The growth of lucerne following defoliation

Much agricultural research has been directed towards maximizing pasture and crop yields. This has involved the identification of factors limiting growth and the formulation of management systems which will assist swards to express their full growth potential under the conditions prevailing. For lucerne, the aim of management has been to obtain high yields of good quality feed as well as maintain plant vigour and stand longevity. This has involved the detailed study of various factors which influence recovery following defoliation. The emphasis has shifted from one aspect to another, and only recently has it been possible to present an analysis of lucerne recovery which integrates all these. For many years, following the classical experiments of Graber et al. (1927) at the Wisconsin Agricultural Experimental Station, a great deal of importance was placed on the level of organic reserves as determinants of plant vigour, including the rate of recovery following defoliation. Quite recently, this interpretation was challenged by the Australian workers May and Davidson (May and Davidson, 1958; May, 1960), who drew attention to the role of reserves in respiration following defoliation. Since then, the studies of Davidson and Milthorpe (1966a, b), Mitchell and Denne (1967), Hodgkinson (1968, 1969), Silva (1968) and Smith and Silva (1969) have enabled a more balanced interpretation of the relative importance of organic reserves in regrowth and respiration. Furthermore, their work has led to an appreciation that the other effects of defoliation on the root system, including nutrient uptake, also influence recovery growth. The studies of Hodgkinson (1968) and Silva (1968) also demonstrated the importance of photosynthates from new leaves as determinants of recovery growth. Following the work of Watson (1947), leaf area index (L.A.I.) has become widely accepted as an important determinant of crop growth rate (C.G.R.). Work by Langer and Steinke (1965) , Silva (1968) and Hodgkinson (1968), has indicated that residual leaves can reduce the stress placed on the root system by frequent defoliation. The importance of such leaves, relative to that of roots, has received little attention. Their effectiveness will depend on their response to the changed environment caused by removing the upper canopy as reflected by their photosynthetic efficiency and longevity. The relationship of C.G.R. to L.A.I. for field swards of lucerne has been investigated by Steinke (1963), Keoghan (1966), and Okubo et al. (1969), while King and Evans (1967) and Wilfong et al. (1967) have investigated the relationship of net photosynthesis to L.A.I. in artificial lucerne communities. All of these studies, except those of Steinke (1963) in which moisture was a limiting factor, have indicated that lucerne does not have a definite optimum L.A.I. type relationship but one in which C.G.R. or net photosynthesis does not decline rapidly after maximal levels of these are reached. A further important factor in the recovery of lucerne after defoliation is the development and elongation of buds and shoots. The importance of coinciding the time of defoliation with the elongation of a new crop of shoots, in order to obtain rapid recovery growth, has been demonstrated by Smith (1962), Nelson and Smith (1968a) and Leach (1968, 1969, 1970). However, only in the studies of Leach (1968, 1969, 1970) has the effect of intensity and stage of defoliation on the relative importance of different types of shoot been examined in detail. The aims of the experiments described in this thesis were to add to our knowledge of the growth pattern of lucerne after defoliation, to provide information on the effect of stage and intensity of defoliation on the source and vigour of regrowth, and to investigate the role of residual leaves after frequent defoliation. 4 Numerous measurements were made to provide information on the effect of changes in L.A.I. and incident light conditions on the transmission of light through various types of sward. A detailed review of the effect of cutting frequency and height on lucerne regrowth (Keoghan, 1967) is presented as Appendix I.