TISSUE TURNOVER WITHIN GRASS-CLOVER MIXED SWARDS GRAZED BY SHEEP - METHODOLOGY FOR CALCULATING GROWTH, SENESCENCE AND INTAKE FLUXES

1. A sustainable, low-input land use might be achieved by the use of g razed grass-legume mixtures. However, maintaining the species balance has proven to be difficult, especially as grazing provides differentia l defoliation and regrowth. Since changes in the sward botanical compo sition can be ascribed to species differences in growth, senescence or intake, a method that yields independent estimates of these three flu xes for both components of a grazed grass-legume mixture is needed. 2. The objective of this paper is to present and assess such a detailed tissue turnover method based on non-destructive measurements of indivi dually marked population units. This method was applied during two spr ing and summer growing seasons to a ryegrass-clover mixture continuous ly or rotationally grazed by sheep and is assessed using these experim ental results. 3. Defoliation was measured on grazed population units (i.e. grass tillers; elongated and axillary shoot growing points of cl over). To calculate the growth rate of defoliated leaves, a sample of ungrazed leaves of each defoliation status (with or without previous d efoliation) was used. Additionally, for clover, a sample of ungrazed l eaves of each 'botanical number' was also required. The mean extension rates of leaves that underwent defoliation during an observed period, and of leaves that did not, was not significantly different during th e preceding period. 4. The estimates of the mixture growth flux varied by less than 10 and 17% when the mean values of the gravimetric coeff icients and of the density of the population units, respectively, were increased by one standard deviation. The net recruitment of new popul ation units led to an underestimation of the growth flux by 8 and 5% w ith grass and clover, respectively. 5. The method was partially valida ted, by comparing independent estimates of the mean dry-weight of the population units, based either on tissue turnover or on destructive me asurements. For vegetative tillers and elongated clover growing points , both estimates were highly correlated with no significant bias. 6. A lthough this method is labour intensive, it yields reliable estimates of the growth, intake and senescence fluxes, at three different levels (square metre, population unit and plant part). Hence, a better under standing of the impacts of herbivory on the grass-legume balance in a grazed mixture can be gained.