PO 54: Know how to calculate total animal manure production on a livestock farm.

Manure quantities and analyses are needed to properly allocate manure to cropland on an annual basis.  Manure production can be measured based on the size of the storage, from records of how many loads of manure of a given size are hauled each year, or from animal inventories and production levels on the farm.

Animal excretion plus other additions to process wastewater uses estimates of per-animal excretion to estimate total manure production.  To use these factors, you need to determine the number of animal units (AU) that are producing manure and the number of days in the manure collection period.  You may need to divide the animals into groups based on different size, age, storage or handling systems, etc.

Animal unit calculation:

• For growing animals, use the average weight during the collection period.
• For example: hogs growing from 45 lb to 245 lb would have an average weight of 145 lb.  With 500 pigs: AU = 500 x 145 / 1000 = 72.5 AU
• Use the table (on the next page) to determine daily manure production rates.
• Finishing pigs produce 11 gal manure/AU/day.  Thus for these pigs: 72.5 AU x 11 gal/AU/day = 797.5 gal/day
• This number is multiplied by the number of days that manure is collected.
• If manure is collected and spread every 4 months (120 days): 120 days x 797.5 gal/day = 95,700 gal to spread every 4 months
• Any additions to the manure, such as rainwater, washwater, bedding, etc. should be added to this total for the collection period.