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T-Sum 200, Coos County Extension Service
Fertilizing Early with Nitrogen for Increased Pasture Growth
By Amy Peters
Livestock Extension Agent
Fertilizer is one of the main costs in maintaining pasture productivity in established pasture. In order to be competitive in today's livestock market, you need to manage nutrients so that the greatest economic return is received. Nutrient management can be divided into four basic areas: 1) the amount of nutrient to apply, 2) timing of application, 3) method of application, and 4) the source to use. The time when you fertilize can make a difference in the amount and quality of pasture growth. Rapid plant growth depletes the nitrogen (N) in the soil. When the N runs out, plant growth slows or stops. Ryegrass and orchardgrass, especially in new seedings, become nitrogen deficient in January. This results in loss of the grass component in pastures if nitrogen is not applied. Previous research (Cannon and Hart, 1989) showed an increase in forage dry matter per acre from the application of N in winter. In our study, we examined a method of determining when to apply fertilizer based on physiological timing of the plant rather than strictly using a calendar date. This method of fertilizer application is called T-Sum 200.
What is T-Sum 200 and how does is work?
T-Sum 200 is a method that can assist in timing initial fertilization with the start of grass growth. T-Sum 200 is a way to add temperature units (heat units) to calculate the best time to put out your N fertilizer for early feed. For the past 10 years in Coos and Curry Counties, that has been Jan. 26 or 27. The T-Sum method can be used to increase the amount of early feed available on those pastures that can be grazed. Studies in southwestern Oregon and previously in the Willamette Valley (Pirelli, 1989) have shown an advantage to an early nitrogen application to improved pasture. The first nitrogen application should be made when plants start to grow. Later fertilizer applications can be made following each grazing or mechanical harvest as additional forage is needed.
January 1 is the starting date each year for adding up heat units. This is a convenient starting date that is based on the assumption that cool or cold winter temperatures in December and January have caused plant growth to slow or stop. At or about 200 °Centigrade (°C) cumulative heat units after January1, plants begin to grow. You want the N available at this time so the plants have N available to them and can then produce feed early in the spring.
The T-Sum 200 method is based on an accumulation of 200 heat units in °C. If the temperature is in Fahrenheit (°F), fertilize when you reach 360 heat units. Heat units are the average of each day's high and low temperatures.
For °C, heat units can be calculated by adding the maximum temperature to the minimum temperature (use 0 for negative values) and dividing by 2.
For °F, add maximum temperature to minimum temperature, divide by 2, then subtract 32.
Beginning January 1, calculate each day's heat units. A quick way to do this is by using this computer-based web address: http://ippc2.orst.edu/OR/
Once at the web site, set the lower threshold to 32°F. Set the method to "simple average". Click on either Bandon or North Bend and the desired year. Click on "calculate" at the top of the page.
This example is in °F. T-Sum 200 (that is, the corresponding 360 in °F) was reached on January 27, which would be the time when N should first be applied.
Coos County Results
Feed was economically produced by the early N application, at less than half the cost of comparable hay. An increase of about 400 lb. more forage is needed to make it a good economic decision if comparable hay costs $60/ton to purchase or an increase of 650 lb. more forage if hay costs $100/ton. There was over 1000 lb. more feed (on a dry matter basis) per acre than with traditional fertilizer application time. So, this method increased the amount of early season feed. The early N application also increased the quality of the feed, which was 10.5% higher in crude protein than feed from traditional fertilizer timing. Application of N at T-Sum increased early forage production and increased the length of the grazing season.
Cannon, L. and J. Hart. 1989. Coos-Curry fertilizer recommendations, Oregon State University Extension Service, Coos County.
Kowalenko, C.G., S. Freyman, D.L. Bates, and N.E. Holbek. 1989. An evaluation of the T-Sum method for efficient timing of spring nitrogen applications on forage production in south coastal British Columbia. Can. J. Plant Sci. 69:1179-1192.
Pirelli, G. 1996. Timing of nitrogen fertilizer for Western Oregon pastures. Oregon State University Extension Service, Polk County.