ECOLOGICAL WEED MANAGEMENT

Summary

Nowadays there is much concern over environmental and human health impacts on weed management practices which has led agricultural producers and scientists in many countries to seek innovative strategies for weed control. As weed management systems are being developed, ecological knowledge will become more and more important and the complexity of weed management must be considered. Therefore understanding weed-crop ecology will lead to more effective weed prevention, management, and control through a full range of factors regulating weed density, growth, and competitive ability. These alternative approaches for suppressing weed growth and reproduction is called “ecological weed management”. Ecological weed management involves the use of different types of information and various control tactics in order to develop strategies for subjecting weeds to multiple variable stresses over time. The main purpose for developing ecological weed management strategies is to integrate the options and tools that are available to make the cropping system un-favorable for weeds and to minimize the impact of any weeds that survive. Many of the components of an ecological management system are inextricably intertwined, thus making it difficult to measure the individual contributions of specific elements of the systems. This Ph.D. project provides an implementation of the knowledge regarding the ecological management of the weed in a Mediterranean area of central Italy. Therefore the main objective of this study was to evaluate and optimize the contribution of several ecological approaches for enhancing weed management in both organic and conventional cropping systems in order to reduce the crop loss due to weed competition and to monitor the evolution of weed community composition. After examining the principles of weed science and evolution on weed control and/or management, the Ph.D. thesis provides information regarding ecological approaches of integrated weed management on chickpea and pepper crops through field and laboratory experiments. Field experiments were planned for evaluating how tillage and cultivation practices, competitive cultivars, cover crops and their residue management can be carried out to reduce weed germination, growth, and competitive ability. The experiments were carried out in the period 2009 – 2011 at the experimental farm “Nello Lupori” of Tuscia University in Viterbo, central Italy (310 m above sea level, latitude 42°24’53’’ North and longitude 12°03’55’’ East) on a clay loam soil classified as Typic Xerofluvent (Soil Taxonomy). Chapter 5.a. describes a field study which was carried out in order to evaluate the competitive ability and the yield response of different chickpea genotypes against one of the main key-weeds (Polygonum aviculare L.) of the Mediterranean environment. Experimental treatments consisted in six chickpea genotypes and four different P. aviculare densities (4, 8, 16, 32 plants m-2). P. aviculare seeds were mixed with dry sand and hand-sown on chickpea rows, just after chickpea sowing. P. aviculare caused an average chickpea seed yield loss of 14, 46, 74 and 88% at the
density of 4, 8, 16, 32 plants m-2 compared to the weed-free crop, although significant losses on yield depended on the combined effect of chickpea genotypes and P. aviculare densities. The results suggest that P. aviculare should have a plant density less than 4 plants per m2 in order to prevent severe chickpea seed yield loss, although the use of highly competitive and weed-tolerant chickpea genotypes could reduce the seed yield loss up to 10%.
