Quick Guide to Calculating the Fertilizer Value in Hay
Hay and other livestock feeds contain fertilizer nutrients. Knowing how to calculate the content of the major fertilizer nutrients nitrogen (N), phosphate (P 2O 5) and potash (K 2O) in feeds can be very useful.
Fertilizer labels give the percentage of nitrogen, phosphate and potash in the fertilizer. For example, the diammonium phosphate (DAP) label is 18-46-0, meaning there are 18 pounds of N, 46 pounds of phosphate, and 0 pounds of potash in 100 pounds of fertilizer. However, feed labels give the nutrient content in terms of crude protein (CP), elemental phosphorus (P) and potassium (K).
Nitrogen from Crude Protein
When analyzing feed for crude protein, the feed is analyzed for nitrogen, and then, the nitrogen content is multiplied by 6.25 to estimate CP. We can calculate the N content in the feed by dividing CP by 6.25. Hay containing 10% CP in the dry matter (DM) has 32 pounds of N per ton of DM.
10% CP in DM = (0.10 CP × 2,000 pounds per ton of hay DM) ÷ (6.25 pounds of CP per pound of N) = 32 pounds of N per ton of DM
If the hay is 90% DM, that is 28.8 pounds of N per ton of hay as fed.
32 × 0.9 pound of DM per pound as fed = 28.8 pounds of N per ton of hay as fed
P 2O 5 from Phosphorus
We convert elemental phosphorus to phosphate (P 2O 5) by multiplying P by 2.3. If the hay has 0.26% P in the DM, there is almost 12 pounds of phosphate in a ton of hay DM.
0.26% P in DM = 0.0026 P × 2,000 pounds per ton of hay DM × 2.3 = 11.96 pounds of P 2O 5 per ton of DM
K 2O from Potassium
We convert elemental potassium to potash by multiplying K by 1.2. If the hay has 1.88% K in the DM, there is over 45 pounds of potash (K 2O) in a ton of hay DM.
1.88% K in DM = 0.0188 K × 2,000 pounds per ton of hay DM × 1.2 = 45.12 pounds of K 2O per ton of DM
To convert this to pounds per ton as fed, this value is multiplied by the percent of DM in the feed. If the above K 2O was from baleage containing 55% moisture, which is 45% DM, it would be:
45.12 × 0.45 = 20.3 pounds of K 2O per ton of baleage as fed
Why It Matters
The content of fertilizer nutrients in feed has practical meaning when we look at their value in a ton of hay. Typical grass hay dry matter contains 10% crude protein, 0.26% phosphorus and 1.88% potassium or in terms of fertilizer nutrients, 32 pounds of N, 12 pounds of P 2O 5 and 45 pounds of K 2O per ton of dry matter. When N costs $0.575, P 2O 5 costs $0.5674 and K 2O costs $0.4075, the total value of nutrients in a ton of this hay is:
(32 × $0.575) + (12 × $0.5674) + (45 × $0.4075) = $18.40 + $6.81 + $18.34 = $43.55 per ton of hay DM
If hay costs $90 per ton, containing 90% DM, then there is $39.20 per ton worth of fertilizer in the as fed hay (43.55 × 0.9 = 39.2), which is 44% of its cost.
Knowing how to calculate the content of fertilizer nutrients in feeds enables the manager to know that the value of fertilizer nutrients is being taken off in a crop or brought on to the farm in purchased feed.
Author: Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy
Last Reviewed: March 2022