Impact of epidemiological, zootechnical, managerial and price factors on the economic efficacy of subclinical mastitis treatment during lactation

A partial budget model was designed to assess the main parameters, not linked to the drug, influencing the economic interest for the farmer to treat subclinical mastitis with antibiotics during Lactation. The model assumes that a treatment curing subclinical infections consequently reduces the herd infection prevalence, the bulk milk SCC, the incidence of new infections and clinical mastitis by contagious pathogens, the rate of incurable infections and increases the incidence of environmental mastitis. These effects on the herd mastitis status result in an improved milk quality for SCC, an extra milk production, Less culled cows and globally Less clinical mastitis treatments. On the other hand, the new treatment generates an extra expense of antibiotics and more unmarketable milk. More than 30 epidemiological, zootechnical, managerial and price parameters were hierarchically ordered according to the amplitude of variation of the economic result when each single parameter varied between extreme values, the other parameters remaining fixed at medium. The three most influencing parameters were the initial CMT scores of the treated quarters, the initial bulk milk SCC and the SCC penalty scheme. The treatment benefit reached a maximum for a BMSCC value situated between the penalty thresholds and decreased towards negative figures when SCC moved away below the lowest or above the highest threshold. The other main influencing parameters were the parity of the treated cows (negative), the rates of spontaneous cure (negative), the on-farm valorisation of the unmarketable milk, the relative risk of new infection in cured quarters (negative), the price of freshening heifer, the treatment cost of a clinical mastitis by contagious pathogens, the cow depreciation time for milk production. Unexpectedly, the proportion of treated infections due to Staphylococcus aureus relative to streptococci and the shortage of milk to produce the farm quota had few influence.