*Agronomy* **2020**, *10*, 364 *Agronomy* **2020**, *10*, x FOR PEER REVIEW 13 of 19

**Figure 5.** Time pattern of field emergence of weeds in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control, a chemical herbicide, and a control with only water. For each weed species, red areas mark the observed presence in the field from February (F) to June (J). **Figure 5.** Time pattern of field emergence of weeds in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control, a chemical herbicide, and a control with only water. For each weed species, red areas mark the observed presence in the field from February (F) to June (J).

time.

Since the Shannon index not only takes into account the number of species, but also the total number of individuals, it may be considered as a representation of the degree of botanical diversity inside each plot. In 2014, the diversity index was rather constantly higher in the untreated plots, and constantly lower in the chemically treated ones. All treatments took intermediate values between these two extreme series; a slight advantage of the *R. coriaria* treatments over the other treatments was detectable, but it must be noticed that in the last part of wheat cycle all water extract treatments exhibited high, and similar, values. In 2016, the diversity index showed a decreasing trend from March onward, homogeneous among all treatments (including chemical) and the controls. *Agronomy* **2020**, *10*, x FOR PEER REVIEW 14 of 19

**Figure 6.** Species richness (n of detected weed species m<sup>−</sup>2) in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control (green line), a chemical herbicide (red line), and a control with only water (blue line). Each value is the mean of three repetitions ± standard deviation. Arrows indicate the date of treatments. **Figure 6.** Species richness (n of detected weed species m−<sup>2</sup> ) in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control (green line), a chemical herbicide (red line), and a control with only water (blue line). Each value is the mean of three repetitions ± standard deviation. Arrows indicate the date of treatments.

This outcome is also evidenced in the graphs in Figure 6, where the detected trend of the number of weed species per area unit (species richness) is reported, throughout all survey dates, and Figure 7, which illustrates the trend over time of the calculated Shannon's index in all plots. In 2014, species richness was initially low, and then shifted to higher values until harvest. Appreciable variations were found among treatments and, noticeably, the chemical treatment showed constantly the lowest values. Monocots and dicots where found in rather the same proportions in both controls and in the

dm weed biomass. Among dicots, wild dill (*Ridolfia segetum*) was certainly the most relevant, found in all plots with highly sized plants, where it represented 23% to 30% of total weed biomass. A significant presence (36%) of *Polygonum aviculare* was found in the plots treated with *E. characias* extracts. In 2016, the opposite trend was evidenced, and weed species number decreased over time. A more simplified weed flora was assessed, and only *Phalaris* and *Avena* were retrieved at harvest

*Agronomy* **2020**, *10*, x; doi: FOR PEER REVIEW www.mdpi.com/journal/agronomy

*Agronomy* **2020**, *10*, x FOR PEER REVIEW 15 of 19

**Figure 7.** Species diversity (Shannon-Wiener index, *H')* in 2014 and 2016 in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control (green line), a chemical herbicide (red line) and a control with only water (blue line). Each value is the mean of three repetitions ± standard deviation. Arrows indicate the date of treatments. **Figure 7.** Species diversity (Shannon-Wiener index, *H')* in 2014 and 2016 in durum wheat treated with five (2014) and two (2016) plant water extracts, compared with an untreated control (green line), a chemical herbicide (red line) and a control with only water (blue line). Each value is the mean of three repetitions ± standard deviation. Arrows indicate the date of treatments.

### Since the Shannon index not only takes into account the number of species, but also the total number of individuals, it may be considered as a representation of the degree of botanical diversity **4. Discussion**

inside each plot. In 2014, the diversity index was rather constantly higher in the untreated plots, and constantly lower in the chemically treated ones. All treatments took intermediate values between these two extreme series; a slight advantage of the *R. coriaria* treatments over the other treatments was detectable, but it must be noticed that in the last part of wheat cycle all water extract treatments This work was aimed to evaluate, in field conditions, the effects on durum wheat of several water plant extracts, applied for weed control. With this purpose, not only the bare conditions of the presence/absence of weeds were accounted for, but also the possible interactions between the supplied extracts and the major growth and yield parameters of the crop.

exhibited high, and similar, values. In 2016, the diversity index showed a decreasing trend from March onward, homogeneous among all treatments (including chemical) and the controls. **4. Discussion**  This work was aimed to evaluate, in field conditions, the effects on durum wheat of several water plant extracts, applied for weed control. With this purpose, not only the bare conditions of the presence/absence of weeds were accounted for, but also the possible interactions between the supplied extracts and the major growth and yield parameters of the crop. The effect of treatments on weed population was variable between years. In 2014, dicots were in general prevailing in plots treated with extracts of *E. characias*, while monocots prevailed after The effect of treatments on weed population was variable between years. In 2014, dicots were in general prevailing in plots treated with extracts of *E. characias*, while monocots prevailed after treatments with *L. camara* and *R. coriaria*. In 2016, when a generally lower weed biomass was present, also a lower diversity level was found, and only the most competitive weed species (*Avena fatua* and *Phalaris paradoxa*) were detected at harvest time. The marked variability expressed by the *A. arborescens* extract on weeds, as revealed by the opposite directions shown by the calculated suppression index in the two years, may be possibly explained by a toxic effect exerted by this extract against wheat in both years and especially in 2016, when this treatment probably induced a less dense wheat canopy (fewer and shorter plants), which allowed weeds to grow and develop even more than in the untreated control.

