the body, and the sensitivity of the individual animal. Indirect effects of these behaviors can vary considerably, depending on the specifics of the contact location, level of current flow, body pathway, frequency of occurrence, and many other factors related to animals' daily activities. All of the documented effects of excessive voltage exposure have been behaviorally mediated.
A review of 22 studies of cows' behavioral and milk production responses to stray voltage found that the most sensitive cows (< 2%) begin to show mild behavioral responses after exposures to 2 mA of current applied from muzzle to hoof or from hoof to hoof (approximately 1 V in wet locations or > 2 V in dry locations) (2). For muzzle-to-hoof exposure of 4 mA (2 V cow-contact voltage in wet locations or 4 V in dry locations), approximately 30% of cows will show some behavioral responses but not avoidance behaviors. The most sensitive 2% of cows begin to exhibit avoidance behaviors at muzzle-to-hoof exposure of approximately 5 mA. Similarly, exposure of cows to 1.8 V (3.6 mA) for 8 weeks did not decrease water intake or milk production when compared to nonexposed cows (3).
Increased levels of stress-related hormones have been documented in some, but not all, cows at voltage/current exposures substantially higher than the threshold required to elicit avoidance behaviors. Despite a large body of research, no studies have supported the hypothesis that voltage exposures of up to 8 V will result in increased somatic cell counts in milk (suggestive of mammary gland inflammation) or incidence of mastitis.
Swine and sheep are similar to cows in the way they respond to voltage/current exposure. Behavioral modification in swine has been observed for exposures of greater than 5 V, with avoidance behaviors at exposures greater than 8 V (4). However, exposures of 8 V or less did not impair the welfare, reproductive performance, or health of sows or suckling pigs. Thus, if stray voltage is suspected to be causing behavioral changes in livestock, practitioners must consider the level of electrical exposure. Lower levels of voltage (approximately 2 V) may cause mild behavioral changes, while exposures of 5 to 8 V may cause transient aversive behavior but are not likely to impair animal performance or well-being.
Pearls & Pitfalls
|
Diagnosis of Stray Voltage in Animal Housing
Electrical testing
Observation of animal behaviors
If a contact voltage (eg, a water trough) reaches sufficient levels, animals may exhibit avoidance behaviors. At watering locations, these behaviors manifest as decreased numbers of drinks per day along with increased length of time per drink. Exposures at feeding locations are uncommon unless metallic feed bowls are used. Hoof-to-hoof exposures at building transitions (eg, entry into milking parlors from a holding pen) may result in reluctance to move in or out of buildings. If avoidance behaviors are observed, applying industry-recognized electrical test methods is necessary to confirm a stray voltage diagnosis.
Standardized test protocols have been developed to determine stray voltage exposure levels at locations that are accessible to animals. Measurements at animal contact locations are required to determine the level of voltage/current that animals might experience. Industry-recognized field investigations include voltage measurements at animal contact locations (cow contact voltage) with an appropriate shunt resistor to estimate current exposure levels (animal contact current).
The standard method to estimate the amount of electrical current flowing through an animal uses a 500-ohm shunt resistor across animal contact points to represent the resistance of a cow’s body plus the contact resistances at the muzzle and hooves. This is meant to estimate the current flowing across the muzzle-to-hooves pathway for a cow standing on a wet floor. Cows standing on a dry surface typically have body plus contact resistance of ≥ 1,000 ohms. Cows standing or lying on dry bedding have a resistance many times higher than this.
Industry standard protocols also include measurements that allow for determination of the source resistance in the animal contact circuit as well as the contribution to animal contact voltage from both on- and off-farm sources. Neutral-to-earth voltage measurements are useful for diagnostic purposes. Animal contact measurements are typically about one-third of neutral-earth voltage levels. Animal contact- and neutral-to-earth voltage levels are typically monitored for 24 hours or more to capture the range of normal electrical system operating conditions on a farm.
All of the voltage and current levels cited above are for steady-state 60 Hz alternating (AC) voltage/current reported as root-mean-square (RMS) average values. As the AC frequency increases or the duration of a current pulse gets shorter, more voltage and current are required to cause the same response. The measurement of high-frequency events requires specialized equipment and careful measurement technique consistent with industry-recognized stray voltage testing methods.
The main cause of high-frequency/short-duration electric pulses on farms is improperly installed electric fences and electrified crowd gates. These devices are designed to produce a powerful electric impulse to control animal behavior. Improper installation of such devices can cause these pulses to appear in unintended areas of the farm. The other common source of high-frequency events is a switching transient that occurs when electric equipment is turned on or off. These high-frequency pulses decay quickly and do not travel far from their source, and it is extremely rare for them to reach problematic exposure levels.
Although stray voltage may lead to behavioral changes such as reluctance to drink or to enter a milking parlor, other possibilities for these changes (hierarchical competition, design of facilities, improper milking protocols) should also be evaluated. In the case of suspected decreased water consumption, water meters should be used to validate actual water intake and to compare to cattle's estimated needs from dry matter intake, milk production, ambient temperature, etc.
