This is an index used to express crop or insect maturity. Growing degree days are those days necessary for crops or insects to complete their growth and development. The basic concept is that development will only occur if the temperature exceeds some minimum development threshold, or base temperature (TBASE). The base temperatures are determined experimentally and are different for each organism.

Reported Base Temperatures for GDD Computations
23,4 °C - sunflower, potato 
32,4 °C - sweet corn, corn, sorghum, rice, soybeans, tomato

To calculate GDD, you must first find the average temperature for the day. The average temperature is found by adding the maximum and minimum temperatures for the day and dividing the sum by two. If the average temperature is at or below TBASE, then the Growing Degree Day value is zero. If the average temperature is above TBASE, then the Growing Degree Day amount equals the average temperature minus TBASE. The values that are greater than zero are added to determine the weekly, monthly or yearly GDD

Accumulated GDD can also be used to:
estimate the growth-stages of crops or life stages of insects; 
predict maturity and cutting dates of forage crops; 
estimate the heat stress on crops; 
help estimate the yields of cereals 
as a planning tool for spacing planting dates to separate harvest dates in vegetable production