Abstract
Sward structure affects herbage growth, pasture species dynamics, and herbage utilization. Defoliation management has a major impact on sward structure. In particular, tiller size-tiller density compensations allow for the maintenance of herbage growth. Tiller size and tiller density are determined by several major morphogenetical components. Defoliation affects these morphogenetical components, depending on its frequency and its intensity, through several direct and indirect physiological and environmental processes. Due to the implications of leaf area removal, defoliation has a direct effect on the mobilization of C and N reserves and their supply to growing leaves. In addition, defoliation has an indirect effect on leaf and tiller morphogenesis, due to its impact on the light environment within the canopy as well as plant responses to light signals (blue light, red far red ratio). Defoliation may also in some cases have a direct negative effect on leaf growth by damaging leaf meristems. Understanding the respective role of these various physiological and environmental processes requires studies where defoliation, photosynthetic active radiation and light signals are manipulated independently. Past and recent knowledge on these direct and indirect effects of defoliation on plant morphogenesis are discussed, leading to an overall integrated view of physiological and environmental processes that lead to adaptations of sward structure in response to defoliation management. Major consequences for herbage utilization efficiency are presented.
Highlights
Maximal daily intake is related to tiller size, the vertical profile of bulk density, which depends on tiller density, and sheath length, which limits grazing in the lower sward layers [6,8,11,12]
Since leaf area is a major determinant of plant growth and since defoliation leads to the removal of part of it, adaptation of sward structure to defoliation management has to be analyzed first in terms of leaf area dynamics
These studies suggest that the decrease in leaf length was associated with a decrease in leaf elongation duration (LED). These results are confirmed by recent data showing a strong relationship between the length of the sheath tube from which a leaf emerges and the final length of this new leaf. This occurs whether the source of variation in sheath length is the ontogenic increase in sheath length during development of an undefoliated tiller, whether it is the sheath length of axillary leaves and the leaves of its daughter tillers, or whether sheath length is reduced by defoliation (Figure 5)
Summary
Plant-herbivore relationships have been the subject of numerous earlier studies These studies have shown that plants and swards have the capacity to adapt their structure (plasticity in size, number and spatial orientation of shoot organs), productivity and persistency, to the defoliation characteristics that result from grazing or cutting management strategies [1,2,3]. Earlier studies have shown that the plasticity of sward structure in response to defoliation allows adaptation in sward productivity and partly determines herbage utilization by animals, in conjunction with animal grazing behavior [4,5,6,7,8,9]. The plasticity of sward structure in relation to defoliation and grazing management, together with the impact of sward structure on herbage utilization of grazing animals, are probably responsible for the observation that within certain limits in defoliation management, herbage productivity and herbage utilization by grazing animals are maintained at a comparable rate, despite variations in herbage mass [13]
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