During the month of September, some producers have asked if they should make a late season application (after September 1) of nitrogen (N) to cool season grasses. They know that the benefits of adding N to forage grasses during the spring and summer months include increased yields, increased plant health, faster regrowth and decreased leaf diseases, and they want to make sure their forage stands are healthy and vigorous for the coming year’s spring growth. As the season changes from summer to fall, temperatures drop and day length shortens, and the plants begin to store carbohydrates and N in the root zone for winter survival and new growth in the spring.

A brief description of the N cycle can help explain how N becomes available in the soil. The main source of N in soils is from organic matter in plant and animal wastes. When these decompose, they add N to the soil. Bacteria in the soil then convert those forms of N into usable forms that plants can use. The plants are either used by animals for food or are returned to the soil to break down again as organic matter.

Sources of inorganic N fertilizer, such as urea, will move into the soil with rainfall and are readily available and do not need to be broken down by bacteria. If inorganic N fertilizers are applied late in the growing season, they may not be used by plants because growth slows when light and temperature decreases. This can result in an excess of N in the soil, leading to leaching and groundwater contamination. Therefore, Michigan State University does not recommend a late season application of N fertilizer to cool season grasses in Michigan.

Nitrogen losses from fall applications vary from 10 to 20 percent on fine-to medium-textured soils (clay, clay loams and loams) and from 30 to more than 50 percent on coarse-textured soils (sandy loams, loamy sands and sands). Although applying N in the fall on fine-textured soils may have certain economic benefits, the environmental risks of this practice generally outweigh the potential economic benefit.

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