Design and performance of statistical process control charts applied to estrous detection efficiency

Statistical process control (SPC) charts to monitor production processes have not been widely used in dairy management. Shewhart and cumulative sum (cusum) control charts were designed to determine true changes in estrous detection efficiency (EDE) amidst normal variation in dairy cattle. A stochastic simulation model was used to track performance over time of individual cows in herds of 100 and 1000 cows. Estrous detection ratios (EDR), calculated as observed estruses divided by estimated estrous days (in periods of 1 to 60 d), were used to monitor EDE. Control charts for EDR, using normal and binomial distributions, were designed at 0.65 EDE for both herd sizes; then EDE was set to 0.65 (no change), 0.55, 0.45, or 0.35 and average days to the first detection signal (ATS) in 400 runs was determined. Observed ATS at 0.65 EDE could differ from the target ATS, depending on the SPC chart design and estimated proportions of estrous days for inseminated cows. Observed ATS were shorter for larger changes in EDE and for the 1000-cow herd. Observed ATS for a change to 0.55 EDE were approximately 300 d (100 cows) or 60 d (1000 cows) with the cusum charts. For a change to 0.35 EDE, observed ATS were approximately 50 d (100 cows) and approximately 11 d (1000 cows). Shewhart charts performed similarly or took longer to signal changes depending on period length. Observed ATS on cusum charts were much longer than minimum when non-optimal reference values were used in the design. Observed ATS were also longer when SPC charts were designed with a longer target ATS and change in EDE was small. Control charts using normal and binomial distributions generally performed similarly. Statistical process control charts detected changes in estrous detection efficiency soon enough to be potentially useful in dairy management.