A direct measure of manure production from storage volume or counting loads spread is the most accurate way to estimate manure production. However, if a direct measurement of actual manure production is not possible, the daily production figures in Table 1.2-13 can be used to estimate manure production for the farm. In calculating manure production, remember to account for animal growth. For example, if you are raising hogs from weaned pigs to market weight, you would need to use an average weight of the growing animals, not the market weight, to estimate manure production. If you don’t know the average weight of your animals, see Table 1 in Agronomy Facts 54: Pennsylvania's Nutrient Management Act: Who Will be Affected?, for typical average animal weights. You also need to account for changing populations of animals on the farm during the year. Examples might include buying feeders and selling steers at different times of the year, or the down-time between broiler flocks. See the example below for estimating manure production using the figures in Table 1.2-13 and Table 1 in Agronomy Facts 54.
Example manure production calculation:
6 flocks of 20,000 broilers per year on the farm for seven weeks per flock
Manure production for medium broilers (Table 1.2-13) is 22 lbs manure/AU/day.
Average medium broiler weight is 2.3 lb/broiler (from Table 1 in Agronomy Facts 54).
Use the following formula:
(Animal Number × weight per animal ÷1000) × Manure produced per AU per day × Days manure is produced
(20,000 broilers × 2.3 lb/broiler ÷1000) × 22 lb manure/AU/day × (6 flocks × 7 weeks per flock × 7 days per week) ÷ 2,000 lb/ton= 149 tons of manure produced per year
Use the total production figure, along with the nutrient contents given in Table 1.2-13 (or better yet, from a manure analysis), to calculate the total nutrients produced in the manure that can, with proper management, contribute to the farm’s fertility program. Dividing this total production figure by the acreage that received manure also is a crude check on the per-acre application rate of manure and the nutrients it contains.
Reference: The Agronomy Guide - Section 2: Soil Fertility Management