Fertilización nitroazufrada en verdeos invernales puros y consociados con vicias y efectos sobre el suelo

Abstract

En los sistemas de producción mixtos del sudoeste de la provincia de Buenos Aires (Argentina), los cereales forrajeros de invierno (puros o mezclados con leguminosas anuales) forman parte de las secuencias o rotaciones de cultivos. En esta región la disponibilidad y calidad del forraje son herramientas críticas para definir la producción de carne. El objetivo de este trabajo fue estudiar el comportamiento productivo de verdeos de invierno, con diferente provisión de nitrógeno (N) y azufre (S), su calidad nutricional y los efectos sobre variables edáficas. Se realizaron dos ensayos, sobre lotes con antecesor trigo, en establecimientos rurales de producción de Bajo Hondo y de Bordenave. El diseño fue de parcelas divididas con factores fijos en bloques completos al azar. Los tratamientos resultaron de la combinación de dos factores: cultivo en parcelas mayores (avena y centeno, puros y en consociación con Vicia villosa y V. sativa) y fertilización en subparcelas (testigo sin fertilización y dosis de 30 y 60 kg N ha-1, con y sin adición de 10 kg S ha-1). Se determinó productividad de materia seca (MS), mediante corte en fechas fijas. En presiembra se extrajeron muestras de suelo, por bloque, para caracterizar los sitios. Durante y después del cultivo, se muestrearon subparcelas en 4 fechas para la determinación de nitrógeno inorgánico, azufre extraíble y una fracción lábil de carbono orgánico.

In mixed production systems of the southwest of Buenos Aires province (Argentina) small grains for forage, in pure stands or mixed with vetch, form part of sequences or crop rotations. In this zone, forage availability and quality are a critical tools to define response in meat production. The objective of this work was to study productive behavior of these forage crops, at different levels of nitrogen (N) and sulfur (S), nutritional quality and residual effects on soil variables. Two experiments were carried out in farmer’s fields, in Bajo Hondo and Bordenave, in which wheat was the previous crops,. A design of three randomized complete block with split plots was used. Treatments resulted from the combination of two fixed factors: crops (oats and rye, pure or mixed with Vicia villosa and V. sativa) and fertilization (check, 30 and 60 kg N ha-1 , with and without 10 kg S ha-1 ). Dry matter (DM) production was evaluated by clipping the forage on fixed dates. Soil was sampled before sowing at block level to characterize the sites. During crop growth and ensuing fallow experimental subunits were sampled on 4 dates for determination of inorganic nitrogen, extractable sulfur and a labile fraction of organic carbon. Vicia roots were extracted and nodules were evaluated. In Bajo Hondo, type of fallow (chemical or mechanical) was included as an additional factor in a split block. Variables of nutritional quality were determined on forage samples to verify fertilization effects on oat in pure stands, differences between genus in the consociations and between vetch species. Statistical procedures included descriptive, univariate and multivariate analysis. Cumulative DM ranged between 1800 and 5300 kg ha-1 . In both sites there was response of DM to fertilization in mixed and pure stands. N agronomic efficiency was between 17 and 32 kg DM kg N -1 and increased up to 66 % due to S addition. Soil characteristics and implantation dates affected crop growth and development, fertilizer response, nutritional quality variables and their relations, as well as changes in soil variables. Mixtures were 49 to 76 % more productive than pure grass stands in Bajo Hondo and Bordenave, respectively, as related to crop and fertilization. N biological fixation and root residues improved soil fertility. Soil labile carbon and potential to mineralize nitrogen were increased, the latter being evident only under mechanical fallow. Rye exhibited greater rusticity than oats through its growth in autumn and winter and was more restrictive on vetch participation. The two species differed in growth cycle, root systems, nodulation characteristics and participation in DM composition but not in DM quality. N and NS applications improved DM quality in oat from pure stands. Crude protein values in pure oat and stands with less than 27 % vetch could restrict high growth rates in ruminants. Fertilization caused temporary changes in soil inorganic nitrogen only during crop growth. S addition increased DM production by up to 780 kg ha-1 , despite relatively high initial soil S. In the mixtures, S improved the competitive ability of V. sativa, thus reducing N negative effects. In pure oat stands nitrogen apparent recovery was 58 %, and S addition raised it to 96 %. These results show the advantages of consociations, over fertilization of pure grasses, as regards use of environmental resources, dry matter and crude protein cost and a more favorable nitrogen balance.