treatments with *L. camara* and *R. coriaria*. In 2016, when a generally lower weed biomass was present, also a lower diversity level was found, and only the most competitive weed species (*Avena fatua* and *Phalaris paradoxa*) were detected at harvest time. The marked variability expressed by the *A. arborescens* extract on weeds, as revealed by the opposite directions shown by the calculated In general, none of the tested treatments (including chemicals) was able to eradicate weeds from the field, and weeds were retrieved at harvest time in all plots. Hence, although chemically-treated plots showed in both years the highest suppression ability, some lately-sprouting weeds were found also therein. However, the fact that in both years grain yield was not significantly different between chemically treated plots and untreated ones, demonstrates that, in the chosen wheat genotype, weed control using chemical herbicide does not necessarily result in a significant increase in grain yield.

Total weed biomass did not appear to be a determinant factor in assessing wheat yields, showing on average—opposite to what was expected—the highest values in the most productive year and treatments. Both measurements (grain yield and weed biomass) were, however, significantly different according to the tested treatment. On average, *R. coriaria* always exerted a positive effect on wheat yields, and *A. arborescens* always a negative effect. A possible explanation could be that the retrieved yield differences are a consequence of the distribution of plant extract itself, rather than an effect exerted on weed biomass. An effect of *R. coriaria* extract on several quality parameters of durum wheat has been already assessed by previous experiments [26]. Further research is needed to explore these aspects.

Noticeable differences resulted in the date of appearance of major weeds, whose flush of emergence was generally earlier in 2016 than in 2014. In all treated plots in the first year, the appearance of wild oat (*Avena fatua*) was delayed with respect to the controls, but this trend in 2016 was confirmed only on plots treated with extracts of *A. arborescens*. Since wild oat and *Phalaris* spp. are among the most noxious weeds in wheat, if confirmed by further experiences, this outcome would have a great practical relevance. The delay of weed emergence is claimed to be a major factor in improving yield levels, since a longer time is at the crop's disposal to enhance its competitivity [52].

The competitive ability of the selected durum wheat genotype (cv Valbelice) resulted in a higher yield capacity even in the presence of a significant weed biomass. To explore this aspect, plant traits correlated to crop competitiveness, i.e., plant population, plant height, and tillering [53,54] were taken into consideration. All of them expressed large differences in consequence of the different climatic pattern of the two years (Y factor always highly significant). As such, climatic conditions acted giving wheat a higher competitive ability in the first year (height values always higher; plant population higher). An advantage of taller plants was evident in both years and in all circumstances, since a general trend of higher productivity with higher plants was rather always recognizable. Similarly, the yield disadvantage of shorter plant size, as retrieved in plots treated with *A. arborescens* extracts, was evident as well.

Both tiller number per plant and number of spikes per area unit resulted to be mostly density-dependent, and did not seem associated with reduced weeds.

### **5. Conclusions**

Although certainly preliminary, this work represents a step forward in the study of weed management through allelochemicals. Although the herbicidal effectiveness of the studied extracts under the given experimental conditions was rather limited, water plant extracts confirmed exerting different—and not always predictable—effects on crop yield and development. By one side, it must be stressed that the goal of weeding is no longer the complete eradication of weeds, rather the containment of weeds population beyond an "acceptability" threshold [55–59]. By another side, the occurrence of significant effects of these extracts on crop open the way to a huge field of investigations involving agronomical, physiological, and biochemical issues. Further studies are necessary, using a broader range of crops and allelochemicals, and pointing out in detail doses and methods of application of the supplied compounds.

**Author Contributions:** Conceptualization, A.C. (Alessandra Carrubba) and A.S.; data curation, A.C. (Alessandra Carrubba), A.L., A.C. (Andrea Comparato), S.M., and A.S.; formal analysis, A.C. (Alessandra Carrubba); investigation, A.C. (Alessandra Carrubba) and A.S.; methodology, A.C. (Alessandra Carrubba) and A.S.; resources, A.C. (Alessandra Carrubba); software, A.C. (Alessandra Carrubba), A.L., S.M., and A.S.; supervision, A.C. (Alessandra Carrubba); validation, A.C. (Alessandra Carrubba), A.L., A.C. (Andrea Comparato), S.M., and A.S.; visualization, A.C. (Alessandra Carrubba) and A.S.; writing—original draft, A.C. (Alessandra Carrubba); writing—review and editing, A.C. (Alessandra Carrubba) and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*
