An existing dynamic and stochastic bio-economic model of intramammary infection (IMI) caused by Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, and Escherichia coli was extended to model the dynamics of IMI during the dry period. These dynamics were modeled on the basis of a Greenwood model. The extended model was used to calculate the cost-effectiveness of different dry period interventions in relation to the annual net costs of IMI in a herd of 100 dairy cows at the start. The interventions were blanket dry cow therapy (BDCT) as the default scenario, BDCT combined with teat sealant (TS), selective dry cow therapy (SDCT) or TS, and SDCT combined with TS. Input parameters of the dynamics of IMI during the dry period and the economic parameters were based on literature. The costs of clinical and subclinical IMI during lactation, clinical IMI during the dry period, and the cost of intervention were used to calculate the combined total annual net cost of IMI per herd. The combined total annual net cost of IMI using the other intervention scenarios was compared to the default scenario (BDCT) to estimate the cost-effectiveness of the other intervention scenarios. Sensitivity analysis was conducted on the parameters involved. The results showed that a considerable number of cows acquire new IMI during the dry period and start the new lactation with IMI. Furthermore, the average combined total annual net cost of IMI per herd using BDCT was € 8800, distributed as € 4561 due to clinical IMI, and € 3063 due to subclinical IMI during lactation, € 88 due to clinical IMI during the dry period, and € 1088 due to antibiotic therapy and labor costs at dry off. On average, application of BDCT combined with TS resulted in € 378 higher combined total annual net cost of IMI compared to the BDCT scenario. Similarly, the SDCT scenarios resulted in higher average combined total annual net costs of IMI compared to the BDCT scenario. The SDCT or TS was on average € 443 higher and the SDCT combined with TS was on average € 635 higher. Sensitivity analysis results showed that the rate of new IMI during the dry period was the most influential parameter on the average combined total annual net cost of IMI. Although the difference between the 4 simulated scenarios was minor, on average BDCT had the lowest combined total annual net cost of IMI.