Physiological and Behavioral Effects

Cows and humans appear to be equally sensitive to electric current; that is, both respond similarly to similar current levels flowing through their respective bodies. However, cows are much more susceptible to stray voltages because their body impedance (resistance) is much lower than those of humans. In accordance with Ohms law, therefore, a given voltage across a lower body impedance results in a larger current.

The voltage needed to deliver these currents depends on the following:

  • The body impedance of the cow,
  • The contact impedance between the cow and the conductive structures,
  • Any impedance of the conductive structures, and
  • The impedance of the voltage source.

A conservative estimate for a worst-case total-series impedance would be to assume that all impedances are zero except for body and contact impedances. The absolute maximum current that a stray voltage can produce is that voltage divided by the sum of the contact and body impedances. Recall that animals respond to the current produced by a voltage and not to that voltage directly. A number of studies have been carried out to characterize impedances for cows. Impedances vary from cow to cow and depend on the points of contact, e.g., mouth to all four hooves or front leg to rear leg. Under conditions likely to be encountered on a farm, a conservative estimate for contact plus body impedances is 500 ohms. A more realistic estimate for total circuit impedance is 1,000 ohms.

The effects of electric current on cows depend on the following:

  • The characteristics (magnitude, duration, and waveform) of the current,
  • The electrical properties (impedance) and sensitivity of the cows through which the current passes, and
  • The environmental conditions.

Furthermore, the sensitivity of cows can vary depending upon their past experiences. In humans, current levels equivalent to those associated with stray voltage are perceived as one or more of the following:

  • Vibration (tingling),
  • Burning sensations, and
  • Pain.

We can only assume that perceptions of cows are similar. Prior experiences will most definitely influence how a cow responds to current. For example, the response to any novel stimulus will be exaggerated; therefore, the initial response of cows to even a low-intensity electric current is often exaggerated and sometimes even appears theatrical. If the current is sustained, the cows will quickly adapt; and after a few minutes, behavior will appear normal.

If, instead, currents of short duration are repeated, the greatest response will be at the onset of each application of current; and after a few episodes, even the onset will not elicit a response. A cow's life history will also influence how she responds to electrical current. For example, previous traumatic experiences will tend to diminish responses to current.

Although cows do not respond uniformly to electrical currents, 97 percent of cows in one test showed a general uniformity in their behavioral responses to currents of different intensities. Researchers have measured physiological variables such as heart rate and endocrine variables such as levels of oxytocin, catecholamines, and prolactin in plasma in a number of stray voltage experiments on cows. In general, no meaningful change in any physiological or endocrine variable was detected. A few cows did show questionable responses, but only when current levels
exceeded 8 mA.