Meta-analysis of stray voltage on dairy cattle.

A quantitative assessment of dairy cow responses to contact current (stray voltage) at 50 or 60 Hz was conducted using meta-analysis and pooled analysis methodology. The objective was to more accurately quantify the minimum exposure level (threshold) at which dairy cows respond and to identify sources of heterogeneity among studies. Several medical and agricultural databases were used to locate individual studies for the systematic literature review, from which 22 published studies of stray voltage and behavioral response or milk yield met our inclusion criteria. Meta-analysis models were constructed to assess the percentage of cows with a behavioral response at documented exposure levels, and the summary relative risk estimate for all exposure pathways combined was calculated for each 1-mA increment from 1.0 through 5.0 mA. The meta-analysis of percentage response showed that cows exhibited statistically significant first behavioral responses at 3.0 mA, response probability increased with exposure levels, and exposure pathways contributed to heterogeneity in the model. The pooled analysis of mean behavioral response threshold was based on experimental studies of ascending series of current exposures on 355 cows. The overall weighted mean for first behavioral response to current was 4.0 mA. Ten of the studies that met the inclusion criteria addressed milk production, but heterogeneity in exposure pathways, patterns, and duration precluded meta-analysis or data pooling. The milk production studies ranged in size from 4 to 48 cows and used switchback or paired design to increase power. A qualitative narrative review of these studies indicated that production was not affected by exposure to contact current at levels of 3 mA or lower for exposures of up to 21 d or 4 wk.