PastureMap calculates Forage Forecasts for your pastures to help you better manage and plan your grazing. These forage forecasts are based on USDA NRCS Web Soil Survey rangeland productivity data. This article explains how these forecasts are calculated and how to use them to improve your grazing management.
Calculating rangeland productivity
First, PastureMap identifies the soil map units that lie within a pasture. Each map unit represents a different soil type. A map unit contains one or more soil components. Each soil component can have annual rangeland productivity estimates in pounds per acre of dry matter for high production (wet), typical, and low production (dry) years. PastureMap calculates a weighted average of the soil components and map units within your pasture to arrive at total forage production for wet, normal, and dry years.
Some soil components such as cropland or forest do not have rangeland productivity data. In addition, USDA NRCS data is only available in the United States. If more than 10% of a pasture is missing NRCS rangeland productivity data, PastureMap cannot provide an accurate Forage Forecast.
For example, if half of a pasture is in a map unit with a component that has 1500 lb/ac of forage and the other half is in a map unit with a component with 500 lb/ac of forage, then the total forage production:
0.5 ⨉ 1500 lb/ac + 0.5 ⨉ 500 lb/ac = 1000 lb/ac
Converting total forage production to forage ingested
The next step is to convert total forage production in pounds per acre to ingested forage, the amount of forage animals actually eat. A rule of thumb for effective grazing management is to “take half, leave half”, or 50% utilization. Following this principle, if a pasture produces 1000 lbs/acre of dry matter during the grazing season, 500 lbs/acre will be grazed and 500 lbs/acre will be left as residual. Not all of the 500 lbs/acre that is utilized will be ingested by the animal, however. Some of that is wasted through trampling, desiccation, manure and urine, and bedding. The fraction of total rangeland productivity that is actually ingested is called harvest efficiency.
Harvest efficiency typically ranges from 25-30% for continuous grazing on rangeland to 35-40% for high intensity managed grazing or grazing cropland. NRCS recommends 25% harvest efficiency as a conservative starting point for forage estimation. The PastureMap Forage Forecast starts with 25% harvest efficiency. If you have different assumptions for utilization and harvest efficiency, you can adjust your estimate in PastureMap accordingly.
For example, if you have 1000 lb/ac of total forage production and 25% harvest efficiency, the amount of ingested forage is:
1000 lb/ac ⨉ 0.25 = 250 lb/ac
Converting forage ingested to animal days
PastureMap can convert forage from lbs/acre to animal unit days (ADs), animal days per acre (ADAs), and animal unit months (AUMs).
One AD is the amount of forage consumed by a standard animal in one day. This typically represents 3% dry matter intake for a 1000 lb animal, or 30 lbs of dry matter per day. You can adjust these values in your ranch settings in PastureMap if you use a different definition for your operation. ADs is calculated by multiplying ADAs by the number of acres in your pasture. For example, if you have a 100 acre pasture with 1000 lb/ac of total forage production, 25% harvest efficiency, and standard animals consuming 30 lbs of dry matter per day, you can calculate ADs as follows:
1000 lb/ac ⨉ 0.25 ⨉ 100 / 30 lb = 833.3 ADs
One AUM is the amount of forage consumed by a standard animal in one month. Thirty ADs equals one AUM. So if you have 833.3 ADs, you can calculate AUMs as follows:
833.3 ADs / 30 = 27.8 AUMs
One ADA is the number of animal days of forage produced by each grazeable acre of pasture. If you have a pasture with 1000 lb/ac of total forage production, 25% harvest efficiency, and standard animals consuming 30 lbs of dry matter per day, you can calculate ADAs as follows:
1000 lb/ac ⨉ 0.25 / 30 lb = 8.3 ADAs
Note that if your pasture has less than 100% grazeable acres, the ADA number will be greater.
Improve grazing management using forage forecast
Once you have estimated the available forage for your pastures, you can plan out the number of grazing days for your herds. You can see if you have sufficient forage to stock additional animals and increase your profits. When adding planned moves, PastureMap provides a stocking calculator to help you calculate how many days your herd can graze in a pasture. You can calculate stocking days based on ADs and the number of animal units in your herd. For example, if you have 100 animals that weigh 500 lbs in your herd and are using 1000 lb standard animal units, you can calculate the number of AUs as follows:
100 ⨉ (500 lb / 1000 lb) = 50 AUs
If your pasture has 300 ADs, you can calculate the number of grazing days for this herd as follows:
300 ADs / 50 AUs = 6 grazing days
When you add planned moves, PastureMap provides a stocking calculator to do all this math for you. Just enter the ADs from the forage forecast and we'll do the animal unit conversions for your herd automatically.
In addition, you can use the forage forecast to adjust your grazing plan during the season. Every time you graze a pasture, PastureMap shows you how much forage your herds have consumed, how much you have planned, and how much is remaining. This lets you identify underutilized pastures that you can graze again so you’re not leaving money on the table. This can also alert you to potential drought conditions so you can destock early.
NRCS provides additional guidelines for forage estimation and grazing planning. If you'd like to learn more, read this article on Estimating Initial Stocking Rates from NRCS.