Abstract
ABSTRACT The objective of this study was to evaluate the phytomorphological parameters of soybean in a crop-livestock system inoculated with Azospirillum brasilense. The experiment was conducted in Santa Maria, Brazil, for two agricultural years (2 Paper extracted from the doctoral thesis of the first author. 012/2013 and 2013/2014) in a randomized complete block design with three replications. In the winter, the black oat and ryegrass pasture was managed with sheep under different grazing systems: (I) - a conventional grazing (CG) system, where the animals remained in the pasture throughout the experimental period without any pasture height control; (II, III, and IV) - systems with post-grazing pasture heights of 10, 20, and 30 cm, respectively; and (NG) - one control without grazing. All treatments received two nitrogen doses (50 and 100 kg ha-1) and inoculation or no inoculation (A. brasilense). The soybean cultivar BMX Potência RR was sown using no-tillage on 16 November 2012 and 1 December 2013. In each plot, seven rows of plants with spacing of 0.45 m were used, and in four rows, the seeds were inoculated with the bacterium A. brasilense. The phytomorphological variables and grain productivity were evaluated. In a croplivestock system, soybean has better productivity when established on black oat and ryegrass pasture managed with post-grazing pasture heights of between 20 and 30 cm. Areas under conventional grazing in the winter led to smaller soybean plants with lower first and last pod heights. Inoculation with A. brasilense should not be used alone in soybean cultivation.
Highlights
Soybean expansion in the state of Rio Grande do Sul, Brazil, is increasing, especially in areas formerly used for cultivating rice and raising beef cattle
In the crop-livestock system established in the winter, the following management practices were used: (I) - conventional grazing (CG), where the animals remained in the pasture throughout the experimental period without any grazing height control; (II) – post-grazing pasture height of 30 cm (A30); (III) - post-grazing pasture height of 20 cm (A20); (IV) post-grazing pasture height of 10 cm (A10); and (V) - no grazing (NG)
This result can be explained because intense grazing maintains a low leaf area index that results in lower shoot (BALBINOT JUNIOR et al, 2009) and root biomass production in forage plants, which can limit nutrient absorption, water infiltration into the soil, gas exchange, and, the development of the successor crop
Summary
Soybean expansion in the state of Rio Grande do Sul, Brazil, is increasing, especially in areas formerly used for cultivating rice and raising beef cattle. With the use of legumes such as soybean, the system benefits because the decomposition of the soybean straw releases nutrients, especially nitrogen, allowing the rotation of crops and improving aspects related to the phytosanitary management and soil fertility (ARF et al, 1999). It is necessary to understand how much residual forage biomass should remain on the soil post-grazing without compromising the productivity of the system (LUNARDI et al, 2008; LOPES et al, 2009) and what this would represent in terms of grazing height management. In the case of soybean, biological fixation supplies all nitrogen demand through the plant’s interaction with Bradyrhizobium bacteria (HUNGRIA et al, 2005), generating annual savings of approximately US $7 million in nitrogen fertilizers (HUNGRIA; NOGUEIRA; ARAUJO, 2013), preserving the production environment, and avoiding water table contamination
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