Plant and Soil 188: 189–198, 1997. 189 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Effects of the previous shoot removal frequency on subsequent shoot regrowth in two Medicago sativa L. cultivars J.C. Avice 1 , G. Lemaire 2 , A. Ourry 13 and J. Boucaud 1 1 INRA Physiologie et Biochimie V´ eg´ etales, Institut de Recherche en Biologie Appliqu´ ee, Universit´ e, 14032 Caen Cedex, France and 2 INRA, Station d’Ecophysiologie des Plantes Fourrag` eres, 86600 Lusignan, France. 3 Corresponding author Received 26 July 1996. Accepted in revised form 6 November 1996 Key words: alfalfa, cutting frequency, light interception, N nutrition index, organic reserves, regrowth Abstract 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. Abbreviations: LAI – leaf area index, NNI – nitrogen nutrition index, PAR i – photosynthetically active radiation intercepted, RUE – radiation use efficiency, VSP – vegetative storage protein. Introduction The regrowth of the shoot following shoot removal in forage legumes is a complex process in which the inter- action of environmental factors (air, soil temperature, solar radiation levels) and endogenous plant factors (reserve levels, number of remaining active meristems) determines the final herbage yield (Richards, 1993; Volenec et al., 1996). Abiotic conditions that limit resource availability (light, water, nutrient) before or after shoot removal can have significant effects on the dynamics of regrowth (see Richards, 1993 for review). FAX no: +3331565360. E-mail: Ourry@ibba.unicaen.fr Competition for light is a major determinant of yield and survival in dense lucerne canopies (Gosse et al., 1988; Lemaire et al., 1991). Lemaire et al. (1991) reported a reduction in the N content of harvestable biomass with depth in dense lucerne canopies, that was related to the increasing attenuation of light, and so to light availability - at a given depth - in the canopy. Under field conditions, Gosse et al. (1984) developed a model assuming a linear relationship between aeri- al dry matter production and the interception of pho- tosynthetically active radiation (PAR ) intercepted by the crop. The slope of the regression between total dry matter and PAR was interpreted as the radiation use