The answer depends on the time of year, number of animals, vigor of the pasture, and animal production goals, including body condition and milk production.
Cool-season pastures grow most rapidly in the spring. Dry matter can increase as much as 100 to 120 pounds per day in a vigorous pasture with good soil fertility. Average growth through the season will be more like 30 to 50 pounds of dry matter per day. Warm-season grasses and annual grasses also show seasonal variation in growth patterns.
It is easiest to work through an example of calculating your rotation rate and rotational grazing paddock size. Assume we have 100 cows, and grass is growing at 60 pounds of dry matter per day (1 inch every 5 days). We want to turn in the cows when the grass is about 8 inches high and move them when they have it down to about 3 inches (5 inches at 300 pounds dry matter per inch = 1,500 pounds dry matter). If we want the cow to eat 40% of her dry matter as pasture, we would expect her to consume about 20 pounds of dry matter per day. So our group will consume 2,000 pounds per day and needs 1.3 acres per day or 2.6 acres for two days. If grass is growing at 40 pounds per day as it slows down later in the summer, it will take 37 days to re-grow the 1,500 pounds dry matter for the next grazing.
When grass is growing faster, for higher milk production, let the milking cows eat the best grass and follow with heifers or dry cows to get it down uniformly to 3 inches. As grass growth slows in the summer, move the cows more often and increase the rest period between grazing.
Many dairy graziers have permanent fencing around large pastures and use temporary electric fencing to subdivide these into a daily allocation. Temporary electric fencing provides great flexibility as pasture conditions change throughout the growing season. The watering system also must be designed with this approach in mind.
More detailed and complete information on labor effieciency and pasture management systems for dairy heifers can be found at the DAIReXNET publication entitled Labor Efficient Pasture Management.
