Hg. Allore et al., DESIGN AND VALIDATION OF A DYNAMIC DISCRETE-EVENT STOCHASTIC SIMULATION-MODEL OF MASTITIS CONTROL IN DAIRY HERDS, Journal of dairy science, 81(3), 1998, pp. 703-717
A dynamic stochastic simulation model for discrete events, SIMMAST, wa
s developed to simulate the effect of mastitis on the composition of t
he bulk tank milk of dairy herds. Intramammary infections caused by St
reptococcus agalactiae, Streptococcus spp, other than Strep. agalactia
e, Staphylococcus aureus, and coagulase-negative staphylococci were mo
deled as were the milk, fat, and protein test day solutions for indivi
dual cows, which accounted for the fixed effects of days in milk, age
at calving, season of calving, somatic cell count (SCC), and random ef
fects of test day, cow yield differences from herdmates, and autocorre
lated errors. Probabilities for the transitions among various states o
f udder health (uninfected or subclinically or clinically infected) we
re calculated to account for exposure, heifer infection, spontaneous r
ecovery, lactation cure, infection or cure during the dry period, mont
h of lactation, parity, within-herd yields, and the number of quarters
with clinical intramammary infection in the previous and current lact
ations. The stochastic simulation model was constructed using estimate
s from the literature and also using data from 164 herds enrolled with
Quality Milk Promotion Services that each had bulk tank SCC between 5
00,000 and 750,000/ml. Model parameters and outputs were validated aga
inst a separate data file of 69 herds from the Northeast Dairy Herd Im
provement Association, each with a bulk tank SCC that was greater than
or equal to 500,000/ml. Sensitivity analysis was performed on all inp
ut parameters for control herds. Using the validated stochastic simula
tion model, the control herds had a stable time average bulk tank SCC
between 500,000 and 750,000/ml.