On Missouri grazing operations, cows that calve in February produce 2,552 more pounds of milk over the course of the lactation than cows that calve in April. Part of the reason, says Stacey Hamilton, extension dairy specialist at the University of Missouri, is that cows reach peak milk when grass quality is high and in abundant supply. Therefore, these cows achieve higher peaks, and that leads to more overall milk production. The chart above shows the difference in the lactation curves.