Growing degree-days (GDD), also called heat units, effective heat units, or growth units, are a simple means of relating plant growth, development, and maturity to air temperature. The concept is widely accepted as a basis for building phenology and population dynamic models. Degree-day units are often used in agronomy, essentially to estimate or predict the lengths of the different phases of development in crop plants (Bonhomme, 2000).
The GDD concept assumes a direct and linear relationship between growth and temperature. It starts with the assumption that the growth of a plant is dependent on the total amount of heat to which it is subjected during its lifetime. A degree-day, or a heat unit, is the departure from the mean daily temperature above the minimum threshold (base) temperature. This minimum threshold is the temperature below which no growth takes place. The threshold varies with different plants, and for the majority it ranges from 4. 5 to 12.5°C, with higher values for tropical plants and lower values for temperate plants.
Many methods for estimating degree-days are available in the literature (Perry et al., 1997; Vittum, Dethier, and Lesser, 1995). The three most dependable and commonly used methods are the standard method, the maximum instead of mean method, and the reduced ceiling method. Numerous others have been proposed, a majority being a modification of one of these three. An exhaustive review of degree-day methods was reported by Zalom and colleagues (1993).
1. Standard degree-day method:
where (Tmax + Tmin)/2 is the average daily temperature and Tbase is the minimum threshold temperature for a crop. 2. Maximum instead of means method:
3. Reduced ceiling method: where Tmax < T
If maximum temperature (Tmax) is greater than the ceiling temperature (Tceiling), then set Tmax equal to TceiUng minus the difference between Tmax
The use of degree-days for calculating the temperature-dependent development of insects, birds, and plants is widely accepted as a basis for building phenology and population dynamics models. The simplicity of the degree-day method has made it widely popular in guiding agricultural operations and planning land use. Most applications of the growing degree-day concept are for the forecast of crop harvest dates, yield, and quality. It helps in forecasting labor needs for factories, and in reducing harvesting and factory costs. A potential area of application lies in estimating the likelihood of the successful growth of a crop in an area in which it has not been grown before. The growing degree-day concept can also be applied to the selection of one variety from several varieties of plants to be grown in a new area. Another application of the concept can be to change or modify the microclimate in such a way as to produce nearly optimum conditions at each point in the developmental cycle of an organism. The concept is also applied to plants other than crop plants and to the issues of growth and development of insects, plant pathogens, birds, and other animals.
Though the degree-day concept is simple and useful, it lacks theoretical soundness and has a number of weaknesses. A range of factors that influence the predictive capability of degree-day accumulations have been identified. Among these are the conditions that impact the physiological state of an organism (such as nutrition and behavior-based thermoregulation), error associated with the assumptions and approximation processes used in estimating developmental rates and thresholds, and the limitations of available weather data. In addition, it is emphasized that regardless of the calculation method, degree-days are never more than estimates of developmental time (Zalom et al., 1993; Perry et al., 1997; Roltsch et al., 1999; Bonhomme, 2000). Specific limitations identified are as follows:
and T ceiling-
• While using growing degree-days, the physiological and mathematical bases upon which they are founded are sometimes forgotten, resulting in questionable interpretations (McMaster and Wilhelm, 1997).
• Except for the modified equations, a lot of weightage is given to high temperature.
• No differentiation can be made among the different combinations of the seasons. For example, the combination of a warm spring and a cool summer cannot be differentiated from a cold spring and a hot summer.
• The daily range of temperature is not taken into consideration, and this point is often more significant than the mean daily temperature.
• No allowance is made for threshold temperature changes with the advancing stage of crop development.
• Net responses of plant growth and development are to the temperature of the plant parts themselves, and they may be quite different from temperatures measured in a Stevenson's screen. Though this difference at a particular time may be small, the cumulative effects through an entire growing period can be very large.
• The effects of topography, altitude, and latitude on crop growth cannot be taken into account.
• Wind, hail, insects, and diseases may influence the heat units, but these cannot be accounted for in this concept.
• Soil fertility may affect crop maturity. This cannot be explained in this concept.
In spite of these limitations, the degree-day or heat unit concept answers a number of questions in plant and insect phenology and growth.
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