Final answer:
The average lactation length for dairy cows is 305 days, which is essential for dairy farmers to manage milk production.
Step-by-step explanation:
The average lactation length for dairy cows is D) 305 days. Lactation is an important aspect of dairy farming, as it determines the period during which a dairy cow produces milk after giving birth. This period is crucial for dairy farmers like Megan who run dairy farms and need to keep track of milk production. After calving, the cow will continue to produce milk for about 305 days before the lactation cycle ends and the cow is given a rest period in preparation for the next calving and lactation cycle.