Increasing the productivity of your pastures can mean the difference between having to destock when drought hits and making it through the grazing season without spending a fortune on hay. By tracking your pasture productivity over time, you can evaluate the effectiveness of your grazing management, prepare for drought scenarios, and create more profitable grazing plans.
This blog post gives a general overview of how to calculate stocking rate for your pastures. This help article will go into more detail about the math.
Over the course of a grazing season, your pastures produce forage, which feeds your animals. The more forage you grow, the more animals your pastures can support. Therefore the amount of forage in a pasture determines its carrying capacity, which is the number of animals that can be supported in that pasture for a given duration. We can measure forage directly in terms of pounds of dry matter. This can be measured by clipping grass and calculating the amount of dry matter contained within the clipping.
Standard animal units
In order to estimate carrying capacity, it is often much more useful to convert pounds of forage into an estimate of the number of animals that amount of forage can support. Before we can do that conversion, however, we need to define the concept of a Standard Animal Unit (SAU). A standard animal unit defines the amount of forage consumed per day by a standard animal. For example, this may be a 1000 lb animal that consumes 3% of its body weight in dry matter each day, or 30 lbs of forage per day. You can change these values in PastureMap to fit your operation under your Ranch Settings.
Converting forage into carrying capacity
Once you know the pounds of dry matter a pasture can produce and the amount of forage per day consumed by a Standard Animal Unit, you can convert your forage into carrying capacity. We can measure carrying capacity in terms of the number of days it would take for a Standard Animal Unit to consume that amount of forage. This is called an Animal Day (AD). This can also be converted into an Animal Unit Month (AUM) by dividing ADs by 30, the typical number of days in a month.
Calculating pasture productivity
In order to compare the productivity of pastures of different sizes, it's useful to measure the carrying capacity per acre. You can do this by dividing ADs by the grazeable acres in the pasture to get Animal Days per Acre (ADA). In PastureMap we assume that your pastures are 100% grazeable, but if you want to exclude a certain fraction of your pasture from your pasture productivity calculations because you have a pond, structure, or other ungrazeable areas in your pasture you can edit the % grazeable acres field for a pasture.
Calculating pasture productivity from grazing records
We described how to calculate your pasture productivity based on the amount of dry matter in a pasture. We can also do the same calculation based on the actual animals that grazed in the pasture. If you know how much grass each animal eats per day and how long it grazed in the pasture, you can add up the total amount of grass consumed to arrive at the amount of forage produced by the pasture. PastureMap can do this calculation for you if you put in your animal weights, average daily gain (ADG), and estimated dry matter intake percent (DMI %). We calculate the amount of forage consumed by each animal by computing:
Dry Matter Weight = (Animal Weight) x (DMI %) x (Grazing Days)
We can then convert this forage dry matter weight into ADAs as above.