In order to get milk from animals, the animals must give birth. At around the time of the arrival of the rainy season, when the fields become covered by green and the nutritional conditions improve, the animals get pregnant. However, even if different types of animals get pregnant at the same time, the gestation duration differs and they give birth at different times. The Figure (below) shows the ratio of adult females producing milk to the total number of adult females, and how this ratio changes over a year. As you can see, having several types of animals is effective in eliminating the period without milk supply.