We use T-Sum 200 as an indicator for applying N to our hay fields. It has proven very effective in increasing growth for first cutting hay.
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 growth. Studies have shown an advantage to an early nitrogen application for improved growth. The first nitrogen application should be made when plants start to grow. Later fertilizer applications can be made following each mechanical harvest.
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 to provide growth early in the spring.
Heat Units
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.