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This article belongs to the Special Issue The Application of Models for Weed Management in Cropping Systems. In weed science and management, models are important and can be used to better understand what has occurred in management scenarios, to predict what will happen and to evaluate the outcomes of control methods. To-date, perspectives on and the understanding of weed models have been disjointed, especially in terms of how they have been applied to advance weed science and management. This paper presents a general overview of the nature and application of a full range of simulation models on the ecology, biology, and management of arable weeds, and how they have been used to provide insights and directions for decision making when long-term weed population trajectories are impractical to be determined using field experimentation. While research on weed biology and ecology has gained momentum over the past four decades, especially for species with high risk for herbicide resistance evolution, knowledge gaps still exist for several life cycle parameters for many agriculturally important weed species. More research efforts should be invested in filling these knowledge gaps, which will lead to better models and ultimately better inform weed management decision making. Muthukumar V. Bagavathiannan’s research program is supported by state and federal funds appropriated to Texas A&M AgriLife Research. Gayle J. Somerville was supported by the UKRI project of evaluating people-environment trade-offs through low-tech intensification of livestock management in communal grazing systems in South Africa (BB/S014683/1) whilst working at Rothamsted. Brian J. Schutte is supported by state and federal funds appropriated to the New Mexico Agricultural Experiment Station. JLG-A was funded by FEDER (European Regional Development Funds) and the Spanish Ministry of Economy and Competitiveness grants (projects AGL2015-64130-R). Guillermo R. Chantre received funding from Agencia Nacional de Promoción Científica y Tecnológica MINCyT (PICT-2016-1575) and Universidad Nacional del Sur of Argentina (PGI 24/A225).

Ponencia presentada en el 2013 Meeting Weed Science Society of America celebrado en Baltimore (EEUU) del 4 al 7 de febrero de 2013., Models based on thermal time describing seedling emergence are valuable not only to understand important aspects of the biology of a weed species, but also for decision making when implementing weed control practices. Most seedling emergence models have been developed in temperate regions where extreme temperatures during the winter and summer prevent seed germination and seedling growth of summer and winter weed species, respectively, creating relatively defined seedling emergence periods. Conversely, to the extent of our knowledge, there are no weed seedling emergence models for tropical conditions, and it is unknown how the absence of extreme temperatures throughout the year might affect weed seedling emergence patterns in agroecosystems. We studied seedling emergence of itchgrass (Rottboellia cochinchinensis), which is an aggressive grass weed in multiple agronomic crops such as corn, rice, cotton, and sugarcane in the tropics. Field experiments were conducted in several locations in Costa Rica from March to August and August to December in 2010 and 2011. Itchgrass seedling emergence timing and soil temperature were monitored after tillage. Most of the emergence occurred two weeks after tillage, regardless of the location and time of the year, suggesting that tillage promoted itchgrass germination. Gompertz and Weibull models were adjusted to the observed cumulative seedling emergence based on thermal time using growing-degree days (GDD) with a base temperature of 20 C, which was determined using an iteration procedure. The models satisfactorily described the observed emergence (r 2=0.79 to 0.93) although the Weibull model best fitted the data based on the Akaike information criterion (AIC). Fifty percent emergence (E50 ) occurred approximately at 115 GDD although there were differences between locations and time of the year, thus E50 ranged between 64 to 300 GDD. Additionally, two phases of rapid seedling emergence occurred consistently in all sites and years, suggesting the presence of two groups of itchgrass individuals with different seed germination requirements within the evaluated populations. The present study illustrates how predictive seedling emergence models can be generated using GDD for tropical conditions.

Foliar fertilization is an effective practice to correct nutritional deficiencies in plants that are under stress conditions or in soils with low nutrient availability, and consists in applying nutrient solutions directly on leaves. Nutrient absorption in the leaf occurs mainly through the epidermis, by diffusion, due to the nutrient concentration gradient established between the leaf surface and the epidermis interior. Once the nutrient has entered the cytoplasm of the epidermal cells, its movement happens in a relatively expedited fashion. The main barrier the nutrient has to pass is the cuticle, which is comprised of waxes. The physico-chemical characteristics of the nutrient, such as size and polarity, control the absorption rate. The present article provides a detailed description of the foliar absorption of nutrients and its influence on the development and use of foliar fertilizers. La fertilización foliar es una práctica efectiva para la corrección de deficiencias nutricionales en plantas que se encuentran bajo condiciones de estrés o en suelos con baja disponibilidad de nutrientes. Consiste en aplicar disoluciones de nutrientes directamente sobre las hojas. Esta absorción en la hoja se desarrolla mayoritariamente a través de la epidermis, por difusión, debido al gradiente de concentración del nutriente que se establece entre la superficie de la hoja y en el interior de la epidermis. Una vez que el nutriente ha ingresado al citoplasma de las células epidermales, la movilización de este ocurre en forma relativamente expedita. La principal barrera que el nutriente debe atravesar es la cutícula, la cual está compuesta de ceras. Las características físico-químicas del nutriente, tales como tamaño y polaridad controlan la tasa de absorción. El presente artículo provee una descripción detallada del proceso de absorción foliar de nutrientes y su influencia en el desarrollo y uso de fertilizantes foliares.