To compare identical plant population x cultivar trials under overhead and surface irrigation

• There was no significant interaction between seeding rate and cultivar with both varieties lower yielding at plant populations of between 15-20plants/m2.
• However, the yield effect of the lower plant population in 45Y28 was less pronounced (0.25-0.3t/ha compared to the optimum than with Trophy (0.45 – 0.8t/ha).
• Plant populations above 35 plants/m2 (60 seeds/m2) and above 30 plants/m2 with 45Y28RR produced the highest yields.
• Plant establishment rate for the trial averaged 75% and there was no significant detrimental yield effect from the highest plant populations in either cultivar, just an additional seed cost that was not cost effective.
• There were differences in biomass (dry matter) at green bud, with the trend to lower biomass at lower seeding rates and resultant plant populations.
• There was no additional biomass response to sowing seeding rates higher than 40 seeds/m2 when assessed at early flower (GS62) or at harvest with crop canopies of 14.5-15t/ha producing the highest yields.
• Oil content was significantly different between varieties but not between seeding rates with higher oil content in the RR hybrid.
• While there was variation in the Harvest Index (HI) from 0.32 to 0.24, the trend was for higher HI at the lower planting populations.
• Water use efficiency was 10.0 kg/mm.