Relationship among grazing management, growing degree-days, and morphological development for native grasses on the Northern Great Plains

Abstract

Air temperature or growing degree-days (GDD) are known to influence morphological development of grass, but the effects of grazing history on grass morphological development has not been established. Morphological development of 5 species located on moderately and heavily grazed mixed prairie sites near Mandan, North Dakota, was determined 3 times per week from beginning of growth in spring to heading. The species were western wheatgrass [Pascopyrum smithii Rydb. (Löve)], blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths], needleandthread (Stipa comata Trin. and Rupr.), green needlegrass (S. viridula Trin.), and prairie junegrass [Koeleria pyramidata (Lam.) Beauv.]. Regression analysis of growth stage with GDD was linear and statistically significant for prairie junegrass (R2=0.62), green needlegrass (R2=0.96), and needleandthread (R2=0.95), and nonlinear for blue grama (R2=0.95) and western wheatgrass (R2=0.97). Prior grazing management had little effect on this relationship. The number of leaves and accumulated GDD required to produce those leaves varied by each species: prairie junegrass (4 leaves, 520 GDD), needleandthread (4 leaves, 640 GDD), green needlegrass (4 leaves, 800 GDD), blue grama (5 leaves, 1,300 GDD), and western wheatgrass (6 leaves, 1,450 GDD). Based on the species and conditions of this study, plant growth stage can be predicted from accumulated GDD and used for predicting grazing readiness and in development of forage growth models.