Yield Components of Contrasting Alfalfa Cultivars Under Rain-fed Conditions in Tasmania, Australia.

The dry matter yield of alfalfa (Medicago sativa L.) is made up of three principle yield components; mass per shoot, shoots per plant and plant density. This study investigated the influence of genotype and environment on alfalfa yield components for one year from establishment in Tasmania, Australia.  Four cultivars were established in the spring and summer of 2006/07 at two contrasting environments, Elliott (41°4’48” S, 145°46’12” E, 1200mm annual rainfall, Humic Eutrodox soil) and Cambridge (42°49’48” S, 147°30’00”E, 500mm annual rainfall, Udic Haplustalfs soil) in Tasmania.  Cultivars chosen to give a wide representation of fall dormancy (FD) were DuPuits (FD3), Grasslands Kaituna (FD4.5), SARDI-7 (FD7) and SARDI-10 (FD10).  Yield and yield components of the rain grown crops were monitored for six harvests at each location. 

Yield was affected (P<0.05) by cultivar in the late spring harvest at Elliott and the late summer harvests at Cambridge.  Yields of all cultivars were consistently greater at Elliott than at Cambridge apart from the late summer harvest where yields were similar.  Cultivar affected (P<0.05) at least one yield component in half the harvests in each environment. However, the yield component that was affected was not consistent, nor was the harvest in which the effects were detected.
Stepwise multiple logistical regressions of yield components against yield at each harvest showed that mass per shoot consistently had the largest influence on yield.  At some harvests it was found that plant density also had a significant influence on yield, however, the effect was not consistent across all harvests.  Shoots per plant rarely made a significant addition to the multiple regression model. 

The development of best management practices for alfalfa in this environment needs to focus on maximising mass per shoot and to a lesser extent plant density to ensure the yield potential of alfalfa is reached.