RESUMEN El desafío de la agricultura actual es maximizar la producción agrícola reduciendo impactos ambientales negativos, optimizando la eficiencia de uso de recursos e insumos. Objetivos i) evaluar emisiones de N2O durante una rotación soja-cebada-maíz y sus respectivos interperíodos -barbechos químicos (ChF) y cultivos de cobertura (CC)-, y ii) comprender las relaciones entre emisión de N2O con N mineral (Nmin), espacio poroso lleno de agua (WFPS) y temperatura de suelo (ST). Las emisiones de N2O se evaluaron semanalmente bajo dos sistemas de manejo: intensificación ecológica (EI) y prácticas agrícolas actuales (FP). EI incluyó mayores tasas de N que FP, aplicación dividida de N, cultivo de cobertura (CC) después de la cebada, entre otras prácticas. Los flujos de N2O fueron mayores durante los cultivos (9-43, 6-61 y 4-104 μg N2O-N m-2h-1, para soja, cebada y maíz, respectivamente), intermedios durante el interperíodo cebada-maíz (2-47 y 3-35 μg N2O-N m-2h-1, para CC y CHF, respectivamente) y menor durante CHF de interperíodos maíz-soja y soja-cebada (1-26 y 5-22 μg N2O-N m-2h-1, respectivamente). El árbol de regresión proporcionó los siguientes umbrales: 7,4 mg N kg-1 para Nmin; 10.35 o 8.7°C para ST, dependiendo de los valores de Nmin y WPFS; 39.6 y 55.4 % para WPFS, ambos con alto N. No hubo diferencias significativas en emisiones acumuladas de N2O entre sistemas de manejo para la secuencia completa (3050 y 3380 g N2O-N ha-1, para EI y FP, respectivamente). Las emisiones de N2O totales a escala rendimiento fueron menores para EI que para FP, con una reducción de 29 % para soja (p<0,05) y cebada (ns) y en 30 % para maíz (p<0,05) en comparación con FP. En conclusión, para la secuencia completa, EI no aumentó las emisiones de N2O por unidad de área y redujo las emisiones por unidad de rendimiento, a pesar de utilizar tasas más altas de N e incluir un CC.

ABSTRACT Agricultural challenge today is to maximize crop production reducing negative environmental impacts by efficiently use of resources and inputs. Objectives: i) to evaluate N2O emissions during a soybean-bar-ley-maize rotation and their respective interperiods -chemical fallows (ChF) and cover crops (CC)-, and ii) to understand the relationships between N2O flow rates with soil mineral N content (Nmin), water-filled porous space (WFPS), and soil temperature (ST). N2O emissions were evaluated weekly under two management systems, ecological intensification (EI) and farmer current practices (FP). EI included higher N rates than FP, split N application, a cover crop (CC) after barley, among other management practices. Simultaneously to N2O emissions, Nmin, WFPS and ST were determined. The N2O fluxes were highest during crop periods (9-43, 6-61 and 4-104 μg N2O-N m-2 h-1, for soybean, barley and maize, respectively), intermediate during barley-maize interperiod (2-47 and 3-35 μg N2O-N m-2 h-1, for CC and ChF, respectively) and lowest during ChF of maize-soybean and soybean-barley interperiods (1-26 and 5-22 μg N2O-N m-2 h-1, respectively). Regression tree provided threshold splits for the effect of soil variables on emission rates: 7.4 mg N kg-1 for Nmin; 10.35 or 8.7°C for ST, depending on Nmin and WPFS values; 39.6 and 55.4 % for WPFS, both with high N. There were no significant differences in accumulated N2O emissions between management systems for the complete sequence (3050 and 3380 g N2O-N ha-1, for EI and FP, respectively). Yield-based N2O emissions for the complete sequence were lower for EI than for FP. EI decreased yield-scaled N2O emissions by 29% for soybean (p<0.05) and barley (ns) and by 30% for maize (p<0.05) compared with FP. In conclusion for the complete sequence, the EI system did not increase N2O emissions per unit area and reduced emissions per unit yield despite utilizing higher N rates and including a CC.

