1. Introduction
Conservation agriculture (CA), a production system based on minimum tillage, permanent soil cover and crop diversification, increases yield and yield stability of maize and wheat under rainfed conditions [
1,
2]. Despite the potential benefits, uptake by smallholder farmers in the tropics and subtropics is limited. Competing interests using residue for fodder make it challenging for farmers to maintain adequate soil cover, and small farmers’ lack of market access restricts the availability of profitable alternative crops for crop diversification [
3,
4]. However, when zero tillage (ZT) is applied without adequate soil cover and crop rotation, this can result in soil degradation, problems with weeds, pests and diseases and, consequently, lower yields [
1,
5,
6]. Furthermore, farmers often lack access to knowledge regarding the management of CA-based production systems, which strongly differ from conventional practices [
3]. Weeds are another major constraint for the widespread adoption of CA [
7,
8,
9]. The main weed control methods in smallholder agriculture include tillage, hand weeding, mechanical control and cheap herbicides. In CA, tillage is not generally seen as an option for weed control. Instead, herbicides, mulching and cultural practices or integrated weed management are the primary means of weed management [
10]. Weeds were reported to be problematic particularly in the initial years after changing from conventional agriculture to CA [
10]. Therefore, the success of CA depends on optimum crop establishment and effective weed management.
With adequate weed control in the first years of CA adoption, weed problems could be reduced in the long term in CA compared to conventional production systems [
10,
11]. All three components of CA have the potential to lower weed pressure if applied correctly [
11]. In conventional production systems, tillage controls weeds but also generates optimum conditions for weed germination and brings weed seeds from the seedbank to the soil surface [
12,
13]. In CA, weeds are not killed by tillage, but conditions favoring weed germination are not generated either [
11]. Weed seeds are not buried but remain on the soil surface, where they are more prone to predation and germination [
14]; therefore, if weeds are prevented from producing more seeds and weed seed predation is augmented, weed seedbanks can be depleted in the long run in ZT [
15]. Soil cover or mulching reduce the number of weed seeds that germinate, and cause delays in weed germination that give crops an early competitive advantage over weeds [
11] and favor increased seed predation [
16,
17]. Some crop residues produce allelochemicals that may inhibit seed germination of many weeds [
18]. The weed-suppressing effect of mulch depends on the amount of soil cover and thus on the productivity of the system [
19]. Crop rotation reduces weed pressure by allowing for contrasting weed management practices in varying crops due to crops being associated with different weeds [
17]. Although these components have beneficial effects on weed management, no studies exist regarding the effect of all three components of CA and their interactions, with most studies reporting only on one or two of these individual components.
Weed community composition and diversity depend on the growing conditions and crop association, which may differ between CA and conventional tillage (CT) and cause a shift in weed flora when changing from CT to CA [
20]. It is possible that CA benefits perennial weeds, since these are not necessarily controlled by tillage [
21,
22]. For example, in a 14-year trial of tillage methods in corn and soybean in Iowa, perennial weeds increased more over time in the reduced tillage treatments than in the moldboard plow system [
21], while no increase in perennials was observed in a similar trial in Argentina [
20]. Mechanical weed control and hand-weeding are more difficult in CA due to the mulch cover, which increases dependence on herbicides for weed control [
7]. The stronger reliance on herbicides in CA favors certain weeds and even generates problems regarding resistant weeds if herbicide management is not appropriate [
23]. Ideally, integrated weed management promotes a diverse weed community to prevent one species from becoming problematic [
24].
Weed control in Mexico relies mainly on the use of herbicides in commercial agriculture and/or on manual or animal-drawn mechanical control in traditional agriculture. Yield penalties due to inadequate weed control can range from 26% to 48% in CT and 58% to 92% in ZT [
25]. Little research exists regarding the effect of CA components and their interactions with weed populations under Mexican highland conditions. Furthermore, very little long-term data about the effect of implementing all three CA components together on weed populations is available in the literature from other regions. The goal of this study was therefore to evaluate the long-term effects of all three CA components (ZT, crop residue and rotation) on weed biomass and density in wheat (
Triticum aestivum L.) and maize (
Zea mays L.). Weed biomass and density were evaluated in a long-term experiment with treatments unchanged for over 20 years. The research hypothesis behind this study was that CA would reduce weed biomass and weed density in the long term. The following specific questions were asked: (1) Were weed biomass and density higher in CA or CT, and what were the effects of the components of CA (ZT, crop residue and rotation) on weed biomass and density? (2) Was there a difference in weed diversity between the treatments with varying crop management? and (3) Did CA favor perennial weeds in maize and wheat systems?
