1. Introduction
A crop rotation, including spring crops, requires an effective weed management strategy during the crop-free period. This might include biological, mechanical, and chemical (also repeated and in combination) weed control tools on the fallow ground not in production in autumn. These tools have the aim to encourage the germination of volunteer crops, remove emerged weeds, reduce available sources, especially for perennial weeds, and to avoid a new weed seed production. The success of a weed management technique during the crop-free period may have a major impact on the weed seed bank, and weed infestation on the subsequent crops. Weeds compete for resources with the main crops and may also act as a host for pests and diseases [
1]. The ergot fungus (
Claviceps purpurea (Fr.) Tul.) for example uses
Alopecurus myosuroides Huds. (
A. myosuroides) as an alternate host [
2]. An effective weed control strategy therefore improves plant health and provides yield stability.
The application of synthetic herbicides is a common weed control practice in conventional farming systems. The use of non-selective herbicides (e.g., glyphosate) is a non-time-intense and efficient weed management practice particularly in conservation agriculture systems.
A. myosuroides, an annual grassy-weed (Poaceae), became a major problem in autumn sown crops in Western Europe [
3]. The increasing impact of
A. myosuroides in agricultural cropping systems can be attributed to the modifications on the current agricultural strategies, like increasing numbers of autumn sown crops, the alteration of cropping and tillage systems and the consequent usage of herbicides with the same mode of action [
3]. Several weed species have developed resistance to herbicides including glyphosate [
4]. Since
A. myosuroides has already evolved field resistance to multiple herbicide modes of action [
5], increasing the reliance on glyphosate can lead to a resistance to it [
6]. The current public concern raised, regarding the use of glyphosate in agriculture and the restrictions enforced in different countries, increases the necessity to search for alternative measures and different weed management tools. Biological and mechanical control methods might be an option to compete with resistant populations as well as to mitigate the development of herbicide resistant weeds.
Mechanical weed control practices, including tillage, might differ regarding the implementation, timing, and frequency [
7]. This might include flat tillage (<5 cm) and as well a deep stubble tillage (>5 cm) [
8]. Ploughing buries the weed seeds and mostly prevents them to emerge from deeper soil layers. Systems with a lower or superficial soil disturbance, compared to ploughing, usually result in a greater weed infestation [
9] and weed seed accumulation near the soil surface [
10]. However, reduced tillage systems have the advantage of decreasing run-off, increasing aggregate stability [
11] and preserving a higher soil moisture [
12]. Repeated flat or medium deep tillage may combine the benefits of reduced tillage systems for soil conservation with a sufficient weed control, yet with a possible negative impact concerning nutrient losses, soil compaction, or carbon gas emissions.
Winter cover crops (CCs), used as a biological weed control measure [
13], may demonstrate several advantages, including nutrient recycling efficiency [
13] and reduced soil erosion [
14]. The success of CCs as an integrated weed management practice, is related to a fast emergence and high soil cover, which depends on the chosen species, soil properties, and the weather conditions at the field location. Using different cover crop (CC) species within a mixture increases the resilience for management failures, bad weather conditions, and combines species-specific benefits [
15]. Seed predation, which may also act as a biological weed control measure [
16], is enhanced in cover-cropping [
17] and no-till systems [
18] and decreases the amount of weed seeds at the soil surface.
The straw management, also in combination with the different weed management practices as mentioned above, has an impact on weed infestation. Generally, straw disposal can for example reduce the number of
A. myosuroides plants, due to weed seed removal from the field [
19]. In no-till systems the straw surface coverage, which generates a physical barrier, is reducing the weed density [
20]. Otherwise, the herbicide efficacy could be reduced by crop residues [
21]. On the other hand, the presence of straw in CC systems might lead to an immobilization of nitrogen, which will then narrow the CC development and their subsequent success for weed suppression [
22].
There is little information available about the potential of repeated flat and deep stubble tillage in comparison to ploughing and cover-cropping to substitute herbicide applications in autumn. In a non-inversion tillage system grass weeds, like
A. myosuroides, might be encouraged [
23]. Furthermore, CCs are a suitable tool for broad-leave weed control [
24]. Within cover-cropping systems grass weeds may also become a severe challenge [
25] which might require the use of herbicides [
24]. The presence and absence of straw will additionally deliver information about the impact of straw management in combination with different weed management treatments on weed infestation.
This study aims at estimating the ability of selected biological, mechanical, and chemical weed control practices on weed suppression before spring cropping. The following hypotheses were investigated: (i) stubble tillage and CCs have similar success in reducing weeds as glyphosate applications; (ii) repeated stubble tillage is a more effective weed suppression measure in comparison to a single deep, turning soil tillage; (iii) the sowing method of CCs (mulch-tillage and no-tillage) has an impact on the success of weed suppression; (iv) the removal of straw after harvest is influencing the weed infestation.
