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Forethoughts on Fertilizing Forages

By Cassandra Schroeder, AAg, Range Management Extension Specialist, Kindersley

March 2021

Forage fertility test plot
A forage plot lacking nutrition next to fertilized plots.

Fertilizing forages can improve forage yields and quality; despite this, many forage crops are under-fertilized. Hay prices, land prices, weather, fertilizer costs, forage species, soil quality and management are some of the many things that will impact the economic benefit of fertilization. These factors change over time and re-evaluating the costs and benefits of fertilization can help you identify opportunities to improve forage fertility and profitability.

Both haying and grazing remove nutrients from the forage system and fertility eventually declines. When grazing, many of the nutrients consumed by animals are returned to the forage through urine and feces. However, grazing still results in a net loss of nutrients through animal growth and production. Haying removes more nutrients than grazing. For each ton of dry matter harvested there is about 35 to 50 pounds of nitrogen and 10 pounds of phosphorous removed. Exact nutrient removal will vary with forage species, forage quality and manure management. If nutrients are not being returned to seeded forages by manure or fertilization, there is a high likelihood that fertility could be limiting forage yields.

Forages, like other plants, require about 17 different nutrients for growth. The most important nutrients required in the largest quantities actually come from carbon dioxide and water. The mineral nutrients required by plants include nitrogen, phosphorous, potassium, sulfur and various micronutrients. The most limiting nutrients need to be addressed first in order to get the most benefit from fertilization. Depending on the region and weather conditions, water might actually be the first limiting nutrient. When this is the case, yields are not greatly improved by mineral fertilization in the year of application. Although most nutrients applied will be available when growing conditions improve and provide later yield benefits, in the meantime, there is an increased risk of nitrogen loss.

After water, nitrogen and phosphorous tend to be the next limiting nutrients. In grass stands, it is usually nitrogen that limits plant growth. Whereas in mixed stands, with a good legume component, phosphorous is more likely to be the limiting nutrient. The legume component is important to consider, because legumes can fix atmospheric nitrogen for the forage system. Nitrogen fertilization of forage stands with significant legume components is not generally beneficial. In grass stands, the benefit of nitrogen fertilization is roughly a 10 (brown) to 30 (black) pounds per acre increase in forage production for each pound of nitrogen applied, depending on the soil zone. Nitrogen fertilization can also improve forage quality by boosting protein levels. The specific benefit of fertilization will vary depending on current fertility, available moisture, species composition and other conditions.

Potassium, sulfur and micronutrients are less commonly limiting in Saskatchewan forage crops. This does not mean that they are less important. If micronutrients are not available, they can also limit forage production. Hence, soil testing is always recommended to identify which required nutrients are needed to meet forage production goals in each unique situation.

After determining which nutrients are needed, it still takes planning to ensure the forage crop can make the best use of the fertilizer application. Nutrients like phosphorous are mostly immobile in the soil and should be incorporated into the root zone. In perennial stands, this tends to mean phosphorous fertility is most easily addressed during forage seeding. Seed-safe rates will limit the nutrient amounts that can be applied with the seed, but phosphorous for the expected stand life can be applied before seeding with other operations during breaking and reestablishment. Phosphorous is stable in the soil and is not easily lost over time. In contrast, nitrogen is less stable and very mobile in the soil. With unfavourable weather, large amounts of nitrogen can be lost to volatilization and leaching. Volatilization is the main concern with broadcast nitrogen applications. To reduce the amount of volatilization, nitrogen should be applied to cool, dry soils prior to expected rainfall or incorporated. There is also an assortment of coated and slow-release nitrogen products that can help to manage nitrogen losses in high risk situations. To get the full benefit of fertilization, the nutrients applied need to make it into the soil where the forage crop can access them.

These are just some of the things to think about when deciding how to apply fertilizer. The costs and benefits for each field need to be considered to ensure that fertilization is economical and nutrients should be applied to address the most deficient nutrients first as determined by a soil test.

For more information of fertilizing forages contact your local range management extension specialist.

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