Fertilizing Forages: Using Nutrients Wisely

Mark Landefeld, OSU Extension Educator, Monroe County (This article appeared first in the April, 2016 issue of The Ohio Cattleman’s magazine)

Applying fertilizers to hay and pasture fields to stimulate plant growth is a common practice to substantially increase forage yields.  This is a sound management practice if application is made in accordance with soil test results and or expected harvest yields.  Applying more nutrients than recommended from a soil test can be expensive and detrimental to the environment.

A single application of 15-15-15, or 19-19-19 fertilizer on each field year after year does not make good use of your purchased nutrients.  Why?  Nutrient levels of the soil may be different in each field and grasses and legumes need nutrients in different quantities for their most beneficial growth. Legume plants do not need the same amount of nitrogen applied to the soil as grass plants because they can make use of nitrogen from the atmosphere. Also, different amounts of each nutrient (phosphorus and potassium) are removed in each ton of forage harvested.

Meadows for example – Each ton of tall grass or legume forage harvested removes approximately 13 pounds of phosphorus(P) and 50 pounds of potassium(K) from the soil.  That means these nutrients need to be replaced, preferably in the ratio of about one part P to four parts K.  A single yearly application of fertilizer, as mentioned above, would not build soil nutrients in the proper ratio to offset those removed in the hay.

If we expect to harvest four tons of hay a year from each acre, we would remove 52 pounds of actual P and 200 pounds of actual K from the soil on each acre. Applying a total of 275 pounds of 19-19-19 fertilizer per acre would meet our P needs (275 x .19 = 52 lbs.), but it would be about 148 pounds short of the K we need to replace (what we removed in four tons of forage).  An additional application of 250 pounds of Potash (0-0-60) per acre should be applied to maintain our nutrient balance.  P and K may be applied to the soil any time of the year if soil conditions are firm enough to hold equipment, but after first cutting has been removed may be the best.

Our 275 pound application of 19-19-19 fertilizer would also contain 52 pounds of nitrogen (N) per acre, adequate maybe for grass stands in one application, but too much N for a field that contains 35-40 percent, or more, legume plants.  As farm managers we need to understand how the wrong fertilizer applications can be costly from the financial aspect and we must minimize unwanted effects to the environment.

That is why split or multiple applications of blended fertilizer with the appropriate analysis should be made. Multiple applications made during the growing season are an effective method to maximize growth and reduce N losses.

Producers should use split applications if you have sandy soils with high potential for leaching, if you run the risk of volatilization of N, or if high rates of fertilizer are to be used. Surface volatilization occurs in some forms of N when it breaks down and forms ammonia gases.  The rate of surface volatilization depends on moisture level, temperature and pH of the soil surface.  If the area is wet when a fertilizer is spread and additional rain is not soon expected, evaporating water can pick up the ammonia released from the urea and be lost.  Temperatures greater than 50°F and pH’s greater than 6.5 can significantly increase the rate of urea conversion to ammonia gasses.  Do not broadcast urea based fertilizers to fields where lime has been surface applied within the last 12 months, especially if two or more tons of lime per acre was applied, unless the fertilizer is incorporated.  Nitrogen loss will occur.

To stop ammonia volatilization from urea, the urea must be tied up by the soil.  To get urea in direct contact with soil it is beneficial to have enough rain to wash the urea from plant residue into the soil.  If residue is very light, .25 to .50 inch of rain is enough to dissolve the urea and wash it into the soil.  If heavy plant residue covers the soil .50 inch or greater rainfall is required.  Other forms of N such as ammonium sulfate or ammonium nitrate could be used to reduce volatilization, but may be hard to find unless you make special purchasing arrangements.

Pastures – Grasses and legumes in a pasture require the same nutrients as those in meadows.  Do you need to put as much fertilizer on your pastures as you do your hay fields?  Probably not, because as cows consume forages they also return approximately 66% of the phosphorus and 90% of the potassium to the field in their manure and urine.

Are the nutrients evenly distributed back on the pasture?  This depends a great deal on your management.  When cows are confined to a specific area for a short period of time, rotationally grazed, studies have shown manure deposits to be evenly distributed over the entire area.  If cows are continuously allowed to roam over large areas as they choose, manure deposits (your nutrients), are moved to the loafing areas where the herd spends most of the day.  This may be around watering troughs or in a tree line/woods if livestock are given the opportunity.

Fertilizer applications in pasture fields should be done at times that are determined by your livestock’s forage needs.  If you need additional forage early in the spring, fertilizing up to one-third of your pastures may be needed.  Generally, grass and legume forages grow quickly in the spring (spring flush) without additional fertilizers.  Early spring fertilization in pastures may compound problems associated with spring grass growth such as grass tetany.  Late May to early June is usually a better time to spread fertilizer on pastures to boost production before hot and dry weather arrives in July and August.

Many farm managers also make fertilizer applications to a portion of their pastures again in early August then remove the livestock. This allows them to stockpile the forage growth for use later in the fall and winter to reduce stored feed needs.

So what does all this mean?  Usually a specific blend of nutrients and proper timing of application are needed in most fields to maximize production.  In most cases this requires a soil test and planning for the application to be done at a time when you will not lose a large portion of the N to volatilization.  Multiple applications may be warranted to make the best use of the nutrients.  Cost of nutrients must also be considered when trying to determine the best fertilizer to use.