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  4. 1996
  5. August

Nitrogen Fertilization of Small Grains for Pasture

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Forage yield data from a replicated nitrogen (N) experiment on a rye/wheat/ryegrass mixture established by Jerry Rogers and Wadell Altom on the Red River Farm in the 1970's show that the response to applied N is linear up to the highest rate used (200 lbs of actual N per acre). The more N one applies, the more forage is produced (see Figure 1).

In other words, the forage response to applied N is a straight line that continues to increase and is highest at the 200 lb per acre rate. Results also show us that we can expect about 15 to 20 lbs of forage dry matter (above the check) per lb of applied N.

Let's make some assumptions: 1) a 400 lb stocker calf converts 10 lbs of forage into 1 lb of gain and has an animalforage harvest efficiency of 75%, 2) N costs $0.28 per lb of actual N using ammonium nitrate. The study results show that the unfertilized check produced 2015 lbs per acre of total dry matter when averaged over the 14 year period. If this forage was converted to gain based on the above assumptions, we could generate about 150 lbs of gain.

When we consider that a fall application of 100 lbs of N per acre produced an average of 3965 lbs of total dry matter and work through the same calculation, about 300 lbs of gain are possible. The difference is 150 lbs of gain resulting from application of 100 lbs of N per acre.

The cost of applied N is $31 (100 lbs x $0.28 per lb of N = $28 per acre + $3.00 per acre application cost = $31 per acre total N cost). If anhydrous ammonia was the N source used and the application combined with a tillage operation prior to seeding, the cost of 100 lbs of N per acre would be around $20.

If custom gain is worth $0.35 per lb, then the value of the additional gain would be about $52.50. When we subtract the cost of N, we get a net per acre benefit of around $21.50 from fertilization. Another way to look at it is the break-even price for gain.

If the benefit from 100 lbs of N is 150 lbs of gain, and for $31 N cost, the break-even price for gain is $0.207 ($31 N cost/ 150 lbs of gain = $0.207 per lb). It is important to note that these numbers include many assumptions, and are based on long-term average data. Under dry conditions, forage production response to applied N can be considerably reduced. However, over the long run, N fertilization pays.

Production Tip: Due to dry conditions and poor yields we experienced this spring and summer, soil samples should be taken from fields which will be planted to small grains in the fall. The results of the residual nitrate-nitrogen analysis should be scrutinized carefully. If significant carry-over N is detected, the residual can be credited directly against N requirements of the next crop.