18,129 research outputs found

    Herbicide-resistant weeds : from research and knowledge to future needs

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    Synthetic herbicides have been used globally to control weeds in major field crops. This has imposed a strong selection for any trait that enables plant populations to survive and reproduce in the presence of the herbicide. Herbicide resistance in weeds must be minimized because it is a major limiting factor to food security in global agriculture. This represents a huge challenge that will require great research efforts to develop control strategies as alternatives to the dominant and almost exclusive practice of weed control by herbicides. Weed scientists, plant ecologists and evolutionary biologists should join forces and work towards an improved and more integrated understanding of resistance across all scales. This approach will likely facilitate the design of innovative solutions to the global herbicide resistance challenge

    Precise Measurement of Gravity Variations During a Total Solar Eclipse

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    The variations of gravity were measured with a high precision LaCoste-Romberg D gravimeter during a total solar eclipse to investigate the effect of solar eclipse on the gravitational field. The observed anomaly (7.0±2.7)×10−8(7.0 \pm 2.7) \times 10^{-8} m/s2^2 during the eclipse implies that there may be a shielding property of gravitation

    The Future – Disruptive Optometry

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    Change is the only constant. How our profession embraces, and indeed leads, disruption will be the key to our collective successful future. The pace of disruption, both in our world and in our industry, is accelerating, forcing independent optometrists to examine their practices with a view to meeting changing patient needs and embracing technology, while also ensuring patient safety. This opportunity allows the profession of optometry to get ahead of the curve and lead change, rather than be a victim of it. The Canadian Association of Optometrists invited four leaders in the industry to provide their perspectives on disruptive optometry and how optometrists can be the disruptor, rather than the disrupted

    Purposeful Ephemera: The Implications of Self-Destructing Space Technology for the Future Practice of Archaeology

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    This chapter is presented from the perspective of a professional archaeologist who specializes in Greek archaeology, intercultural contact and exchange, and the ethics of cultural heritage. His chapter investigates the mandates for discard and “design for demise” of space objects in the wider context of cultural phenomena from all cultures. The chapter finds comparanda for purposeful ephemera in examples from the media of performance, architecture, and visual art

    Riemannian Optimization via Frank-Wolfe Methods

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    We study projection-free methods for constrained Riemannian optimization. In particular, we propose the Riemannian Frank-Wolfe (RFW) method. We analyze non-asymptotic convergence rates of RFW to an optimum for (geodesically) convex problems, and to a critical point for nonconvex objectives. We also present a practical setting under which RFW can attain a linear convergence rate. As a concrete example, we specialize Rfw to the manifold of positive definite matrices and apply it to two tasks: (i) computing the matrix geometric mean (Riemannian centroid); and (ii) computing the Bures-Wasserstein barycenter. Both tasks involve geodesically convex interval constraints, for which we show that the Riemannian "linear oracle" required by RFW admits a closed-form solution; this result may be of independent interest. We further specialize RFW to the special orthogonal group and show that here too, the Riemannian "linear oracle" can be solved in closed form. Here, we describe an application to the synchronization of data matrices (Procrustes problem). We complement our theoretical results with an empirical comparison of Rfw against state-of-the-art Riemannian optimization methods and observe that RFW performs competitively on the task of computing Riemannian centroids.Comment: Under Review. Largely revised version, including an extended experimental section and an application to the special orthogonal group and the Procrustes proble

    Stochastic Frank-Wolfe Methods for Nonconvex Optimization

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    We study Frank-Wolfe methods for nonconvex stochastic and finite-sum optimization problems. Frank-Wolfe methods (in the convex case) have gained tremendous recent interest in machine learning and optimization communities due to their projection-free property and their ability to exploit structured constraints. However, our understanding of these algorithms in the nonconvex setting is fairly limited. In this paper, we propose nonconvex stochastic Frank-Wolfe methods and analyze their convergence properties. For objective functions that decompose into a finite-sum, we leverage ideas from variance reduction techniques for convex optimization to obtain new variance reduced nonconvex Frank-Wolfe methods that have provably faster convergence than the classical Frank-Wolfe method. Finally, we show that the faster convergence rates of our variance reduced methods also translate into improved convergence rates for the stochastic setting

    Farmer seed networks make a limited contribution to agriculture? Four common misconceptions

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    The importance of seed provisioning in food security and nutrition, agricultural development and rural livelihoods, and agrobiodiversity and germplasm conservation is well accepted by policy makers, practitioners and researchers. The role of farmer seed networks is less well understood and yet is central to debates on current issues ranging from seed sovereignty and rights for farmers to GMOs and the conservation of crop germplasm. In this paper we identify four common misconceptions regarding the nature and importance of farmer seed networks today. (1) Farmer seed networks are inefficient for seed dissemination. (2) Farmer seed networks are closed, conservative systems. (3) Farmer seed networks provide ready, egalitarian access to seed. (4) Farmer seed networks are destined to weaken and disappear. We challenge these misconceptions by drawing upon recent research findings and the authors’ collective field experience in studying farmer seed systems in Africa, Europe, Latin America and Oceania. Priorities for future research are suggested that would advance our understanding of seed networks and better inform agricultural and food policy
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