Effects of the previous shoot removal frequency on subsequent shoot regrowth in two Medicago sativa L cultivars

The frequency of shoot removal in lucerne (Medicago sativa L.) has long been recognized as a key factor in its management and productivity. The present study was undertaken to determine the impact of cutting interval during spring (30 or 45 days) on the subsequent summer regrowth, in contrasting lucerne cultivars (cv. Europe and Lodi). In particular, the dynamics of shoot regrowth (leaf area index, radiation use efficiency, N accumulation in harvestable biomass) and its relationship with taproot organic reserves (starch and N contents) were studied. Results showed that increasing the duration of the spring regrowth had a positive effect on subsequent summer regrowth, but there were also effects of cultivars. During the first 14 days of summer regrowth, the ''Lodi'' cultivar showed higher leaf area index (LAI) and greater photosynthetic active radiation interception than the ''Europe'' cultivar. The organic reserve level was also affected by the length of the previous spring cutting treatment (45 days treatment > 30 days treatment) and cultivar (Lodi > Europe). Lodi accumulated a larger amount of starch and N reserves which were subsequently mobilized to a greater extent in the first three weeks of regrowth and this contributed to its faster initial shoot growth rate. Our results confirm the important role played by N and C taproot reserves in shoot growth rate in the first days following shoot removal. Results are discussed in relation to recent studies on the role of storage N compounds in regrowth, and the concepts of radiation use efficiency (RUE); nitrogen nutrition index (NNI), and the decline in N content seen during the accumulation of biomass in lucerne canopies.