Nitrogen – the N in nutrient management
Date of publication : 11/12/2008
Source : Cornell University PRO-DAIRY
As environmental regulations require dairies to control nutrients, producers must find ways to decrease nitrogen and phosphorus excretions.
To control nitrogen (N) and phosphorus (P) excretions, managers must formulate rations that minimize the import of these nutrients. And they must do that without sacrificing milk production, herd health, reproduction and profit. This is possible, as answers to the following questions prove.
Q: What is the N balance on New York dairy herds?
A: A number of surveys show that 60 to 75% of the total N imported onto dairies, primarily in feed and fertilizer, never leaves. Feed N accounts for 45 to 85% of the total N imports. Between 25 to 35% of the total N intake ends up in milk.
Q: Can producers lower the N balance?
A: Yes, as a survey conducted in Maryland shows. Herds surveyed were feeding N at levels of 8 to 17% in excess of requirements.
Q: What opportunities exist to shift N balance?
A: By fine-tuning protein feeding, producers decrease the quantity of N imported via feed and lower the quantity of N excreted. The Cornell Net Carbohydrate and Protein System (CNCPS) model can be used to make ration adjustments. The net effect of using the model is to increase the efficiency of N use and microbial protein synthesis in cows. The latter is done by adjusting carbohydrate metabolism in the rumen.
Microbial protein is a high rumen bypass feed that is about 65% crude protein on a dry matter basis. It also has a methionine to lysine ratio similar to that found in milk. The CNCPS also lowers ration crude protein levels by 1 to 2 percentage units.
Q: How much N excretion can be shifted?
A: Computer simulations have shown that yearly N excretion can be decreased by 40 to 60 pounds per cow. This is 4,000 to 6,000 pounds of N per year for a 100-cow herd. If a dairy applies 100 pounds of N per acre from manure, producers would need 40 to 60 fewer acres with this decrease in N.
This assumes no loss between the cow and the field. Realistically, 40 to 60% of the N excreted by cows can be lost, primarily through volatilization, before manure is spread.
Predicted N excretion has been reduced by 20 to 30% in herds where the CNCPS model is used to adjust rations.
Q: What do field research results indicate?
A: A trial at the U.S. Dairy Forage Research Center in Wisconsin evaluated four protein-feeding strategies over a full lactation with cows producing about 25,000 pounds of milk per lactation. Total N intake was decreased about 50 pounds per cow without affecting milk production. Yearly N excretion was also reduced by about 50 pounds per cow. This research data fits with CNCPS simulations.
Decrease N excretion
1. Have a consistent supply of high quality forage available.
2. Use silage harvest and storage methods that lower soluble protein levels.
3. Feed forage NDF at 0.95 to 1.05% of bodyweight.
4. Balance the rumen proteins and carbohydrates to maximize microbial protein synthesis.
5. Use feeding management practices to stimulate high dry matter intake.
6. Consider multiple total mixed rations for lactating cows
By Larry Chase, associate professor
Department of Animal Science - Cornell University
Total Dairy Management (The Manager newsletter - Northeast Dairy Business)
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