3. Results
Over the years, 40 different weed species were observed in the trial (
Table S1). Most species were annual dicots or broadleafs. Of all the identified species, 31 were annuals and 9 were perennials, with 30 dicots and 10 monocots. Only 10 weed species (
Portulaca oleracea, Amaranthus hybridus, Sonchus oleraceus, Verbena bipinnatifida, Euphorbia stictospora, Galinsoga parviflora, Malva parviflora, Oxalis corniculata, Eragrostis mexicana and
Bromus carinatus) were observed every year; of these,
O. corniculata and
E. mexicana were the most dominant.
O. corniculata was mainly associated with maize, while
E. mexicana was mainly associated with wheat.
Herbicides were generally effective in weed suppression. The subplots with herbicides showed significantly lower weed biomass (31 ± 5 g m−2) than the subplots without herbicide application (188 ± 16 g m−2). Lower weed biomass was recorded in maize treated with herbicides than in wheat treated with herbicides (17 ± 5 g m−2 and 34 ± 8 g m−2, respectively, excluding observations from 2014, when data were only collected from wheat). There was a significant relationship between herbicide application and the tillage system on weed biomass, so data with and without herbicide were analyzed separately. Weed control efficiency was 93% in maize on average, but lower in wheat at 68%, mainly due to presence of E. mexicana; the minimum occurred in 2014, when it was only 33%. Tillage, residues and rotation did not influence weed control efficiency of herbicide applications. The treatment with the lowest weed control efficiency was W-Mon-ZT-K with an average of 52%, which demonstrated significantly lower weed control efficiency than all maize rotation treatments. Weed density was significantly higher without herbicide application than with herbicide application, resulting in 208 ± 29 plants and 134 ± 23 plants m−2, respectively.
3.1. Effects of Conservation Agriculture and Its Components on Weed Biomass and Density
In maize with herbicide application, a significant effect of crop rotation was shown only on weed biomass, while the effects of tillage practice, residue management and interaction effects were not significant. Crop rotation in maize significantly reduced weed biomass by 87%, resulting in 31 ± 10 g m
−2 in monoculture and 4 ± 1 g m
−2 in rotation on average (
Figure 1,
Table 4). ZT and CT in maize resulted in similar weed biomasses on average (18 ± 6 g m
−2 in CT and 18 ± 9 g m
−2 in ZT). Treatment effects were generally consistent over the measured seasons, although weed pressure magnitude differed widely throughout the years (
Figure S1).
In herbicide-applied wheat, a significant effect of rotation was observed, with a weed biomass of 58 ± 13 g m
−2 in the monoculture treatments and 24 ± 5 g m
−2 in the treatments with rotation. Weed biomass was also significantly higher in wheat with CT than with ZT. There was no overall significant effect of residue in wheat, although weeds were significantly reduced in W-Mon-ZT-K compared with W-Mon-ZT-R (treatment abbreviations in
Table 1). Weed biomass was higher in the treatments with wheat monocropping, except in W-Mon-ZT-K, mainly because of an infestation with
E. mexicana which was not controlled by the clodinafop or fenoxaprop herbicides used.
In maize subplots without herbicide application (
Figure 1), a significant interaction was observed among all factors (tillage, rotation and residue), making interpretation of the overall effects complicated. Treatments with CT resulted in higher weed biomass than ZT treatments (220 ± 27 compared to 178 ± 43 g m
−2), with treatments with rotation having lower weed biomass on average than monocultures (176 ± 29 compared to 222 ± 41 g m
−2), but no apparent difference was observed due to residue retention (200 ± 35 and 198 ± 37 g m
−2 with K and R, respectively). Effects of the treatments varied over the years; in 2005, the effects of all factors were significant, but this was not the case for 2013 and 2015 (
Figure S1).
In wheat without herbicide application, weed biomass was significantly higher in the CT treatments compared to the ZT treatments throughout all four years, but there were no significant effects due to rotation or tillage. Treatment effects varied over the years; in 2005 and 2014, there was only an effect due to crop rotation, but 2005 demonstrated higher weed dry biomass with crop rotation, probably due to a combination of initial drought and higher soil fertility with crop rotation, whereas lower weed biomass with crop rotation was observed in 2014. In 2013, all factors produced significant effects on weed biomass in wheat, and in 2015, rotation and ZT resulted in significantly reduced weed biomass (
Figure S1).
In maize and wheat, weed biomass was markedly lower when crops were planted in CA (a combination of ZT with retained residue and crop rotation) compared with treatments representing the conventional production system of the Mexican highlands, i.e., CT with residue removal and monoculture of maize or wheat. When herbicide was applied, the weed biomass was 34 ± 18 g m−2 in M-Mon-CT-R, with M-Rot-ZT-K showing a biomass of 3 ± 1 g m−2, 91% lower; similarly in wheat, weed biomass was 77 ± 33 g m−2 in W-Mon-CT-R, while W-Rot-ZT-K showed a result 81% lower at 15 ± 13 g m−2. Differences were similar without herbicide application; weed biomass was 299 ± 79 g m−2 in M-Mon-CT-R, while in M-Rot-ZT-K this was 66% lower at 102 ± 29 g m−2. Similarly, in wheat, weed biomass was 288 ± 37 g m−2 in W-Mon-CT-R, while this was 59% lower in W-Rot-ZT-K at 118 ± 75 g m−2.