Low levels of electrical exposure have not been shown to be the primary cause of such pervasive problems as low milk production, poor growth, high somatic cell counts, and increased mortality rates. For example, if a dairy client is frustrated with decreased milk production, veterinarians should consider the multifactorial, and more typical, causes of this problem and carefully identify them with prescribed methods that rely on evidence-based diagnosis. Similarly, no published reports have linked stray voltage to impairment of the immune system in livestock.
Prevention and Control of Stray Voltage in Animal Housing
Regular maintenance, inspection, and testing of farm electrical equipment
The best way to prevent stray voltage problems is to ensure that the farm's electrical system meets electrical code requirements. Equipotential planes are required at critical animal contact locations in animal confinement facilities and effectively eliminate contact voltage even when substantial levels of neutral-to-earth voltage are present. Additionally, all farm equipment requires maintenance and repair when damaged, and the farm electrical system is no different. Periodic inspection and testing of voltage/current levels by the electric power supplier or a qualified electrician are recommended, especially when changes to the farm's electrical system occur.
Most US states and Canadian provinces use 2 mA at cow-contact locations as the level of concern for stray voltage exposure. This 2 mA standard is further defined to include a standardized measurement of 1 V of steady-state 60 Hz AC voltage/current measured across a 500-ohm resistor at animal contact locations. This exposure level is recognized as a conservative, preventive level, below the point at which moderate avoidance behavior is likely to occur and well below the level at which a cow's behavior or milk production would be affected. This level is not a damage level. As stated in the USDA publication on the effects of electrical voltage, “research indicates that current levels below 6 mA have no direct effect on production, reproduction, or animal health; furthermore, there is no evidence that hormones naturally released during milking and stress are adversely impacted by elevated current levels. Some moderate behavioral changes are observed in cows exposed to currents of between 3 mA and 6 mA” (1).
The summary recommendation from the USDA is that contact voltages should be kept below 2–4 V (steady-state 60 Hz measured across a 500-ohm resistor) on dairy farms to avoid any adverse effects on animal behavior, milk production, and animal health (1).
Pearls & Pitfalls
|
It is typical for voltage levels to vary considerably through the normal daily operation of electrical equipment on a farm. Decreased feed and/or water intake will result only if current exposure levels at watering and feeding locations are sufficient to produce avoidance and if they occur often enough to interfere with drinking and eating behaviors. If an avoidance-level current occurs only a few times per day, it is not likely to have an adverse effect on cow behavior. The more often avoidance-level current exposures occur in areas critical to drinking or feeding, the more likely that activity is to affect the cows.
Common deficiencies in farm electrical systems include the following:
loose connections
ground faults (shorts)
undersized wiring
unbalanced loads or wiring damaged by animals
accidents
moisture
corrosion
Faults or electric code violations that could pose an electrocution hazard should be corrected immediately to protect both animals and humans. If stray voltage levels are excessive, a competent investigator should assess the situation to determine the most practical, safe, and efficient way to decrease animal contact voltage.
Key Points
The causes of and solutions for stray voltage are well understood, as are the effects of electrical exposure on farm animals.
Research shows that dairy herds display no adverse responses when cow contact voltage levels are below 2 V.
Investigation of stray voltage problems should be systematic; solutions should follow accepted codes and practices for farm electrical distribution systems.
Before attributing impaired livestock performance to low levels of electrical exposure, consider all other possible contributing causes.
For More Information
Minnesota Stray Voltage Guide: A Guide Addressing Stray Voltage Concerns. 2015.
Hoben PJ, Hendrickson RC. Final Report of the Science Advisors to the Minnesota Public Utilities Commission: Research Findings and Recommendations Regarding Claims of Possible Effects of Currents in the Earth on Dairy Cow Health and Milk Production. Minnesota Public Utilities Commission. 1998.
Reinemann DJ. Stray voltage and milk quality: a review. Clin North Am Food Anim Pract. 2012 Jul;28(2):321–45.
Stray Voltage. Public Service Commission of Wisconsin,
References
Lefcourt, AM, Ludington D, Aneshansley DJ, et al, eds. Effects of Electrical Voltage/Current on Farm Animals: How to Detect and Remedy Problems. USDA Agricultural Research Service. Agriculture Handbook No. 696. 1991. https://docs.idahopower.com/pdfs/Safety/StrayVoltageBooklet.pdf
Erdreich LS, Alexander DD, Wagner ME, Reinemann D. Meta-analysis of stray voltage in dairy cattle. J Dairy Sci. 2009;92(12):5951-5963. doi:10.3168/jds.2008-1979
Rigalma K, Duvaux-Ponter C, Barrier A, et al. Medium-term effects of repeated exposure to stray voltage on activity,stress physiology, and milk production and composition in dairy cows. J Dairy Sci. 2010;93(8):3542-3552. doi:10.3168/jds.2009-2903
Robert S, Matte JJ, Martineau GP. Sensitivity of reproducing sows and suckling pigs to stray voltage. Am J Vet Res. 1996;57(8):1245-1249.
