Tillage rotation and biostimulants can compensate for reduced synthetic agrochemical application in a dryland cropping system

Abstract

The effects of four continuous tillage regimes; mouldboard ploughing, tine-tillage, shallow tine-tillage, no-tillage; and three tillage rotations (involving shallow tine-tillage once every two, three, and four years in rotation with no-tillage), and two rates of synthetic agrochemicals (standard: with regular application of synthetic agrochemicals; and reduced: fewer synthetic agrochemicals in combination with biostimulants) on wheat and canola yield and quality were investigated between 2018 and 2020 under typical Mediterranean climatic conditions in South Africa. It was hypothesised that a combination of tillage rotations and the application of reduced synthetic agrochemicals will improve crop yield and quality relative to mouldboard ploughing or no-tillage. Results showed that a combination of reduced application of synthetic agrochemicals and tillage rotation practices maintained but did not significantly increase crop yield and quality, relative to no-tillage and mouldboard ploughing. Results also showed that intensive ploughing is unnecessary as it did not significantly increase yields (p > 0.05). In addition, it is possible to reduce the quantity of synthetic agrochemicals applied by partially replacing them with biostimulants without significant changes in grain or seed yields and quality. We, therefore, suggest that producers opt for biostimulants in combination with no-tillage or tillage rotation as a sustainable way of farming.

Keywords:

• Biostimulant
• rotational tillage
• crop yield
• mouldboard plough
• no-tillage

REVIEWING EDITOR:

• Tejada Manuel, Universidad de Sevilla, Spain

SUBJECTS:

• Agriculture & Environmental Sciences
• Environmental Management
• Environmental Issues
• Sustainable Development

1. Introduction

Different tillage intensities and frequencies can exert positive and negative effects on soil quality and crop productivity. For example, a mouldboard plough can effectively control weeds by overturning soil and burying weeds and their seeds to a depth where their germination becomes improbable (Conyers et al., 2019; MacLaren et al., 2021). Conversely, deep tillage may also expose buried viable weed seeds and facilitate their germination. Continuous tillage with aggressive primary tillage implements such as the mouldboard plough may lead to the breakdown of soil aggregates (Bottinelli et al., 2017; Gao et al., 2017). Smaller soil aggregates can increase the chances of soil erosion by wind and water, leading to loss of the fertile topsoil (Bogunovic et al., 2018; Vach et al., 2018) and a decline in soil organic matter and aggregate stability (Derpsch, 2004). Poor soil quality typically translates into diminished crop productivity. To improve soil quality and crop productivity, reduced tillage and no-tillage have been widely advocated (FAO, 2017) and adopted in South Africa (Findlater et al., 2018) and across the world (Derpsch, 2004).

The benefits of no-tillage have been widely published (Bottinelli et al., 2017; Gao et al., 2017; Kibet et al., 2016; Swanepoel et al., 2015; Tshuma et al., 2021; Vach et al., 2018). No-tillage can lead to improved soil aggregate stability, reduced soil erosion, improved water infiltration, increased soil microbial activity and a reduced carbon footprint, to name a few benefits. Despite these benefits, prolonged no-tillage has been associated with increased weed pressure, soil nutrient stratification, inability to incorporate lime and fertiliser to deeper depths, and soil compaction (Blanco-Canqui & Wortmann, 2020; Conyers et al., 2019; Dang et al., 2018; Liebenberg et al., 2020). Consequently, producers who practise no-tillage may be compelled to resort to the increased use of herbicides and insecticides, for crop protection management. Excessive and repeated use of herbicides has led to the development of herbicide-resistant weeds, such as ryegrass (Lolium spp.) (Pieterse, 2010), plantain (Plantago lanceolata L.) (Ndou et al., 2021), and horseweed (Conyza spp.) (Heap, 2021). Furthermore, agrochemicals have come under scrutiny for their environmental impact (Lackmann et al., 2021; Le Du-Carrée et al., 2021) including the harm they inflict on beneficial insects and human health (Curl et al., 2003).

There is, therefore, an impetus to reduce the quantity of agrochemicals applied in agriculture and to adopt ecologically friendly farming systems (MacLaren et al., 2020). One option for reducing the quantity of agrochemicals can be to substitute them with some environmentally-friendly biostimulants or organic compounds (Cao et al., 2023). A biostimulant can be described as any ‘substance or micro-organism that, when applied to seeds, plants, or the rhizosphere, stimulates natural processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, or crop quality and yield’ (Sleighter et al., 2023). The term, biostimulant can be used to refer to organic acids such as humic and fulvic acids, seaweed extracts, microbial inoculants, beneficial fungi, and amino acids and protein hydrolysates (Sleighter et al., 2023). Biostimulants can improve crop growth and productivity by enhancing the crop’s physiological, biochemical and molecular aspects such as photosynthesis and other metabolic pathways (Maksoud et al., 2023). In this article, the term ‘biostimulant’ does not imply organic certification. A particular biostimulant can have multiple functions (Lamlom et al., 2023). For example, Trichoderma, a beneficial fungi species that is naturally found in the soil, can be used as a biocontrol agent for a wide range of plant diseases as well as a plant growth enhancer (Manzar et al., 2022; Saadaoui et al., 2023).