RESUMO: Hipomagnesemia é um distúrbio metabólico de ruminantes devido a uma redução na absorção de magnésio (deficiência primária), digestibilidade incorreta ou fatores metabólicos associados que reduzem a ingestão de Mg (deficiência secundária). Hipomagnesemia é uma doença de produção responsável por importantes perdas econômicas em rebanhos de corte da Argentina. Vários fatores influenciam o desenvolvimento da hipomagnesemia em bovinos, incluindo fatores fisiológicos, clima, solo e forragem. Este trabalho descreve uma análise retrospectiva dos últimos 20 anos, revisando os casos de hipomagnesemia clínica em bovinos de corte registrados no Serviço de Diagnóstico Veterinário do INTA Balcarce, Argentina.

ABSTRACT: Hypomagnesaemia (grass tetany) is a metabolic disorder of ruminants due to a reduced dietary intake of magnesium (primary deficiency), incorrect digestibility or associated metabolic factors reducing Mg intake (secondary deficiency). Grass tetany is a production disease responsible for important economic losses in beef herds from Argentina. Several factors influence the development of grass tetany in cattle, including physiological status, weather, soil and forage. This research described a retrospective analysis over the past 20 years, revising the cases of beef cattle clinical hypomagnesaemia registered at the Veterinary Diagnostic Service in INTA Balcarce, Argentina.

Soil total (COT) and particulate (COP) organic carbon (C) contents decrease over the years under cropping. Its variation affects many soil processes that determine soil productivity (e.g. nitrogen (N) supply). COT and COP changes depend on the balance between C input and loss. For mollisols of the southeastern region of Buenos Aires province, it was hypothesized that: 1) COT and COP content decrease in the arable layer and vary as a function of the crop sequence regardless of the tillage system (SL), ii) COP content depends on the immediate preceding crop residue characteristics, and iii) changes in COT and COP are related to the soil N supplying capacity, regardless of the SL and crop sequence. Total organic C and COP were determined in soil samples taken annually at 0-5 cm and 5-20 cm depths in a long-term experiment in Balcarce between 2005 and 2011. The experiment comprised two SL (conventional (LC) and no-till (SD) and three crop sequences (with three alternatives (Series) for each one), including corn (M), wheat (T) and soybean (S): MMT, MST y SST. Corn and T were fertilized with N (FN). There were differential decreases in COT and COP among SL (no decrease under SD and decreases of 0.34 and 0.39 Mg ha-1 yr-1 under LC, respectively) only at the 0-5 cm layer. In the whole arable layer COT and COP decreased over the years under cropping (0.30 and 0.26 Mg ha-1 yr-1, respectively), regardless of the SL. No differential COT nor COP decrease was detected related to crop sequences, regardless of the SL. Neither was there an effect of the immediately preceeding crop residue characteristics on COP content. Likewise, COT and COP showed similar dynamics than reported for NAN. Relationships between NAN and COT and NAN and COP were significant although with low R² (0.55 and 0.57 (0-5 cm), 0.09 and 0.10 (5-20 cm), 0.18 and 0.17 (0-20 cm), respectively). Frequency and rate of FN may have masked the expected effects of the amount of residue C input on COT and COP and, besides, on the relationship of these two variables and NAN. Nitrogen fertilization affects organic material decomposition and transformation dynamics. The experimentally obtained evidences did not support the posed hypotheses.

Los contenidos de carbono (C) orgánico total (COT) y particulado (COP) del suelo disminuyen con los años bajo agricultura y su variación afecta muchos procesos edáficos que determinan la productividad (e.g. la provisión de nitrógeno (N)). Las variaciones de COT y COP son dependientes del balance entre los aportes y la pérdida de C del suelo. Se hipotetiza que en molisoles del Sudeste Bonaerense: i) los cambios en COT y COP en la capa arable, son más dependientes del aporte carbonado de la secuencia de cultivos utilizada que del sistema de labranza (SL) empleado, ii) el contenido de COP es dependiente de las características de los residuos devueltos por el cultivo antecesor inmediato, y iii) las variaciones de COT y COP se relacionan con la capacidad del suelo de proveer N, independientemente del SL y de la secuencia de cultivos. Se determinaron el COT y el COP de muestras tomadas anualmente de 0-5 y 5-20 cm de un ensayo en Balcarce entre 2005 y 2011. El ensayo comprendió dos SL (convencional (LC) y siembra directa (SD)) y tres Secuencias de cultivos (con tres alternativas (Series) cada una), incluyendo maíz (M), trigo (T) y soja (S): MMT, MST y SST. El M y el T fueron fertilizados con N (FN). Hubo disminuciones diferenciales en COT y COP entre SL (sin disminución bajo SD y con disminuciones de 0,34 y 0,39 Mg ha-1 año-1 bajo LC, respectivamente) sólo en la capa de 0-5 cm. En la capa arable completa, aquellas variables disminuyeron con los años de agricultura (0,30 y 0,26 Mg ha-1 año-1, respectivamente), independientemente del SL. No se observó una disminución diferencial de COT ni de COP asociada a las combinaciones de cultivos, independientemente del SL. Tampoco se observó un efecto de las características del cultivo antecesor inmediato sobre el contenido de COP. Asimismo, el COT y el COP presentaron una dinámica similar a la reportada para NAN. No obstante, las relaciones entre NAN y COT y NAN y COP, si bien fueron significativas, presentaron bajos R² (0,55 y 0,57 (0-5 cm), 0,09 y 0,10 (5- 20 cm), 0,18 y 0,17 (0-20 cm), respectivamente). La FN (frecuencia, dosis) puede haber enmascarado los efectos esperados en la relación del COP y COT con el aporte de C por los residuos y, además, de aquéllas con el NAN, por afectar las condiciones de descomposición y transformación de los materiales orgánicos. Las evidencias reunidas no sostuvieron las hipótesis planteadas.

