High Yield Grass Seed Production and Water Quality Protection Handbook

Nutrient Management

Prudent fertilizer management is essential for preserving water quality in the agricultural landscapes of the Willamette Valley. Understanding how grass seed plants grow is key to developing a fertilizer program that provides the correct amount of nutrients at a time when they are most beneficial and are least subject to environmental losses. Especially important is knowing when the precise period of rapid plant growth and peak nutrient uptake occurs.

For most grass species, including perennial ryegrass and tall fescue, 80 percent of seed and straw production occurs between April and early June. The pattern of nutrient uptake begins before the period of rapid growth. Peak uptake for most nutrients occurs in April. Very little uptake of nutrients commonly applied as fertilizer occurs in May and June, even though the crop continues to grow during these months (Figures 1-3).

Nitrogen

Nitrogen uptake in grass seed crops is
rapid during April and essentially complete
by mid-May.

Nitrogen (N) is the most important fertilizer nutrient used in grass seed production. Efficient use of this fertilizer is crucial for economic seed production and protection of groundwater and surface water. Temperate grasses use both ammonium and nitrate forms of N very effectively, but regardless of the type of N fertilizer applied, bacteria eventually converts soil ammonium to nitrate. Nitrate is very water soluble and mobile in the soil. For this reason, proper applications that coincide with plant use are essential.

The key to optimizing yields and preventing N losses is to apply fertilizer at a time when the crop is physiologically ready to produce seed heads (e.g., proper day length) and when climatic and soil conditions exist to move nutrients to the roots and provide the substantial heat units necessary for growth. For the most part, N uptake precedes rapid tiller elongation and crop growth.

N should be applied at a rate that does not leave excess nitrate in the soil after harvest that is subject to leaching with winter rains. In grass seed crops, N uptake is rapid during April and essentially complete by mid-May. Total uptake typically ranges from 120 to 190 lb/a, of which 40 to 130 lb/a comes from the soil itself through breakdown of organic matter. Very little uptake occurs in the fall and winter; soil mineralization is usually adequate to meet plant needs during those months.

Recommended rates

Recommended N fertilizer rates depend on the particular seed crop being produced and soil conditions and range from 50 to 80 lb/a for fine fescue to 140 to 160 lb/a for perennial ryegrass. Application should occur between early March and mid-April, or between 400 and 850 growing degree days because this is when day length is adequate to stimulate flowering tiller production and temperatures are on the increase. Research has not shown a seed yield advantage to multiple applications; however, splitting N applications helps ensure uniform coverage on the field and allows for greater flexibility in working around rainy spring weather conditions. Any advantage of multiple N applications though must be weighed against higher equipment and labor costs, plus the added soil compaction that is likely to occur from early season applications on wet fields.

Additional N will not stimulate plants when soils are saturated even if temperatures are warm enough for growth. Grass seed crops exhibit more yield component compensation than other crops in the grass family such as corn or wheat. Consequently, there is more flexibility in the timing of N application in the spring. Delaying the entire N fertilizer application until weather conditions are favorable and soils are no longer waterlogged and prone to runoff does not result in reduced seed yields. Even on better-drained soils, there is little advantage to applying N before early March in the Willamette Valley.

Nitrification

The conversion of urea and ammonium based N fertilizers to nitrate-N is temperature and moisture dependent. The conversion to nitrate-N is called "nitrification," and the bacteria that are responsible for this chemical change require oxygen. Nitrification does not occur in saturated soils regardless of the soil temperature. When oxygen is present in the soil, the rate of nitrification increases with increasing soil temperature; at soil temperatures above 45 degrees F, nitrification is not a limiting factor for crop demands on Willamette Valley soils. (The weekly average maximum 2-inch soil temperatures in the Willamette Valley are near 45 degrees F by mid-February; by early March the weekly mean is above 45 degrees F).

Grasses grown for seed in Oregon are cool season species. They begin to grow and use nutrients at approximately the same temperatures that allow microbial N transformations to also begin.

Nitrogen fertilizer and mineralized N not used by the crop remains in the soil in the leachable nitrate form after harvest. If it is not taken up during the fall by crop re-growth, it is subject to leaching losses. Extensive research across a range of soil types in the Willamette Valley has shown that N fertilizer application rates below 200 lb/a do not result in elevated nitrate levels in the soil. At normal use rates, fall soil samples are consistently below 10 ppm nitrate-N, a level considered low by soil scientists. Nitrate leached to shallow groundwater and surface waters in near proximity to grass seed fields result more from mineralized N than directly from fertilizer N applications.

