Experimental data confirm the importance of appropriate selection environments for breeding maize adapted to organic farming systems

Abstract

Organic farming systems, characterized by special attention to soil fertility, recycling techniques and low external inputs, gained increased significance in recent years. As a consequence, there is a growing demand for varieties adapted to organic and/or low input farming. In several studies modern varieties as well as landraces have been tested for their suitability for organic farming (OF), whereas special breeding programs aiming at genotypes optimized for this farming system are scarce. Comparing the testcross performance of unselected recombinant inbred lines derived from the widely used US hybrid B73 x Mo17 under conventional (CF) and organic farming systems, Lorenzana and Bernardo (2008) concluded that high yielding maize hybrids for OF can be developed by screening under CF. In contrast, Burger et al. (2008) found moderate phenotypic between OF and CF in unselected breeding material indicating strong genotype x farming system interactions. Apparently, specific yield associated characteristics are necessary to reach maximum performance under OF and CF, respectively. Such traits include early vigour, competitiveness to weeds, and high nitrogen use efficiency under OF on the one hand, and resistance to green snapping and early root lodging under CF on the other. Thus, selection under OF conditions might be needed to fully exploit the genetic potential of maize for this management system. This is in line with previous studies demonstrating that breeding maize for improved nitrogen use efficiency was more efficient under low nitrogen levels compared to indirect selection under high nitrogen conditions (Presterl et al. 2003). But organic farming conditions are very difficult to characterize since they strongly depend on factors like crop rotation, soil fertility, organic fertilizer input, weed, pest and disease pressure, making it challenging to choose a representative selection environment. The objectives of the present study were to (i) compare the testcross performance of segregating maize populations under established organic and conventional farming (CF) systems, (ii) determine quantitative genetic parameters decisive for the selection response under OF versus CF conditions, and (iii) draw conclusions for breeding new varieties optimally adapted to OF systems