Final answer:
A cow is most susceptible to develop a new intramammary infection during the dry period and the early postpartum period.
Step-by-step explanation:
The two most susceptible times for a cow to develop a new intramammary infection are during the dry period and during the early postpartum period.
During the dry period, which is the period between lactation cycles, the cow's udder is not being milked and is more prone to bacterial invasion. This can lead to new infections if proper hygiene is not maintained.
During the early postpartum period, which is the time immediately after calving, the cow's udder is undergoing significant changes as it transitions from a non-lactating state to producing milk. This makes it more vulnerable to infection.