Fertilizing forage crops in 2009

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By Rick Greenwell

Forage producers around the world have struggled over the last year with high fertilizer prices. I’ve had many tell me, “I can’t afford to fertilize my hay or pastures this year. I’ll go broke.” While there is some truth to this statement, in the long term, you will definitely “go broke” if your forage crops don’t have the necessary nutrients for sustained growth. I don’t claim to be a soil science expert, but I would like to provide some basic principles on how to manage the nutrient inputs and outputs on your farm.

First of all, everyone should take soil tests on a regular basis. In hay fields, soil tests should be taken every year, since nutrient removal in hay is high. For well managed pasture fields, soil tests are only recommended about every three years. I have heard some of you say, “No point in taking a soil test, since I know I can’t afford to apply expensive fertilizer.” I would counter that you can’t afford not to take a soil test, especially when fertilizer costs are high. A soil test may tell you that one field does not need any fertilizer, another field only requires P, another field only needs lime, and a fourth field has very low nutrient levels so you may consider planting a forage like annual lespedeza which will grow in low pH and low P soils. Taking a soil test will allow you to make educated decisions on your farm rather than adding fertilizer that is not needed, or reducing fertilizer applications on fields that need it the most. And some of you may be fortunate and find that you can “mine” the nutrients for a few years and hope that fertilizer prices come down (as they have recently for certain nutrients).

If you decide that you are going to cut back fertilizer applications remember that hay crops remove more nutrients than almost any other commercial crop. In the book “Southern Forages” nutrient removal from various forage crops is shown in a table on page 85. For example, a 5 ton/acre alfalfa hay crop removes 280 lbs. of nitrogen/acre, 75 lbs. of phosphate, and 300 lbs. of potash. Nitrogen removal is not a problem with alfalfa because N fixation occurs in the nodules on alfalfa roots, but P and K level will drop in that field. Fortunately, it takes the removal of 5 to 7lbs./acre of potassium/acre and about 10 lb./acre of phosphate for soil test levels to drop by 1 lb/acre. In other words, there are more nutrients tied up in the soil than show up on the soil test report. Remember though that even if your soil test levels are high today, hay crops will eventually deplete the nutrients in the soil if you are not replacing them with commercial fertilizers or manure.

In pastures approximately 80 percent of the nutrients consumed in the forage are returned to the pasture in the manure and urine. Therefore, fertilizer requirements on pasture are lower than for hayfields, but this is only true in well managed rotationally grazed pasture where manure and urine are equally distributed throughout the pasture. If livestock continuously graze one field all season, then nutrient redistribution occurs because much of the manure and urine end up around the pond and shade trees and the rest of the field becomes nutrient deficient.

During the last couple of years the price of nitrogen increased rapidly (N fertilizer is produced using natural gas) and therefore the interest in planting legumes also increased. Nitrogen fertilizer prices have dropped, but it still makes more sense to add legumes than to rely on only N fertilizer. Not only do the legumes provide N “for free” from the air, but legumes are usually higher quality than grasses resulting in higher rates of gain and higher milk production. Legumes have tremendous advantages, but they also require high pH levels (with the exception of the lespedezas). Just like the rest of life, “you don’t get something for nothing.

(Source: Ray Smith)