The optimisation of the seed production techniques in alfalfa is difficult due to the lack of diagnosis tools for analyzing the actual seed yield and investigate the limiting factors. Little data have been provided on the relationship between the biomass production, the harvest index and the seed yield for a given cultivar across a range of environments.

Two cultivars, Europe and Magali, official controls for the variety registration in the French list of the Flemish and Provence types respectively, were analyzed in a range of 27 environmental conditions. This set of conditions was obtained through combinations of four locations, three harvest years and different crop ages. In each condition, three replications were studied. At harvest, the seeds were collected as well as the whole aerial biomass. The harvest index was calculated as the ratio between seed yield and the aerial biomass.

For each variety, a strong relationship was observed between the seed yield and the aerial biomass at harvest (figure 1). The regression slope was steeper for Europe than for Magali. This is relevant with the lower harvest index reported on this variety. No relationship was found between the seed yield and the harvest index.

The lower seed yield found on average on the crops in their planting year is explained by the lower biomass found on these crops that is due to the later achievement of the full radiation interception and the lower conversion efficiency of the PARa into above ground biomass (Khaiti and Lemaire, 1992).

This relationship was validated using four data sets from previous trials where different agronomic practices were tested : clipping date, row spacing, irrigation. For three of the experiments, the data fitted well with the regression developed in the present experiment. For the last one, the seed yields were lower than expected according to the biomass available at harvest. This trial had actually been badly damaged by Bruchophagus.

Thus, the presently developed regression could be used as a diagnosis tool to investigate the limiting factors of seed yield in alfalfa and to optimise the agronomic practices.