Fall Fertility in Cool Season Pastures

September 9, 2014 03:30 AM
Fall Fertility in Cool Season Pastures

The optimum application window for fall nitrogen in fescue pastures starts around Aug. 1 and responses to fall-applied nitrogen decreases incrementally after Sept. 1. In southwest Missouri mid to late-August is optimum according to Jill Scheidt, agronomy specialist with University of Missouri Extension in Barton County.

"The average yield response to fall-applied nitrogen typically is less than response to spring-applied nitrogen. Depending on rainfall the fall nitrogen response can be quite variable. A favorable distribution of precipitation can lead to a large yield response," said Scheidt.

Research from Missouri and other states shows a positive response to rates of nitrogen over 100 lbs per acre, but the amount of response decreases at higher rates. Higher rates are also more likely to reduce legumes in mixed fescue-legume pasture.

"A rate between 50 and 100 lbs. nitrogen per acre is justified in most pastures. Producers should choose the lower rate on pastures looking to maintain a high legume component," said Scheidt.

The argument supporting fall nitrogen applications is the high quality and high use of properly managed stockpiled production.

"The ability to stockpile fall forage production and then strip graze it in late fall and winter insures most farmers will make good use of any increased yield in response to applied nitrogen," said Scheidt.

Research at the University of Missouri has shown stockpiled fescue maintains quality through March while the toxic element of ergovaline in tall fescue decreases by half by mid-January.

"The decision to purchase nitrogen fertilizer or purchase hay will depend on the type of operation you have and what type of risk you want to take," said Scheidt.

A question to ask would be: are legumes a cheaper source of nitrogen than synthetic fertilizers?

Fertilizer recommendations for pasture are actually based on hay harvest concepts. When hay is harvested and feed elsewhere, the mineral elements are literally carried away from the field.

"Each ton of mixed grass- legume hay will typically contain 10 - 12 lbs of phosphorus, 40 - 50 lbs of potash, and varying amounts of other elements. Continually harvesting hay without replacing the P and K removed will deplete the fertility of most Missouri soils," said Scheidt.

If hay harvesting is contrasted to grazing, some fertility credit must be given to the manure that is being returned to the soil and the plant litter that is trampled down but left on-site.

"This simple concept suggests that pasture fertility rates should be lower than hay field rates. The effectiveness of returned manure in maintaining pasture fertility is highly dependent upon a number of factors," said Scheidt.

MU Extension research (from 1981-1989) compares the cost of applying nitrogen to fescue compared to the cost of adding a legume as a source of nitrogen. The research explains the cost per grazing day is typically lower for grass-legume pastures than for nitrogen-fertilized pastures. Animal performance is generally 5 to 20 percent higher on grass-legume pastures compared to nitrogen-fertilized grass.

"The cost per pound of gain will be proportionally lower for grass-legume pastures as long as carrying capacity is similar," said Scheidt.

Another question to ask this year with the dry spell is: will it pay to apply nitrogen now, even if it doesn't rain for a couple weeks?

Applying nitrogen from mid to late-August encourages fall growth in wet and dry summers.

"Producers are better off to apply for autumn growth even if the summer is dry because the value of that forage is worth more in dry year than a wet year," said Scheidt. "There will be a smaller response in a dry year, but it would still more than pay for the application."

Fall growth is driven mainly by water and nitrogen. If the pasture has good soils and good grass, like Max Q, a producer can apply 75-80 units of nitrogen. In a lower quality pasture, such as a toxic fescue, 40 units of nitrogen would be appropriate.

Dry forms of nitrogen such as: Agrotain, SuperU, ammonium sulfate and ammonium nitrate are the safest forms of nitrogen to apply in a dry year in order to avoid loss. Dry urea that is untreated can be lost easily because it has a weak chemical bond. Nitrogen fertilizers will also suffer less loss in dry weather than in humid weather.

Source: University of Missouri Extension

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