Regardless of herbicide application, no significant differences were observed in weed density between factors or between treatments throughout the whole time period (
Table 5) due to the large variation in weed density between the years (
Figure 2,
Figure S2). Without herbicide application, significantly fewer weeds were produced in ZT compared to CT in 2013, but no significant effects of the factors were observed in the other years. With herbicide application, significantly fewer weeds were observed in maize than in wheat in 2015, with 63 ± 21 and 136 ± 49 plants m
−2, respectively. In maize, treatments with rotation showed similar weed densities, while maize monoculture produced a significantly lower weed density in CT and with residue retention. In wheat, there were significantly more weeds after monoculture treatments than with rotation. Treatments with monoculture resulted in significantly more weeds in ZT than in CT, while rotation treatments with tillage did not affect weed density. In 2013, significantly fewer weeds were observed in maize with crop rotation than with monoculture, 14 ± 4 and 122 ± 30 plants m
−2, respectively, while rotation did not affect weed density in wheat. In contrast, in 2004 and 2014, no significant effects on weed density were observed as a result of any of the factors.
3.2. Effects of Conservation Agriculture and Its Components on Weed Diversity
Overall, in ZT, perennial weeds were of minor importance in the weed community counts. Of the perennial weed species, Oxalis spp. was the most common, followed by Alternanthera caracasana and B. carinatus. With herbicide application, only the number of perennial weeds was significantly influenced by crop rotation and were shown to be significantly more abundant in treatments with maize monoculture compared to the other treatments. This was mainly due to the high incidence of O. corniculata in the maize monoculture treatments. Without herbicide application, there was a significant effect from the rotation × tillage interaction on the density of perennial weeds, with perennial weeds more abundant in maize monoculture with CT, while in the maize–wheat rotation there were no differences in perennial weed density.
In maize, an average of 45% of counted weeds were Oxalis spp., with 13% E. mexicana. In wheat, 30% of the weeds were E. mexicana and 24% were Oxalis spp. Other common weeds included Euphorbia stictospora, with 5% of observations in maize and 7% in wheat. In maize, Portulaca oleracea was another common weed, making up 5% of the observations, and in wheat Acalypha mexicana was common, with 7% of the observations. None of the other weeds observed formed more than 5% of the counted weed population in any of the years of observation. Because of the dominance of Oxalis spp. in maize and E. Mexicana in wheat, monocots were more dominant in the weed community in wheat, while dicots were more dominant in maize. Without herbicide application, significantly more monocots were observed in ZT, while there were no differences in monocot density due to tillage when herbicides were applied. Sedges were not commonly observed in the trial.
The weed community diversity per treatment was rather low in the trial, with the number of species observed per plot ranging between 0 and 12 in any given year. When herbicides were applied, the number of species was significantly impacted by management factors (
Table 6). In wheat, the number of species was significantly higher in treatments with rotation compared to monoculture and with CT compared to ZT. In maize, the number of species was higher in ZT than in CT. There was also a significant rotation × residue interaction observed, with the number of species higher with residues than without in maize monoculture treatments, while rotation treatments demonstrated no effect of residue management. Without herbicides, species diversity and evenness only differed significantly with rotation, with the effect of residue management on species diversity appearing marginally significant (
p = 0.06). There was higher species diversity and evenness in treatments with rotation compared to monoculture, and higher species diversity and evenness in treatments with residue retention compared to residue removal.
4. Discussion
This study demonstrated that, at least in the long term, weeds are not necessarily problematic in CA, and CA can lead to lower weed pressure compared to CT if adequate weed management is performed, as proposed in literature reviews regarding this subject [
8,
10]; this is true both with herbicide control and without. All three CA components showed potential to decrease weed populations. Even when the herbicides could not control weed populations adequately (e.g., the treatments with wheat monocropping showed that herbicides did not control all weed species), the combination of ZT and residue retention resulted in a significantly lower weed biomass, thereby demonstrating the usefulness of the individual CA components in weed suppression. The treatments with all three CA components generally resulted in the lowest weed biomass over the years, which may have been a result of the weed-reducing effect of all three components over the 25-year course of the experiment, leading to reduced weed seedbanks and lower weed populations in the treatments with adequate weed control through the CA components, while weed problems accumulated in treatments without adequate control, particularly in wheat.
