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 grazed grass-legume mixtures. However, maintaining the species balance has proven to be difficult, especially as grazing provides differential defoliation and regrowth. Since changes in the sward botanical composition can be ascribed to species differences in growth, senescence or intake, a method that yields independent estimates of these three fluxes 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 individually marked population units. This method was applied during two spring and summer growing seasons to a ryegrass-clover mixture continuously or rotationally grazed by sheep and is assessed using these experimental results. 3. Defoliation was measured on grazed population units (i.e. grass tillers; elongated and axillary shoot growing points of clover). To calculate the growth rate of defoliated leaves, a sample of ungrazed leaves of each defoliation status (with or without previous defoliation) was used. Additionally, for clover, a sample of ungrazed leaves 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 the preceding period. 4. The estimates of the mixture growth flux varied by less than 10 and 17% when the mean values of the gravimetric coefficients and of the density of the population units, respectively, were increased by one standard deviation. The net recruitment of new population units led to an underestimation of the growth flux by 8 and 5% with grass and clover, respectively. 5. The method was partially validated, by comparing independent estimates of the mean dry-weight of the population units, based either on tissue turnover or on destructive measurements. For vegetative tillers and elongated clover growing points, both estimates were highly correlated with no significant bias. 6. Although 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 understanding of the impacts of herbivory on the grass-legume balance in a grazed mixture can be gained.