Calculating the correct carrying capacity of a pasture will help you stock to a level that maintains the health and productivity of both your land and animals. Following are six steps for calculating carrying capacity.
Taking samples of forage in the field is the first step in calculating the capacity of the pasture. Clip samples of forage within an area 50 x 50 cm (about 20 inches square) from different areas of the pasture that have not been grazed. Dry the samples and weigh once dry.
Calculate the approximate forage yield based on the weight using the formula:
Dry forage grams x 35.6 = lbs./acre. (For example, if you had 50 grams of dried forage x 35.6, that equals 1,780 lbs. of dry forage per acre).
It’s best to clip in mid July and mid September to build up data from a number of years and get a picture of the average forage yield that is representative across varying conditions.
2. Utilization Rate
The utilization rate determines how much forage is used or lost to grazing, trampling, insects and wildlife. This helps determine how much material needs to be left behind to maintain future production. (see table 1 and 2)
Take clippings from grazed areas and compare yield to the clippings from non-grazed areas, to give an idea of the amount utilized during grazing. This shows how much plant material was left behind after grazing, and the ideal amount to allow for rapid, good-quality re-growth will vary depending on the time of year. For example, in mid-July leaving about two thirds of the plant material behind after grazing is ideal to maintain the quality and volume necessary for the herd.
Utilizing pasture at a rate that exceeds the plant communities’ ability to cope will promote weeds, lower forage production and encourage less palatable and productive species to invade the pasture.
Determining a suitable utilization rate can be a bit tricky but here are some general guidelines:
3. Dry Matter Intake
Determining the dry matter intake means taking into account, as an animal increases in size, so does its feed requirement. Cattle will eat between 1.5 – 3.5 per cent of their body weight per day on a dry matter (DM) basis. The palatability, nutrient profile and availability of the feed will determine where along this scale your cattle will fall.
For a cow with a calf, 2.5 per cent is commonly used DM intake average. Calves don’t need to be accounted for until they reach about 600 lbs. For grassers three per cent is commonly used figure.
750 lbs. steer x 3.0% = 22.5 lbs. DM/day
1850 lbs. cow x 2.5% = 45 lbs. DM/day
The mature large cow will eat over twice as much as the steer even though she consumes less feed as a per cent of her body weight. (see table 3)
4. Carrying Capacity
Once the amount of forage yield and utilization rate have been determined, the carrying capacity can be calculated, which can be used to determine either the total forage available and/or the livestock forage requirements.
There are then two ways to use the information, either to determine the number of head a system can carry or to determine how many days a specific herd can graze in the system. (see table 4 and 5)
To determine litter yield, rake all of the brown litter in an area 50 x 50 cm square (about 20 inches square), making sure not to include any green material. There are two ways to determine whether the amount of litter present is optimal, either by visually comparing the amount collected or by calculating the weight of the litter using the same formula as used for the grass yield lbs./per acre.
The amount of litter left on the land can make a big difference in the performance of the pasture. Litter includes ungrazed residue from previous year’s growth, residue from bale grazing, fallen stems, leaf material and other partially decomposed material. Litter helps to conserve moisture by reducing evaporation, improving infiltration and cooling the soil surface.
There are a number of Canadian websites that provide more information on calculating forage and litter values. Manitoba Agriculture has an excellent site at: http://www.gov.mb.ca/agriculture/crops/forages/bjb00s17.html
Alberta Agriculture has a good website for calculating forage litter at: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/for8656
Saskatchewan Agriculture has good information at: http://www.agriculture.gov.sk.ca/Default.aspx?DN=94318c24-72b0-4b0e-814e-f476fec9e2c6
And of course, if you just Google the topic of calculating carrying capacity, it gives you many more website options.
Managing pastures to get the maximum benefit from them is crucial to meet the needs of the individual herd and farm. It’s not possible to have maximum lbs./day weight gain per cow and maximum lbs./acre gain for the pasture. Producers will need to determine what they want to achieve and manage accordingly.
It is important to rotate the cow herd through pastures at intervals that allow adequate rest periods to achieve rapid, high quality re-growth.
Pasture management must take into account many variables, like how much bush pasture is on the land, the type of operation and goals of the individual farmer, but trying to maintain and balance the resource against the needs of the cows will produce better long term results. †