The purpose of this study is to investigate the effect of changing the planting date on the length of the triticale growth period based on growing degree days at the provincial level. Knowledge of growing degree days improves crop management. For this purpose, the average daily temperature data of 13 synoptic meteorological stations in Guilan province, including: Astara, Anzali, Talesh, Jirandeh, Deylaman, Airport, Rudbar, Rudsar, Keshavarzi, Kiashahr, Lahijan, Masouleh, and Manjil, were used over a 15-year period from 2008 to 2022. In this study, triticale growth degree-days were considered as 960 units with a physiological zero of 5 degrees Celsius for 8 planting dates with 15-day intervals from September 1 to December 15. Harvest dates with corresponding to probabilities of 25%, 50%, and 75% were calculated using the normal distribution function and interpolated for Guilan province using the IDW algorithm. The results showed that in the western regions of the province, delaying planting significantly increases the growth period. Also, in the cold regions of the province where rice cultivation is not common, there is a good opportunity for spring planting. Overall, it is recommended that triticale be planted in the province before the beginning of autumn, preferably in the first half of September. For example, in Rasht, if the planting date is September 1, there is a 50% probability that the growth period will be 53.4 days with 960 degree-days of growth, and if the planting date is December 15, the growth period will be 157.2 days with 960 degree-days of growth. By choosing the appropriate planting date, maximum productivity in livestock fodder production can be achieved while taking advantage of the appropriate temperature and sufficient GDD.