Grazing part two - paddock sizes and number of acres
AND NOW… THE TECHNICAL PART OF GRAZING MANAGEMENT!
HOW TO ESTIMATE DRY MATTER INTAKE AND CALCULATE THE SIZE AND NUMBER OF PADDOCKS NEEDED.
HOW MUCH PASTURE (DRY MATTER) ARE THEY EATING?
Ideally, a pasture should be grazed rapidly down to 1 - 4 inches, and then permitted to grow back up to 6 to 10 inches. A good quality pasture (with high plant density) which is 6 to 8" tall has about 2400 lbs. of dry matter (D.M) per acre. If it is grazed down to a 1 to 2" height, it has about 1200 lb. D.M./acre left. This means there is 1200 lb. D.M./acre available to the grazing livestock to eat in that paddock.
The density of the plants in the pasture will have a large influence on how much dry matter there is in your pastures. If the plants are spaced far apart (low density) so that you can easily see soil between the plants, there may only be 2000 lbs. of dry matter per acre when the pasture is 6" tall (instead of 2400 lbs.). The best way to learn how to estimate dry matter is to go to some pasture walks, or host a walk on your farm. The density and quality of your pastures will increase as you practice good grazing management!
The amount of pasture D.M. an animal will eat depends on a lot of factors (lactation, growth, animal size, supplemental feed ...). Three percent of body weight in D.M. intake is a reasonable estimate for lactating cows grazing on well managed pasture. This means a 1000 lb. cow will eat 30 lb. of pasture D.M. per day.
To prevent overgrazing of plants and maximize dry matter intake, move animals frequently, so that each paddock is not grazed for more than three consecutive days. Grazing periods of 12 to 24 hours result in much higher pasture and livestock productivity.
WHAT SIZE SHOULD THE PADDOCKS BE?
Paddock size depends mostly on how many animals you have, and how long they will be in the paddock. Refer to the pasture worksheet at the back of this article. EXAMPLE: You have 50 cows, each weighing 1000 lbs., so each cow will need an estimated 30 lbs. of pasture DM per day (3% of 1000 lbs.). You plan on giving them a fresh paddock after each milking. You let the paddocks grow up to 6 to 8" (2600 lbs. DM/acre) before grazing, and graze them down to 1 to 2" (1200 lbs. DM/acre), so you have 1200 lb. of D.M. per acre (2600 - 1200 = 1200) for the cows to eat (this is a high quality pasture with a high plant density). The 50 cows need 1500 lb. D.M. per day (30 lbs. DM x 50 cows = 1500 lbs.). So they need 1.25 acres each day (1500 lbs. required per acre / 1200 lbs. available per acre = 1.25 acres). Since you will give them a paddock after each milking, you need to set up paddocks which are 0.63 acres of an acre each (1.25 / 2 = .63). An acre is 43,560 square feet, or about 210 feet on a side if it is a square. 0.63 acre is 27443 square feet (43560 x .63 = 27443), which is a square that is about 165 feet on a side (the square root of 27443).
HOW MANY PADDOCKS TO I NEED?
Research done by Bill Murphy on a Fairfax Vermont (Champlain Valley) Farm on high quality pasture produced the following average regrowth periods during a 3 year period (of reasonably average rainfall). The regrowth periods on your farm may be shorter or longer... in a dry summer, the regrowth periods may easily reach 60 days or longer!
12 to 15 days in late April to early May
18 days by May 31
24 days by July 1
30 days by August 1
36 days by September 1
42 days (and longer) by October 1
These numbers are just averages, the actual amount of time needed for complete regrowth will probably be different on your farm. However, in general the pastures grow quite quickly in spring and early summer, and more slowly as the growing season continues.
EXAMPLE: In this example, on this Fairfax farm, if there are 50 cows that need 1.25 acres per day, total acreage needed to graze in spring will be 19 acres. (1.25 x 15 days = 19 acres) When pasture growth slows down to 35 to 40 days, acreage needed will increase to 44 to 50 acres. (1.25 x 35 days = 44 acres and 1.25 x 40 = 50 acres).
If total acreage is not increased as regrowth periods increases, plants won’t get enough rest, and dry matter intake by animals will drop, resulting in poor animal and pasture performance.
For grazing planning on your farm, it is helpful to keep records of how often, and for how long your graze each paddock each year. Then you can know how long it takes the paddocks on your farm to grow back during the grazing season. You can keep these records on a copy of your farm map, or by using a notebook or worksheet where you write down the date, where the animals grazed, and for how long. These records will allow you to know exactly what the rest periods are in your pastures.
Records of past grazing are only useful if you use them! Walk through all your pastures at least once each week, and record how much each pasture has grown back. This way you can plan what order to graze your paddocks in.