Soil physical condition is essential to define soil quality/health and is affected by management. Hence, it is necessary to monitor physical status of soils under cropping through sensitive and simple indicators. One of them is bulk density (DA), which is related to many soil functions and is sensitive to management practices. Reference methods to determine DA are tedious and time consuming and make difficult the frequent DA monitoring. The aim of this work was to compare the reference method to determine DA (cylinder method, MC) with a simpler alternative method (tubular sampler method, MM) which facilitates soil sampling. The experiment was carried out on Mollisols of Balcarce county under nine different soil use situations. We also evaluated method performance when applied by persons with different expertise in soil sampling. Bulk density obtained with MM was acceptably similar to that obtained with MC since variability in DA measured with MM explained 93% of the variability of DA measured with MC. On the other hand, the analysis of the mean of the differences of DA between methods showed a significant effect (p<0.05) of the level of experience of the operator, being higher when the operator had low expertise. Our results indicate that MM could be used regularly to measure DA in soils similar to those used in this study and to compare management situations and/or to calculate carbon or nutrient stock seeking for balances. However, the reliability of the measurements done with MM depends on the expertise or the training received by the person that apply it

La condición física del suelo es fundamental para definir su calidad/salud y es afectada por el uso. Por ello, es necesario el seguimiento de los cambios en la condición física de los suelos bajo producción, a través de indicadores sensibles y fáciles de determinar. Uno de ellos es la densidad aparente (DA), la cual se relaciona con la funcionalidad del suelo y es sensible a las prácticas de manejo. Los métodos de referencia de determinación de la DA son tediosos y dificultan el monitoreo frecuente. El objetivo de este trabajo fue comparar el método de referencia de determinación de DA (método del cilindro, MC) con un método alternativo (método del muestreador tubular, MM) más sencillo para la obtención de las muestras. La experiencia se realizó en Molisoles del partido de Balcarce, en nueve situaciones de uso de suelo. Se evaluó también el comportamiento de los métodos con operarios con diferente nivel de experiencia en la toma de muestras. La DA obtenida con el MM tuvo correspondencia aceptable a la DA con el MC, ya que la variabilidad de ésta explicó el 93% de la de aquella. Asimismo, la diferencia entre valores promedio de DA obtenidas con ambos métodos fue dependiente del nivel de experiencia del operario (p<0,05), siendo mayor esta diferencia para operarios con baja experiencia. Los resultados de este estudio indican que el MM podría ser utilizado de manera regular para la determinación de la DA en suelos similares a los analizados, con fines agronómicos tales como la comparación de situaciones de manejo o el cálculo de la masa en un determinado espesor de suelo para la realización de balances de nutrientes o de carbono. No obstante, ello es válido si el operario encargado de las determinaciones posee experiencia o recibe suficiente entrenamiento para la realización de muestreos.