Nitrogen recycling

At harvest, straw contains about 70 percent of the N taken up by the crop, and when crop residue is flail-chopped back on the field, the N is gradually recycled. When straw is chopped back the first time, there is some immobilization or "tie-up" by microbes as the straw breaks down, but uptake in the spring is reduced only 10 to 20 lb/a. After two to three years of chopping straw, there is a net release of N from the decomposing straw and a corresponding increase in N uptake by the crop. These effects are not large and normal N fertilizer rates are usually satisfactory on fields where the full straw load is being flail chopped.

Soil testing for N has not been an effective diagnostic tool in the Willamette Valley to determine crop needs in the spring. Soils with a high organic matter level generally require less spring N, but for most valley soils there is too much variability in organic matter mineralization rates and moisture levels in the spring to make very precise predictions on N fertilizer needs. However, testing in the fall can be useful as a post-harvest "report card" to determine the efficiency of the spring N fertilizer program. If residual nitrate levels consistently exceed 20 ppm, it may be wise to reduce fertilizer N inputs in future seasons.

Phosphorus and potassium

Adequate phosphorus (P) and potassium (K) are essential for optimum grass seed production. Compared to N, these nutrients are not mobile in the soil and do not pose a threat to groundwater.

Significant P loss in runoff from grass seed fields has not been observed. Soils in the Willamette Valley have a high clay and organic matter content, enabling them to adsorb P tightly in the surface layer of the soil. However, excessive P application in the form of fertilizer or manure can increase the risk of runoff, especially on newly planted fields. According to current concepts of P indexing for environmental protection, the critical soil test value for P loss in runoff is well above the level considered necessary for optimum seed yields on Willamette Valley floor soils. Thus, use of soil analysis to determine P fertilizer needs will help avoid excessive concentrations in the surface soil.

Potassium has not been related to water quality concerns in Oregon agriculture. Grass seed plants remove large amounts of K from the soil. They can remove several times more than needed if it's available. This is called "luxury consumption" and represents an economic loss for growers if K fertilizer is applied when it's not needed and straw is removed from the field. If soils test near the adequate level for K, maintenance applications are a good practice, but not on fields that have a high soil test. Especially where the full straw load is chopped, very little K is removed with the seed at harvest, and K in the straw is rapidly recycled and available to plants.

In contrast to N, soil testing for both P and K is very reliable and is recommended to determine optimum fertilizer rates for seed production and resource conservation.

Other nutrients

Sulfur, calcium and magnesium are commonly applied to grass seed crops with mixed fertilizers or lime applications. These nutrients do not pose a threat to groundwater or surface water given their low mobility in the soil, low rates of application and efficient use by grass seed crops.

Most soils in the Willamette Valley are not deficient in micronutrients for grass seed production, although boron does test low in many fields. For insurance, B may be applied on a trial basis at a rate of 1 lb/a where soil tests are less than 0.3 ppm. To avoid potential toxicity problems, boron should not be band applied or used on an annual basis.

Soil acidity

Efficient nutrient use in crops is partly dependent on maintaining a soil pH that allows the development of a good root system. In the Willamette Valley, soils can become extremely acid over time. Under grass seed production, soils consume the equivalent of 500 or more lb/a of lime yearly. Eventually, the soil pH falls to a critical zone of acidity (pH< 5.0) in which aluminum and manganese become so soluble that they severely inhibit root growth. Not only are yields reduced under these conditions, but also the efficient use of potentially leachable nutrients such as N becomes jeopardized. Soil testing for pH and lime requirement is recommended. While grass seed crops vary in their sensitivity to low pH, good root growth is only obtained at pH levels above 5 for most grass species.

Mark Mellbye, Gale Gingrich and John Hart; Oregon State University Extension Service
Stephen M. Griffith; USDA, Agricultural Research Service

Grass seed is an integral part of Oregon's economy

Oregon growers enjoy an international reputation for producing high-quality grass seed. Here, approximately 60 percent of the world's cool season grass seed is produced. The many species and varieties grown in Oregon are used for beautification, recreation, erosion control and forage in North America, South America, Europe and Asia.

More than 95 percent of the grass seed grown in Oregon is produced in the Willamette Valley. At any one time, approximately 50 percent of the tillable land in the Willamette Valley-about 500,000 acres -is planted to grass seed.

Grass seed contributes significantly to Oregon's economy. Farm gate receipts for grass seed-nearly $340 million in 2000-make up 10 percent of the total farm gate receipts for Oregon. And grass seed injects hundreds of millions of dollars into peripheral industries such as shipping, packaging, sales and labor.

A percentage of straw left over after seed harvest is used as livestock feed. Straw dealers shipped nearly 500,000 tons of straw to Japan, Taiwan and Korea at a farm gate value of $50 million in the 2000 market year.