Cows, ground surface potentials and earth resistivity

The "stray voltage" problem on dairy farms is discussed briefly. By reference to published literature it is shown that a "step voltage" (SV), i.e., a potential difference between front and hind hooves of a dairy cow, much less than the often quoted threshold value of 0.5 V, when applied for long periods of time, could possibly affect cow health and milk production. Values as low as approximately 10 mV could conceivably be significant. A measurement program carried out on 19 representative Minnesota dairy farms during the summer of 1997 is described. Nine farms had above average (HP) and 10 below average (LP) milk production. Results show that SV was 4.2 times higher on the LP than HP. However, only three farms had SV greater than 9 mV, and low milk production on these farms could possibly be due to absence of recommended vaccinations rather than high SV. Soil resistivity rho measured in the farm fields was 3.4 times larger on the LP than on HP. The possible origin of SV in relation to electric distribution systems within and to farms is discussed. Relations between SV and rho are analyzed. Conditions are specified under which SV in the barn must be related to rho measured in the field, rather than the rho of the concrete floor of the barn. It is suggested that laboratory research is necessary to establish the significance for cow health and milk production of long term exposure to low SV levels.