Chapter 5.b. reports a field experiment carried out in order to assess the competitive ability of selected chickpea genotypes grown as pure stand and in mixture with natural weed infestation partially suppressed by inter-row tillages. Experimental treatments consisted in six chickpea genotypes and four different weed managements [no weed control (weedy); 1-hoeing performed at 25 DAE (days after chickpea emergence); 2-hoeings, one performed at 25 and one at 50 DAE; weed-free]. Weed competition did not appear to affect the length of the chickpea cropping period, but resulted in reductions in both chickpea aboveground biomass and grain yield, even if the negative effect of weed competition was firstly observed in the seed yield reduction and later in the accumulation of the dry matter, as confirmed by the significant and positive correlations between the weed aboveground biomass and the number of fruitless pods. However, the chickpea yield loss, due to natural weed infestation, varied according to the level of mechanical weed management. As expected, the mechanical weeding caused a general reduction in weed density and biomass, an increase in both the competitive ability and the relative biomass of chickpea, and a decline of the relative biomass of the weeds. However, the chickpea genotypes greatly differed in their competitive ability against the weeds at different weed management levels. The chickpea aboveground biomass in the earlier stages, the ground coverage and the plant height were the
traits that positively associated with competitive ability, and the increase of these parameters could lead to the improvement of the competitive ability of chickpea genotypes against the weeds. Therefore, our results highlight that these screenable traits could be positively evaluated in breeding programs with the aim of developing high competitive chickpea cultivars. It is clear that competitive ability alone is not sufficient for suppressing weeds in chickpea crops and it needs to be combined with other types of weed management such as mechanical weeding. The results of this research show that seed yield loss due to incomplete weed control performed with inter-row tillage, could be considerably reduced by choosing highly competitive chickpea
genotypes which could be an integral part of an environmentally-friendly weed management strategy. Chapter 5.c. reports the results of a cover crop/pepper sequence in order to evaluate the effect of different cover crop species and their residue managements on weed control and fruit yield of a pepper crop. The treatments consisted in: (a) three winter cover crops (hairy vetch, oat, canola) and bare soil; (b) three cover crop residue managements [residues left in strips on soil surface in no-tillage (NT), green manure residues at 10 cm of soil depth in minimum tillage (MT), and green manure residues at 30 cm of soil depth in conventional tillage (CT)]; (c) three levels of weed management applied to the pepper crop [weed free (WF), inter-row mechanical control applied at 30 days after pepper transplanting (WH), and weedy (W)]. Oat was the most weed suppressive cover crop compared to canola and hairy vetch both throughout the cover crop growing period and in the following pepper crop regardless the different cover crop residue managements, probably due to its severe chemical and physical effects. The conversion of cover crop aboveground biomass in mulch strips in NT conditions was clearly the most effective weed management strategy compared to MT and CT especially when hairy vetch and oat were adopted. Even if the inclusion of an inter-row hoeing in the early growing stage of the pepper determined a strong weed reduction in all residue management treatments, it only proved to be a suitable weed
control practice in NT, while in MT and CT conditions it may still be necessary to use additional means for controlling the weeds, such as herbicides or other tillage operations. Regardless cover crop management, hairy vetch determined a marketable pepper yield of about 40 t ha-1 of FM in weed-free conditions and in any case almost twice than canola and oat in presence of weeds. The inclusion of legume cover crops in vegetable crop sequences in no-tillage systems could be a part of an ample strategy for reducing the amount of chemical inputs used both for controlling weeds and for N-fertilizing vegetable crops in the Mediterranean environment. Chapter 5.d. describes the results of a field experiment carried out in order to evaluate the effect
of cover crop species and their residue management on weed community composition and weed species diversity in a winter cover crop/pepper sequence. The treatments consisted in: (a) three winter cover crops [hairy vetch (Vicia villosa Roth.), oat (Avena sativa L.), canola (Brassica napus L.)] and bare soil; (b) three cover crop residue managements [residues left in strips on soil surface in no-tillage (NT), green manure residues at 10 cm of soil depth in minimum tillage (MT), and green manure residues at 30 cm of soil depth in conventional tillage (CT)]. Oat residues generally showed a higher reduction of weed species density, weed species richness and Shannon's index compared to hairy vetch and canola. The conversion of cover crop aboveground biomass in mulch strips (NT) greatly reduced weed species density but this did not always imply a reduction of weed species diversity in pepper compared to shallow and deep green manuring (MT and CT). In fact, the weed species richness and Shannon’s index were reduced inside the pepper rows, where the mulch was left on the soil surface, while a richer and more diverse weed community was found outside the pepper rows where the soil was not mulched. These results suggest that the cover crop residues converted into mulch strips can represent a more sustainable approach of weed management compared to green manure. In fact in NT it is possible to reduce the weed density and consequently to increase the pepper yield while maintaining a high level of weed diversity indices and evenness achieving a low agro-ecosystem disturbance.