Simulating the hierarchical order and cow queue length in an automatic milking system

The effect of cow hierarchical order on cow queue length in an automatic milking system (AMS) was quantified using a validated simulation model.A typical herd of 67 Israeli Holstein–Friesian cows was observed during 3 months (15 000 milkings, June–August 2006). Milking data were quantified in terms of a gamma probability distribution, which is an input to the simulation model. The herd population was divided into three hierarchical ranks; the lower-tenth percentile (10th percentile) comprised the “lowest-ranked cows”, e.g., heifers, those with leg or hoof problems, or weaker animals that could not fight their way to the robot. A low-rank cow would wait until the queue had cleared before it entered a milking stall. A dominant cow (the upper 10th percentile) would not wait in a queue, but would push aside the lower-ranked cows until it reached the head of the queue. Within a rank, the cows enter a robot according to their arrival sequences.Two typical days were simulated: (1) a regular day on which the cows arrived randomly throughout the day and night, (2) a crowded day caused because the robot has been stopped or blocked for a certain period, or all the cows pass through the robot to reach the pasture.The use of a simulation model allows for full repeatability, so that a long experiment can take only few seconds of computer simulation time. It also enables parameters that are difficult to control in reality to be controlled. The results showed that that social competition between cows influences the timing of visits to the robot i.e., the cow milking frequency. Low-ranking animals generally have to wait longer than dominant animals before they get access to the robot. The simulations showed that during a regular day a low-rank cow waited 68.9 min in the queue, compared with only 10 min for a middle-rank animal, and 3.5 min for the dominant rank. On a crowded day, an average middle-ranking cow queues for 93.5 min while a low-ranking cow waited about 412 min. Introducing a software feature that prevented return milking before 80% of the cows have milked reduce the length of queue by 1 h.