Richard Taylor, Extension Agronomist; rtaylor@udel.edu
This year, with very high hay prices and short supplies, there is a need for early pasture for grazing to stretch tight budgets and short hay supply. One of the few ways to stimulate growth in pasture is the application of nitrogen (N) at, or just before, pasture spring greenup. Even when N was applied in the early to mid-fall period to stimulate root system expansion and provide pasture grasses with stored N for early spring regrowth, an additional application of N just at greenup can be useful in promoting early pasturage.
A question often asked is whether it’s economical and safe to use granular urea on pastures at this time of year. To answer the economics in the question you need to understand what happens when urea is applied over top of a pasture. If conditions are favorable, urea applied to a pasture can react with water from the soil or vegetation and the ever present enzyme, urease, to convert into ammonium carbonate. Ammonium carbonate is a very unstable form of fertilizer N that breaks down spontaneously into ammonium (NH4+) or ammonia gas (NH3), if the pH is alkaline, water, and carbon dioxide. The ammonium then is either taken up by plants, or it attaches to the cation exchange sites on clay and soil organic matter, or is acted on by the nitrifying bacteria to become nitrate (NO3–). If conditions favor it staying ammonia, this is lost to the atmosphere and effectively raises your cost per pound of N. Urea frequently has the lowest cost per pound of N but if much N loss occurs the savings will be eliminated.
Conditions that favor ammonia loss, besides the presence of plant material that provides the urease enzyme, include warm temperatures (especially 70°F. and higher), high humidity or a moist soil surface, and high soil pH where the prill or urea granule rests on the soil. On Delaware soils where the pH is often maintained between 5.5 and 6.5 for pastures and where air and soil temperatures are cool to cold at this time of year, the loss of N from urea fertilizer is minimal. In fact when I worked in the Deep South, pastures or hay fields were fertilized with urea rather than ammonium nitrate all the way into April as long as the temperatures did not warm up into the mid to upper 70s. Through March at least in Delaware, fertilization with urea should be the most cost effective way to provide N for pastures since losses will be minimal.
What about animal health concerns? Since urea, like other fertilizers, is a salt, animals can become ill if they gain access to bags of urea fertilizer and consume too much of it. As long as the applicator practices safe handling and storage principles and ensures that the fertilizer is evenly spread without large clods, animal safety should be ensured. For those that prefer to err on the side of more caution, we suggest that they keep animals off a fertilized field until it has received from ¼ to ½ inch of rainfall. Rainfall or irrigation water will move the urea quickly into the soil eliminating any concerns for animal health; and, at the same time, will reduce or eliminate the concern with ammonia volatilization.
Another way to get pastures off to a fast start, which also plays into the above health concern, is to keep animals off pastures early in the greenup period to promote more growth. As an analogy, think of a tiny little tomato seedling. It can double in size a number of times but until it reaches a critical size the doubling amounts to only a very small increase in dry weight of the plant. Pastures that are grazed even before the permanent grasses green up in the spring will produce little useable forage compared with a pasture that is fertilized and then allowed to grow to a height of 3 to 4 inches before being lightly grazed, rested a couple of weeks and then grazed again. If the grazing animals are removed when 3 inches of pasture remains, recovery and the pounds of dry matter produced per day will be much greater than that of a pasture kept constantly at a grazed height of 0.5 to 1 inch. It may mean using more hay initially but once the pasture reaches that 3 to 4 inch height, it often will produce more feed per day than your animals will consume.
Once you begin grazing a pasture, the best thing you can do to promote growth is to practice rotational grazing where you allow animals on a subdivision of your pasture for a short period, usually no more than 3 to 5 days at most, and then remove the animals to another subdivision while the plants in the recently grazed subdivision rest and recover and renew growth.
Another suggestion is to take that soil test sample you’ve been meaning to get and send it in for analysis. Soil tests should be taken at least every three years and as often as every year at the same time of year each time. The soil test will help you decide if you need to correct a pH problem or apply nutrients to relieve any nutrient deficiencies. If the pasture soil pH level has declined below 6.0, an application of lime will help both grasses and legumes grow better.
I mentioned N fertilization earlier. How much N should you apply? This does depend a bit on the pasture you are fertilizing and your goal for that pasture. Where you either have too much legume (clover) or where you have so little clover that is isn’t contributing N to the surrounding grass, an application of about 100 lb urea per acre (this is about 46 lb N/acre) will stimulate grass growth helping to reduce the percentage legume in the pasture or will replace the N lacking when legumes are grown with grasses. This rate should be enough to jump start the pasture grasses without a risk of overfertilization and risking damage to the environment. On pastures where maintaining legume presence is important, you should apply only half the rate of urea (50 lb urea per acre). At this rate of N, the legume can continue growing and will not slough off the bacteria nodules that help the legume by fixing atmospheric N (N2 gas) in a plant available form.