Plevele v ekologickém zemědělství

Plevele jsou specifickou skupinou rostlin, kterou příroda sama nevytvářela. Jejich vznik je spojen s činností člověka – zemědělce. Jejich původ můžeme odvodit od „pionýrských“ rostlin. Ty se vyskytují v počátečním stadiu rostlinné sukcese. Právě podmínky tohoto stadia jsou velmi podobné podmínkám panujícím na orné půdě. Část druhů se tedy přizpůsobila podmínkám opakované kultivace půdy a staly se z nich polní plevele

Results of the CoreOrganic-Workshop on animal based parameters in Trenthorst, Germany (04.02.08-08.02.2008)

It was the aim of this meeting to train the assessors of several CoreOrganic-Partners (i.e. Austria, Germany, Denmark and Norway) with regard to the methods which are going to be applied in the course of the project on-farm assessments. Training involved animal-based parameters, a resource checklist and a management questionnaire. After a theoretical and practical introduction, inter-observer reliability (IOR) of the main animal-based parameters was tested. This included gait scoring/lameness, body condition, cleanliness and integument alterations. The scoring systems for gait scoring, cleanliness and integument alterations had been adapted from the WelfareQuality® protocol; body condition was scored according to Metzner et al. (1993). Subjective scoring systems which are used in many epidemiological studies have the advantage that they do not require any equipment but - due to the subjectivity of the assessment - it is necessary to achieve acceptable inter-observer reliability (IOR) before and after the study to ensure valid data

Hybridization and spin decoherence in heavy-hole quantum dots

We theoretically investigate the spin dynamics of a heavy hole confined to an unstrained III-V semiconductor quantum dot and interacting with a narrowed nuclear-spin bath. We show that band hybridization leads to an exponential decay of hole-spin superpositions due to hyperfine-mediated nuclear pair flips, and that the accordant single-hole-spin decoherence time T2 can be tuned over many orders of magnitude by changing external parameters. In particular, we show that, under experimentally accessible conditions, it is possible to suppress hyperfine-mediated nuclear-pair-flip processes so strongly that hole-spin quantum dots may be operated beyond the `ultimate limitation' set by the hyperfine interaction which is present in other spin-qubit candidate systems.Comment: 7 pages, 3 figure

Tunable quantum spin Hall effect in double quantum wells

The field of topological insulators (TIs) is rapidly growing. Concerning possible applications, the search for materials with an easily controllable TI phase is a key issue. The quantum spin Hall effect, characterized by a single pair of helical edge modes protected by time-reversal symmetry, has been demonstrated in HgTe-based quantum wells (QWs) with an inverted bandgap. We analyze the topological properties of a generically coupled HgTe-based double QW (DQW) and show how in such a system a TI phase can be driven by an inter-layer bias voltage, even when the individual layers are non-inverted. We argue, that this system allows for similar (layer-)pseudospin based physics as in bilayer graphene but with the crucial absence of a valley degeneracy.Comment: 9 pages, 8 figures, extended version (accepted Phys. Rev. B

The effect of three soil tillage treatments on weed infestation in forage maize

This study assessed the impact of using soil tillage in maize crops on weed infestation intensity and weed species composition. A field experiment was established as a model example of livestock production management in drier climate conditions where maize was grown in seven-step crop rotation sequence: alfalfa - the first year, alfalfa - the second year, winter wheat, forage maize, winter wheat, sugar beet, and spring barley. Three soil tillage treatments were applied: conventional tillage (CT), minimum tillage (MT), and no-tillage (NT). An arithmetic method and multivariate analyses of ecological data were used. The highest weed infestation, mainly due to late spring species, was recorded in MT. Perennial and overwintering species were frequently observed in NT. Early spring weed species were abundant in CT. Different tillage treatments cause a significant change in the weed species spectrum in maize. A study of the relationship between tillage and the level of weed infestation requires long-term monitoring which will allow us to predict the intensity of weed infestation in particular locations.O

A double critical mass phenomenon in a no-flux-Dirichlet Keller-Segel system

Derived from a biophysical model for the motion of a crawling cell, the system $$(*)~\begin{cases}u_t=\Delta u-\nabla\cdot(u\nabla v)\\0=\Delta v-kv+u\end{cases}$$ is investigated in a finite domain $\Omega\subset\mathbb{R}^n$, $n\geq2$, with $k\geq0$. While a comprehensive literature is available for cases with $(*)$ describing chemotaxis systems and hence being accompanied by homogeneous Neumann-type boundary conditions, the presently considered modeling context, besides yet requiring the flux $\partial_\nu u-u\partial_\nu v$ to vanish on $\partial\Omega$, inherently involves homogeneous Dirichlet conditions for the attractant $v$, which in the current setting corresponds to the cell's cytoskeleton being free of pressure at the boundary. This modification in the boundary setting is shown to go along with a substantial change with respect to the potential to support the emergence of singular structures: It is, inter alia, revealed that in contexts of radial solutions in balls there exist two critical mass levels, distinct from each other whenever $k>0$ or $n\geq3$, that separate ranges within which (i) all solutions are global in time and remain bounded, (ii) both global bounded and exploding solutions exist, or (iii) all nontrivial solutions blow up in finite time. While critical mass phenomena distinguishing between regimes of type (i) and (ii) belong to the well-understood characteristics of $(*)$ when posed under classical no-flux boundary conditions in planar domains, the discovery of a distinct secondary critical mass level related to the occurrence of (iii) seems to have no nearby precedent. In the planar case with the domain being a disk, the analytical results are supplemented with some numerical illustrations, and it is discussed how the findings can be interpreted biophysically for the situation of a cell on a flat substrate