The study was implemented at field sites with an increased population of
A. myosuroides. The results may clarify if tillage, herbicide application, or cover-cropping can reduce the number of
A. myosuroides plants. CCs were sown within a mulch-till and no-till systems to evaluate if no-till systems lead to an increasing number of weeds in comparison to stubble tillage systems as shown by Gruber et al. [
26].
4. Discussion
Stubble management can have a big impact on weed dynamics [
30]. The result of postharvest tillage on annual weeds mainly relates to the weed flora, the seed bank, and the dormancy status of the seeds [
30]. Flat postharvest tillage incorporates crop residues and stimulates volunteer wheat to germinate. Multiple soil tillage induces weed seeds for germination and destroys and buries them at the subsequent tillage. This might decrease the total weed seed amount in the soil.
No clear differences concerning weed suppression were found between flat (FST) and deep stubble (DST) cultivation, which had also been demonstrated by Boström [
31]. In the past, the shallow plough, as stubble tillage practise, was seen as most effective tool for weed management in Germany and Austria, as reported by Gruber et al. [
26]. Within our study ploughing (PL) showed worst results concerning WCE among all treatments. It is therefore not reasonable to recommend a deep soil tillage (including ploughing), which is labor intense and costly and does not provide benefits for weed control and soil conservation. In this study, ploughing was done early after harvest. However, a late treatment before weed seed maturity might improve the performance. The generally moderate performance of all mechanical treatments in comparison with the chemical and the biological treatments might be caused by the wet weather conditions during autumn. Cirujeda and Taberner [
32] who harrowed in cereals and state that a high WCE of harrowing is attributed to dry conditions after harrowing. From time to time inversion tillage or stubble management might be useful in order to control weeds in highly infested fields [
26]. Ploughing, especially in combination with stubble tillage [
33], is a useful tool against perennial and root spreading weeds. At both field trials, annual weeds were dominant, which allows a reduction of the tillage intensity [
34] and a conservation stubble management with reduced soil disturbance. Pekrun and Claupein [
7] recommend to leave the stubble undisturbed. In terms of a biological weed control strategy, keeping the freshly produced weed seeds on the soil surface enhances biodiversity and increases seed predation as biological weed control [
35].
The experiments had shown that both CC treatments (CC+NT and CC+MT) achieved an effective weed control during the crop-free period from August until December. In contrast to Brust et al. [
36,
37] CCs were also able to suppress volunteer wheat. Especially
A. myosuroides, which tend to be the most challenging grass weed, was successfully controlled by CCs, whereas stubble tillage and glyphosate application mostly failed this effect. The CC treatments reached an ACE up to 100% and a WCE up to 94%. The weed suppression potential of CCs has been proven by several studies [
30,
37,
38]. Winter CC cultivation has the potential to shift the use of herbicides towards a postemergence herbicide program [
24]. Weed seed germination and establishment is reduced in cover-cropping systems, but the amount of weed seeds in the soil may increase in the upper layer, especially in no-tillage systems. The success of CCs concerning their WCE is site specific and relates to the CC chosen. Further, it depends on the present weed species and the management at the field site [
39]. The weather conditions at both field sites, with sufficient amount and distribution of rainfall and the long growing period, were very suitable to achieve a dense canopy and competitive plant stand to suppress weeds. The biomass production of CCs, does not necessarily need to correlate with their weed suppression ability [
38,
40]. However, biomass-driven CCs are generally more competitive [
24,
40]. Instead of single CCs species, a species mixture was used within this study. By combining different CC species with specific advantages in CC mixtures, the benefits concerning weed, soil, nutrient, and pest management may increase [
41,
42].
The continuous loss of herbicides in the EU [
30] and the increasing problems with herbicide resistant weeds will raise the awareness of producers to strengthen their focus on non-chemical weed management. The GLY+GLY treatment achieved the significantly highest WCE within this experiment. However, a single glyphosate application (GLY) was not sufficient, in particular, to control
A. myosuroides weeds.
A. myosuroides plants emerge in several flushes during autumn, when the climate conditions are favorable [
10]. Applying glyphosate too early might miss most of the plants. Furthermore, this study demonstrated that at both trials, during autumn and at the end of the growing period, CCs (especially CC+MT) had similar effects on WCE, ACE, and VCE as the chemical treatments. The CC+NT treatment was only showing a slightly weaker WCE and VCE than the CC+MT treatment. Nevertheless, glyphosate is a useful tool within no-till and reduced tillage systems.
The wheat residue management (presence or absence of straw) had a minor effect on the success of either mechanical, chemical, or biological weed control practices. Even though burning the straw on the field is used in some regions and it might result in decreasing weed numbers, it can have some negative side effects especially concerning the carbon gas emissions. The baling of straw is not achieving a decrease of the weed infestation [
19], which was also demonstrated within this study.
Within this study, the effects of postharvest weed control on the previous spring crop season were not evaluated, but might deliver interesting insights to see whether the CC treatments preserved weed seeds, instead of reducing the weed seed bank for the repeated stubble tillage treatments (FSL, DST).