O presente trabalho foi realizado para estudar a persistência dos herbicidas atrazine, metribuzin e simazine em solos do sudeste da Província de Buenos Aires, Argentina, por meio de um bioensaio com aveia. Doses de atrazine de 0, 0,58, 1,16 e 2,32 mig g-1 de ingrediente ativo (i.a.), doses de metribuzin de 0, 0,14, 0,28 e 0,56 mig g-1 de i.a., e doses de simazine de 0, 0,72, 1,45 e 2,9 mig g-1 de i.a. de solo seco, foram aplicadas em vasos que continham solos de Balcarce e San Cayetano. O conteúdo de matéria orgânica (MO) e o pH do solo de Balcarce foram 5,5% e 5,8%, enquanto no solo de San Cayetano foram 2,9% e 6,7%, respectivamente. Com a parte aérea das plantas de aveia calculou-se o peso relativo da matéria seca (PSR) em relação ao tratamento sem herbicida. Considerando uma redução de 20% do PSR, as persistências nas doses recomendadas para os solos de Balcarce e San Cayetano foram, respectivamente: atrazine (1,16 mig g-1 de i.a.), 78 e 130 dias após tratamento (DAT); metribuzin (0,28 mig g-1 de i.a.), 63 e 77 DAT; simazine (1,45 mig g-1 de i.a.), 81 e 156 DAT. Os resultados indicam que a persistência foi maior com altas doses dos herbicidas, com baixo conteúdo de MO e alto pH dos solos.

An oat bioassay was conducted in pots under greenhouse conditions to determine the persistence of atrazine, metribuzin and simazine herbicides in soils of the southeast of Buenos Aires Province, Argentina. Atrazine rates of 0, 0.58, 1.16 and 2.32 mug g-1 of active ingredient (a.i.), metribuzin rates of 0, 0.14, 0.28 and 0.56 mug g-1 of a.i., and simazine rates of 0, 0.72, 1.45 and 2.9 mug g-1 of a.i. dry soil weight were applied to pots containing soils from Balcarce and San Cayetano sites. Organic matter (OM) content and pH of Balcarce soil were 5.5% and 5.8%, while for San Cayetano soil were 2.9% and 6.7%, respectively. Relative dry weight (RDW) of oat shoots was calculated as percentage of control. Considering a 20% RDW reduction of oat shoots, persistences of recommended rates for the region were: atrazine (1.16 mug g-1 of a.i.), 78 and 130 days after treatment (DAT) for Balcarce and San Cayetano soil, respectively; metribuzin (0.28 mug-1 of a.i.), 63 and 77 DAT for Balcarce and San Cayetano soil, respectively; simazine (1.45 mug g-1 of a.i.), 81 and 156 DAT for Balcarce and San Cayetano soil, respectively. Results show that persistence of atrazine, metribuzin and simazine in soil increased with high rates, low OM content and high pH.

Conduziram-se estudos no campo durante três anos no sudeste da Província de Buenos Aires, Argentina, para determinar o período crítico de controle de plantas daninhas no milho. Os tratamentos consistiram de dois períodos diferentes de interferência das plantas daninhas: um período crítico livre de plantas daninhas, e um período crítico de remoção de plantas daninhas. O período crítico livre de plantas daninhas foi ajustado por meio da equação Gompertz, enquanto o período crítico de remoção de plantas daninhas foi ajustado mediante a equação logística. Foram utilizadas as unidades térmicas para descrever cada período de interferência de plantas daninhas. O período crítico livre e o período crítico de remoção de plantas daninhas variaram de 222 a 416 e de 128 a 261 unidades térmicas acumuladas, respectivamente, para impedir uma perda de rendimento de 2,5%. A biomassa das plantas daninhas variou ao contrário do rendimento do cultivo, em todos os anos estudados. Quando as plantas daninhas competiram com o cultivo desde a emergência, foi alcançado um grande incremento de biomassa aos dez dias após a emergência do cultivo. Porém, poucas plântulas de plantas daninhas emergiram e prosperaram depois do estádio de 5-6 folhas de milho (10-20 dias após a emergência).

Field studies were conducted over 3 years in southeast Buenos Aires, Argentina, to determine the critical period of weed control in maize (Zea mays L.). The treatments consisted of two different periods of weed interference, a critical weed-free period, and a critical time of weed removal. The Gompertz and logistic equations were fitted to relative yields representing the critical weed-free and the critical time of weed removal, respectively. Accumulated thermal units were used to describe each period of weed-free or weed removal. The critical weed-free period and the critical time of weed removal ranged from 222 to 416 and 128 to 261 accumulated thermal units respectively, to prevent yield losses of 2.5%. Weed biomass proved to be inverse to the crop yield for all the years studied. When weeds competed with the crop from emergence, a large increase in weed biomass was achieved 10 days after crop emergence. However, few weed seedlings emerged and prospered after the 5-6 leaf maize stage (10-20 days after emergence).