University of Vermont

Nitrogen Management of Pasture Nitrogen Management on Pasture

Sid Bosworth, Extension Associate Professor, Department of Plant and Soil Science, University of Vermont sid.bosworth@uvm.edu



Grasses have a high demand for nitrogen (N).  Grass hay trials have shown that some grasses will respond in yield up to 300 and even 400 lbs of actual N per acre per year.   The economic “break even” application rate of N depends on the cost of N fertilizer and the value of the hay crop.  For hay, this is generally between 100 and 200 lbs N/acre split two to four times per year, depending on the particular grass specie, soil type and weather conditions.  For pasture, the decision to apply N becomes more complicated.  Here are a few points to consider when making N fertilizer decisions.
 
  • A Management Tool - Think of N fertilizer as a short term management tool for producing temporary increases in pasture dry matter.  It usually shows an immediate response and then it is gone.  For your farm, you may decide that you don’t need N at all or you only want to use it on certain acres for certain times of the year.
  • Stocking rate - Do you need the extra forage that will be produced by adding N fertilizer?  Some farms have more land than their animals need.  Adding N would just add to the pasture waste.  On the other hand,  if your operation is stocked right at that point where most summers are a stretch, then N fertility may be an option to increase forage dry matter.
  • Legume content - Yields from legume forages such as clover, alfalfa or trefoil are usually not limited by a lack of nitrogen since they “fix” N from the air.  If your pasture has over 30% legume (which is a lot more than you might think), you are not as likely to see a yield increase by adding additional N fertilizer.  In fact, continual additions of N will cause a shift from a legume-based to a grass-based pasture, thus, increasing your reliance on N fertilizer to maintain production.   You may decide that a $30 per acre investment in lime which boosts your legume growth is a better way of adding “N” to your pasture compared to a $30 investment in N fertilizer which only has a short term effect.  The difficulty is that you can see the immediate affect of N, where as, the response to lime is much more subtle.
  • Application Rate - For most applications, a rate of 50 lbs N per acre per application works well.  You will usually “see” a good yield response at this rate in terms of growth and color and yet it is not a high enough rate that you would need to be concerned with nitrate accumulation.  Your actual fertilizer rate will depend on the fertilizer formula you are using.  To get 50 lbs of N, divide by the % N in your formula.  For instance, urea is 46-0-0; therefore, you would need to apply 109 lbs (50 divided by 0.46) to get 50 lbs N.  For ammonium nitrate (34-0-0), apply about 150 lbs. for a 50 lbs. N rate.
  • When to Apply N and When to Graze - If you are rotating pasture, you get your best growth response by applying right after the animals leave the pasture.  For continuous pasture, you may want to split your applications at lower rates, perhaps 30 lbs N, just to reduce any risk of nitrate accumulation.
  • Early Spring Application - Applying N in early spring can boost early spring growth for earlier grazing.  However, you would not want to treat too many of your acres or you will have a mountain of pasture on your hands come late May!   If possible, use your best drained land to do this.  Extremely wet soils causes N in the soil to convert to a gas (called “denitrification”) which is lost to the air.
  • Summer Boosts -  Applying N in early to mid June can usually give a growth boost in summer pasture growth unless it is an extremely dry year.  On average, you might expect a 20 lb increase in dry matter for every pound of N applied; therefore, a 50 lb rate would boost yields by 1000 lbs per acre.  With good grazing management, this could potentially provide 30 to 35 additional grazing days per animal unit (1000 lb animal).  Remember, N is not the answer to grass growth during dry years.
  • Fall Stockpiling - Allowing certain pastures to accumulate growth in late summer and fall is a good way to “extend” the grazing season into late fall.  If your pasture is predominately orchardgrass or tall fescue, applying 60 lbs of N per acre by early to mid August, can really boost your stockpiled forage.  If your pasture is mainly bluegrass, apply 40 to 50 lbs N.   Remember, this needs to be done by mid-August to really see the yield increases for late fall grazing.


This site is maintained by Sid.Bosworth@uvm.edu, Plant & Soil Science Department, University of Vermont.

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Last modified May 26 2004 01:29 PM

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