|| Brejda, J.J., Brown, J.R., Lorenz, T.E., Henry, J., Lowry, S.R.
||Environmental variation, and the interaction of environment with management practices, can significantly affect perennial forage yields. The objective of this research was to evaluate the importance of environmental main effects and the interaction between environments, N rates, and harvests on eastern gamagrass [Tripsacum dactyloides (L.) L.] forage yields. Established eastern gamagrass stands were fertilized with 0, 168, or 336 kg N ha-1, and harvested three times during the growing season for three consecutive years at two locations in northern Missouri. Year and the year x location interaction were significant environmental sources of variation affecting forage yields. The year main effect had the larger variance component and accounted for 9% of the total variation in forage yields. The harvest main effect and year x harvest interaction had the largest variance components, and together accounted for more than 67% of the total variation in forage yields. Most of the variation between harvests resulted from differences in first-harvest yields, taken at the reproductive growth stage, compared with regrowth harvest yields which were at vegetative growth stages. Variation in precipitation amounts between harvests significantly influenced regrowth harvest yields, causing a significant year x harvest interaction. Forage yield responses to N rates also varied between years, causing a significant year x N rate interaction. Location was not an important source of variation in eastern gamagrass forage yield in this study, nor was there a significant interaction between N rates and harvests. Results suggest that supplemental irrigation and longer rest periods between harvests may help improve forage yields and distribution.