The effect of residue retention on weeds was dependent on tillage; in CT, crop residues were incorporated by tillage, therefore, they did not form a mulch layer that suppressed weeds. Treatments with ZT resulted in low weed biomass, regardless of residue retention or removal. Weed germination, growth and development were lower in ZT, which may have been dependent on the residue management. In treatments with residue retention and ZT, soil fertility and soil water content were considerably higher compared to treatments with residue removal [
5], leading to better crop growth and an improved ability of the crop to compete with weeds. The mulch cover, especially after maize, prevents weeds from germinating and may facilitate higher seed predation due to favorable conditions for soil fauna, such as ants and beetles [
8,
37]. In treatments with ZT but without residue retention, the soil was degraded and showed poor fertility and lower infiltration [
5], which created an unfavorable environment for weeds to germinate and grow. Wheat was shown to leave more stubble when harvested than maize, and previous studies showed that ZT-treated soil with wheat monoculture or maize–wheat rotation without residue was less degraded than treatments with maize monoculture [
38,
39]. Therefore, in ZT treatments with residue removal, weed biomass was the lowest in M-Mon-ZT-R treatments, higher in M-Rot-ZT-R and W-Rot-ZT-R treatments, and highest in the W-Mon-ZT-R treatment, proving that weed management must be considered when designing CA systems, since the crops available for rotation and the residue that they produce affect weed control and growth.
The dominant weed species were different in maize and wheat, and their presence was related to management factors. In wheat,
E. mexicana was the dominant weed, with mulching with residues helping to suppress this problem. In the W-Mon-ZT-R treatment, weed biomass was high due to the infestation with
E. mexicana, while in W-Mon-ZT-K, it was significantly lower due to the presence of residues. This may have been due to the residues impeding germination, the small seeds having insufficient reserves to push through the residue cover, higher seed predation with residues present, or because of the residues’ allelopathic effects. In wheat, the same herbicides (e.g., fenoxaprop or clodinafop) were used since the beginning of the experiment for weed control, but they did not control
E. mexicana, leading to an increase in the presence of this weed. Alternative active ingredients available on the local market were tested in separate experiments, but none were able to control
E. mexicana. In maize,
E. mexicana was not a problem due to S-metolachlor effectively controlling the weed, combined with the higher competitiveness of maize with
E. mexicana. Similar results were obtained in an experiment with winter wheat in Alberta, where populations of
Bromus tectorum increased over the course of six years in wheat monoculture treatments, but not in a wheat–canola rotation, where it could be controlled with herbicides in canola [
40]. In maize,
Oxalis spp. weeds were the most common. As
Oxalis spp. do not compete strongly with maize, especially later in the growing season, applying additional herbicides to control
Oxalis spp. in maize was not considered to be necessary, leading to an increase in
Oxalis spp. propagules in maize monocultures. In maize, treatments with monoculture had higher weed density on average due to presence of
Oxalis spp., especially in CT. Crop rotation helped suppress the presence of this weed in maize;
Oxalis spp. was not common in wheat.
Treatments with rotation resulted in larger weed diversity, which may have helped the crops avoid weed problems. The more diverse the weed community in a field, the less likely one dominant weed is to cause serious problems [
41]. The diversity of weeds in a field depends on the local ecology and on station conditions. Changes appeared to occur in the weed species present in the trial since its initiation in 1992. While
P. oleracea, A. hybridus, S. oleraceus, V. bipinnatifida, E. stictospora, G. parviflora, M. parviflora, O. corniculata, E. mexicana and
B. carinatus were the most common weeds observed in this study,
Elusine, Eragrostis, Setaria, Cyperus, Oxalis, Alcalypha, Galinsoga, Taraxacum, Portulaca and
Sonchus were reported to be the most common weed genera in the 1992–1995 period [
42].
Taraxacum was only observed in 8 out of 256 plots sampled over the years, while
Setaria was not observed during our study. These shifts in populations were similar to those reported by [
43,
44], who reported rapid changes in dominant weed species during a five-year trial with tillage and herbicide treatments.
No indication of a higher incidence of perennial weeds in CA was found, which was similar to the findings of other studies; for example, a study on tillage and rotation in Argentina did not find a consistent relationship between tillage reduction and the presence of perennial weeds [
20]. This is, of course, not a general conclusion, as it is highly dependent on the weed species present, the climate and the field management. The most common perennial weed species at the site,
O. corniculata, was more abundant in CT, especially in maize. These observations were considered to be long-term effects of CA, as the treatment was in place for more than 25 years and occurred in a research station in which weeds were controlled for over 50 years. In Mexico, agroecological conditions vary widely across the country. Therefore, to gain more conclusive results, it is necessary to study the effects of CA in on-farm trials, different agroecologies and on different time scales.