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.

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Defoliation, Shoot Plasticity, Sward Structure and Herbage Utilization in Pasture: Review of the Underlying Ecophysiological Processes

Morphogenetic and structural variables were obtained from measurements performed on marked tillers and were quantified during August to October 1999 in a ryegrass pasture grazed by sheep under continuous stocking and variable stocking rate in order to maintain the sward at four different heights (5, 10, 15 and 20 cm). The experimental design was a randomized complete block with three replicates. Fifty marked tillers per experimental unit were assessed and distributed along transects lines. Sward measurements were performed in two periods: the first one, from the start of grazing until the point where the intended grazing heights were established, with no differences among treatments. In the second (after achieving the intended grazing heights), leaf elongation rate, total green leaf lamina length per tiller and leaf size showed a positive response to increasing sward heights, while the time to leaf lamina elongation decreased. The variables leaf appearance rate and leaf life span, tiller population density and number of live leaves per tiller were not affected by treatments.

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Variáveis morfogênicas e estruturais de azevém anual (Lolium multiflorum Lam.) manejado em diferentes alturas

Summary


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In grasses the leaf growth zone is the main site of shoot growth where anatomical and chemical characteristics of leaves originate. Yet, there is insufficient information to generalize as to whether the leaf growth zone reflects habitat characteristics, whether leaf growth traits are regularly interrelated, and whether they coincide with characteristics of mature leaves.

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Here the contribution of both length of the leaf growth zone (region where cell division and expansion occurs) and relative elemental growth rate to the variability in leaf elongation rate (LER) were examined in eight grass species from habitats with different soil fertility. Further, we examined the relationship of the above traits with specific leaf area (SLA) and its components.

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Growth zone length differed significantly among species (P < 0.05) and it was the trait contributing the most to LER. Using LER and derived components it was possible to classify seven out of eight species into two groups related to soil fertility. Leaf elongation rate exhibited a positive correlation to SLA and a negative correlation to leaf dry matter content. A significant relationship existed between size of the growth zone and leaf dry matter content.

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The results suggest that the leaf growth zone is a critically important leaf trait that explains inherent differences in LER and other plant characteristics of grass species.

Components of leaf elongation rate and their relationship to specific leaf area in contrasting grasses

Apparent digestibilities of feedstuffs can be estimated using either total faecal collection (TFC) or digestibility markers. However, it is still debatable whether the use of markers results in a reliable prediction of digestibility. Four adult ponies were fed four iso-energetic rations for 24 days with insoluble ash (AIA) and titanium dioxide (TiO2). The mean digestibilities of the organic matter based Mean faecal recovery of markers as percentage of intake was 107.9 (SE 1.588), 118.9 (SE 1.726) using ADL and TiO2 variance in faecal TiO2 2 seemed to be a suitable alternative to the TFC method in order to estimate apparent organic matter digestibility of diets differing in haylage to concentrate ratios.

Forages and grazing in horse nutrition

The objective of this experiment was to evaluate the structural characteristics of young, mature and old tillers on continuously stocked marandu palisade grass maintained at 30 cm and fertilized with nitrogen. Treatments corresponded to three nitrogen application rates (150, 300 and 450 kg/ha of N) plus control (no fertilization) and were assigned to experimental units (1,200-m2 paddocks) according to a completely randomized block design, with four replications. Measurements were performed during fall/winter 2007 and summer 2008. During summer, young tillers showed longer leaves and shorter stems relative to old tillers. During fall/winter, young and old tillers showed shorter leaves, while mature and old tillers showed longer stems. Tiller age interfered with the number of leaves per tiller mainly through variations in the number of expanding and senescing leaves during both fall/winter and summer. The structural characteristics of individual tillers are affected by plant age, with nitrogen favouring the production of leaf tissue in young tillers during summer.

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J. Nelson

Papers

An integrated view of C and N uses in leaf growth zones of defoliated grasses