Yield components of lucerne were affected by sowing dates and inoculation treatments

This study analysed the effects of sowing dates and inoculation treatments on yield components of rainfed lucerne (Medicago sativa L.) crops grown at Lincoln University, New Zealand. Crops were sown in a split-plot design on 26 January, 21 February, 15 March and 3 April 2012, combined with five inoculation treatments (ALOSCA® coated seed, Nodulator® peat slurry and bare seed). Shoot biomass was 90% higher in the earliest spring sowing date (19.8 ± 0.14 t ha⁻¹) compared with the latest (11.9 ± 0.15 t ha⁻¹). Yield variability was explained by differences in light interception and its conversion efficiency (RUEshoot) in the first spring. A 40% reduction in RUEshoot was estimated for March and April (autumn) sowing (0.93 ± 0.05). This was probably caused by greater continued partitioning of biomass to roots. A longer phyllochron for autumn sowing reduced leaf area expansion rates. Root biomass was higher from early sowing dates, but by the end of the experiment all treatments had >3 t ha⁻¹ of perennial biomass. Inoculation increased shoot yield by 40% compared with the uninoculated control (16.6 ± 0.1 vs. 13 ± 0.2) in the first year. Bare seed treatments fixed 50% less nitrogen during the winter and early spring of 2012. This was linked to less nodulation and reduced nitrogen nutrition index (NNI) in shoots of the uninoculated treatment. However, after late spring, nitrogen fixation was not different among treatments, which was consistent with effective colonisation by naturalized rhizobia strains in the bare seed treatment. A 20% decrease in RUEshoot was measured in uninoculated crops compared with inoculated ones. This was probably caused by reduced photosynthesis due to the lower NNI. Root biomass was unaffected by inoculation and all treatments reached >3. t ha⁻¹ by the end of the experiment. Uninoculated lucerne crops appeared to have similar accumulation rates of perennial biomass despite lower N fixation and N content in shoots. These results explain the physiological response of lucerne crops to sowing date and inoculation and can be used to inform management decisions and crop simulation models