White Clover Control in Integrated Crop-Livestock Systems: Effect on Soybean Nodulation, Development and Production.

The search for more sustainable alternatives in agriculture has led to greater demand for biodiversity. In subtropical climates, integrated crop-livestock systems with grazing in winter and grain production in summer have been widely adopted as an important agricultural model. Forages of the Poacea plant family are the main winter forage in those systems and their combination with white clover (Trifolium repens L.) (WC) is an alternative for increasing the nutritional quality of winter pastures in addition to the contribution of biological nitrogen fixation in the system. However, the perennial life cycle of WC and its tolerance to a wide range of herbicides turns difficult its control, creating the possibility of its coexistence with the subsequent summer grain crop, but its control is necessary to prevent grain yield reduction. Assuming that different active ingredients in herbicides can generate variable levels of WC control, an experiment was conducted at Guarapuava in Paraná state, Brasil (25º23'36"S, 51º27'19"W) to evaluate soybean production after a WC winter crop using several post-emergent herbicides, with or without sequential applications of glyphosate plus two control treatments (without WC control and complete WC control by hand-weeding). Treatments were arranged in a  7 x 2 + 2 factorial in randomized complete block design with 3 replications. The aim was to generate a gradient of WC control in order to (i) understand the interference of WC on the development and production of soybean and (ii) identify possible post-emergence herbicides for soybean that control WC. Less control of WC resulted in lower soybean nodulation, which affected soybean development, and significantly decreased dry mass of the roots and above ground plant. The reduced number of nodes m^-2 directly reduced soybean production. The post-emergent herbicides chlorimuron-ethyl and lactofen, with a sequential application of glyphosate, provided good control of WC in the soybean crop.