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Management of volunteers derived from imidazolinone-tolerant oilseed rape
(2016) Huang, Shoubing; Claupein, Wilhelm
Oilseed rape (OSR) has become the second most important oilseed crop after soybean worldwide, producing 70.95 million tons of seed yield, and providing 13.4% of world supply of oilseeds in 2014. The demand for OSR is expected to increase due to protein meals/cakes used in animal feed and vegetable oils/fats for biodiesel and human consumption. With increasing cultivation area, concern over volunteer OSR is rising, particularly if the variety in question is tolerant to specific herbicides. Currently, the introduction of imidazolinone-tolerant OSR (commercially named Clearfield® OSR; CL OSR) into Europe poses new challenges for chemical control of CL OSR volunteers because of their tolerance to imidazolinone herbicides and other acetolactate synthase (ALS) inhibiting herbicides. Additionally, the potential of gene dispersal in time and space by persistent dormant seeds in the soil and by volunteers is increasing. Volunteers emerge from the soil seed bank, the volume of which is predominantly dependent on seed dormancy. Therefore, the objectives of this study were (i) to investigate seed dormancy and dormancy formation of CL OSR, and (ii) to find out suitable agricultural strategies to reduce volunteers by growing OSR genotypes with low potential for seed dormancy and seed survival, and by implementing appropriate tillage operations. Focusing on these aims, several experiments were carried out with different methods, namely field experiments, germination tests in the laboratory, and genomic analysis, providing data for three scientific articles. Experiment 1. A 3-year field trial in south-west Germany investigated dormancy dynamics during seed development (primary dormancy and potential secondary dormancy; tested with an existing standard method in the laboratory) of 10 non-CL OSR varieties (lines) in 2009 and 2010, and of five CL OSR varieties (hybrids) in 2014. Experiment 2. A total of 15 CL OSR genotypes grown at two locations in south-west Germany in 2012/2013, and eight genotypes (two CL genotypes included) grown at 12 locations across Germany in 2011/2012, were tested for potential secondary seed dormancy with the aim to investigate dormancy traits of CL OSR and maternal environmental effects on dormancy formation. Experiment 3. A 5-year experiment (2011–2015) was conducted in south-west Germany with non-CL OSR and CL OSR (two CL varieties: high dormant and medium dormant) in the same rotation (non-CL winter oilseed rape - winter wheat - CL winter oilseed rape - winter wheat - corn) to investigate OSR volunteer dynamics under different modes of tillage (inversion tillage, non-inversion tillage, no-till, with or without additional stubble tillage prior to primary tillage). Following hypotheses were tested: Experiment 1. (i) There is primary (innate) and secondary (induced) dormancy in oilseed rape; (ii) primary dormancy decreases during seed development, the potential secondary dormancy increases; (iii) at maturity, the level of the remaining primary dormancy and the varietal potential to secondary dormancy correlate. These hypotheses have been approved. Primary dormancy decreased from a high dormancy level (ca. 99%) at about 30 days after flowering (DAF) to a quite low level (< 15%) at late seed development. Embryo growth probably regulates the dynamics of primary dormancy, at least during early seed development. Depending on variety and year, potential secondary dormancy initially increased from nearly 0% to the highest level (up to 90%) at about 70 DAF, and then slightly decreased with further seed development. The correlation between primary dormancy and potential secondary dormancy was high at early seed development, but was quite low at late seed ripening. Experiment 2: (i) There is variation in potential seed dormancy of CL OSR; (ii) F1 (seeded) and F2 (harvested) generations of hybrid CL-OSR show similar dormancy levels although changes through environmental effects are known; (iii) the environment (location) during seed development and maturation has an effect on the potential dormancy. The hypotheses were approved. The CL OSR genotypes differed in potential secondary dormancy from 0.0 to 95.7% in the F1 generation and from 3.5 to 77.9% in their corresponding offspring (F2). Out of the 15 CL genotypes, nine were considered to be low dormant (<30% dormancy level). High correlation (r = 0.96) between F1 and F2 generations indicates a strong inheritance of seed dormancy. Precipitation during seed development is thought to be a contributor to dormancy formation, e.g. the higher the precipitation the higher the dormancy level. These results indicate that selection or breeding for low dormancy CL OSR is feasible. A direct comparison of varieties by dormancy is only possible if they have been grown and harvested at the same location, due to environmental effects. Experiment 3: (i) The soil seed bank size of OSR is determined by post-harvest tillage (particularly tillage time) and seed dormancy traits of the cultivated variety; (ii) the emergence of volunteers from the seed bank also depends on the mode of tillage; (iii) gene segregation in herbicide-tolerance might occur among CL volunteers. These hypotheses were partly approved. There was no significant effect of tillage on the soil seed bank, but the soil seed bank was visibly higher if stubble tillage was done prior to primary tillage (179 vs. 56 seeds m-2; treatments with stubble tillage vs. corresponding treatments without stubble tillage). There were significant effects of tillage in general on volunteers in the next crop. Non-inversion tillage resulted in 30 times more volunteers in the following winter wheat crop than inversion tillage due to shallow seed burial depth. A high dormancy OSR variety resulted in a significantly larger soil seed bank than a medium dormancy variety (147 vs. 58 seeds m−2) but in fewer volunteers (0.9 vs. 1.9 volunteers m−2) in the first following crop winter wheat, probably due to slow release of seeds from dormancy. Hypothetically speaking, seeds from low dormancy varieties seem to be released from dormancy more rapidly than seeds from high dormancy varieties. Gene segregation with 10 zygosities of the imidazolinone-tolerance genes PM1 and PM2 was detected in the CL volunteers in the first following crop winter wheat. Approximately 90% of sampled plants were homozygous for PM1 and PM2, still conferring a high tolerance to imidazolinones. Overall, a high variation in potential secondary dormancy was detected for CL OSR, which is similar to non-CL OSR. The contribution of seed dormancy to the soil seed bank was confirmed. During seed development, maternal environment can influence seed dormancy dynamics to some extent. Tillage operations, particularly tillage time, can also influence the soil seed bank and the emergence of volunteers. A very new aspect is that the disposition of seeds to release from dormancy (instead of induction of dormancy) should be considered in further studies. Sound strategies to control volunteers should include (1) the use of low dormancy varieties with a low potential to establish a seed bank and with a fast release from dormancy, and (2) a combination of different tillage operations in the years following OSR cultivation, e.g. delayed inversion tillage with a deep burial depth in the first year, followed by shallow non-inversion tillage in subsequent years. Combined with a thorough knowledge of seed dormancy, of the development of the soil seed bank and of the release from dormancy, the occurrence of CL volunteers in following crops can be reduced or even avoided by a scope of practical methods and approaches proposed in this study.