Daily feed costs don't rise as fast as milk production following calving and don't fall as fast after peaking. Higher producing cows maintain a profitable level of production over feed costs for a longer interval after calving than lower producing cows. Putting a number to 'how many days is fewer' is not simple because of the intricacies of these complex factors. Further research is needed to clarify this question. However, sharp dairy managers continually monitor the production and reproduction of their cows to make economically sound decisions on when to quit breeding cows and assign them to the 'cull' list.
Most cows decrease in milk production with advancing lactation.
Several factors contribute to this decline, but one factor is pregnancy. Cows that are pregnant will normally decline in production faster than cows that are open. This affects the economics of culling cows in two ways. Cows that are left open during their lactation can be expected to produce more milk than cows who become pregnant. Recent research from Iowa State University used about 175,000 records to evaluate the effect of days open on milk production during the current lactation.