The combination of biostimulants with reduced tillage practices (including tillage rotations), can offer benefits for wheat (Triticum aestivum) and canola (Brassica napus) production, especially in dryland farming systems where water and other biotic factors can be limiting. In literature, the term ‘tillage rotation’ refers to conducting different forms of soil disturbances through the use of various tillage implements. For example, Townsend et al. (2016) described tillage rotation by using the terms ‘mixed tillage’ and ‘rotational ploughing’ as ‘a system where the land is ploughed at specific points in the rotation with other tillage practices used in between.’ Zhang et al. (2022) described rotational tillage as two consecutive years of no-tillage followed by a year in which ploughing was conducted. Tshuma et al. (2021) used the term ‘infrequent tillage’ to refer to a similar tillage routine as described by Zhang et al. (2022) except that tillage was conducted to a depth of 75 mm without soil inversion. In this paper, the term ‘tillage rotation’ is as described by Tshuma et al. (2021) and it involves the application of alternating tillage regimes which include tillage and no-tillage. The phase of no-tillage can be one, two, or three consecutive years which are followed by a year in which shallow-tillage with a chisel plough was conducted to a depth of approximately 75 mm.

Tillage rotation is different from strategic tillage in that strategic tillage refers to once-off tillage, which is intentionally applied to solve specific problems that are identified in a no-tillage field (Blanco-Canqui & Wortmann, 2020; Conyers et al., 2019; Dang et al., 2018; Kirkegaard et al., 2014; Labuschagne et al., 2020; Peixoto et al., 2020) whereas tillage rotation is a continuous pattern of tillage. Although some studies have reported on tillage rotations (Agenbag, 2012; Bai et al., 2020; Maali & Agenbag, 2006, 2003), there is a paucity of information on the combined effects of biostimulant application and tillage rotation practices on crop yield in a dryland cropping system.

This research aimed to determine the short-term effects of biostimulant application on wheat and canola yield and quality in a dryland cropping system employing different historic tillage practices. Three objectives were examined by determining the combined effects of reduced application of synthetic agrochemicals (with biostimulants) and different tillage regimes on (1) aboveground plant biomass production, (2) seed and grain yield, and (3) seed and grain quality. It was hypothesised that a combination of tillage rotations and the application of biostimulants (reduced synthetic agrochemicals) will increase crop yield and quality relative to continuous tillage with a mouldboard plough or no-tillage treatment in a system with standard synthetic agrochemicals.

2. Materials and methods

2.1. Site description

The trial was conducted under a dryland condition at the Langgewens Research Farm (33°17ˈ0.78ˈˈ S, 18°42ˈ28.09ˈˈ E) of the Western Cape Department of Agriculture, in the Swartland region of South Africa. The region has a Mediterranean-type climate, characterised by wet winters and hot, dry summers, with a 55-year mean annual rainfall of 395 mm (standard deviation = 101.1 mm). Almost 80% of the rainfall occurs during the crop-growing season between April and September (ARC-Small Grain Institute, 2020). The trial site has shallow (300 mm) Haplic Cambisols (IUSS Working Group WRB, 2015). The soils have a clay content of 14.7% (excluding the gravel and stone content), whilst the gravel and stone content in the A horizon was 44.6% (Maali & Agenbag, 2003). A detailed description of the soil nutrient status of this research site can be found in Tshuma et al. (2021).

2.2. Trial history and treatments

The study was laid out as a factorial randomised block design with four replications. The experiment had two factors: tillage (summarised in Table 1), and agrochemical application (standard, and reduced). There were four blocks, and each had 14 plots. Each plot measured 50 × 6 m. The blocks were separated by a buffer zone of at least 9 m, and plots were separated by a 1 m buffer zone.

Table 1. Summary of tillage treatments, abbreviations and the implements used at Langgewens research farm.

Tillage treatment	Abbreviation	Description
Mouldboard	MB	Tillage with a chisel (tine) plough to a depth of 150 mm, followed by the mouldboard plough to a depth of 200 mm and field cultivator to a depth of 50 mm.
Tine-tillage	TT	Non-soil inversion tillage with a chisel plough to a depth of 150 mm, followed by field cultivator to a depth of 50 mm.
Shallow tine-tillage	ST	Non-soil inversion tillage with a chisel plough to a depth of 75 mm.
No-tillage	NT	Tillage was not conducted.
ST applied every 2nd year in rotation with NT	ST-NT	Tillage rotation consisted of one year of no-tillage, followed by one year in which non-soil inversion tillage with a chisel plough was conducted to a depth of 75 mm.
ST applied every 3rd year in rotation with NT	ST-NT-NT	Tillage rotation consisting of two consecutive years of no-tillage, followed by one year in which non-soil inversion tillage with a chisel plough was conducted to a depth of 75 mm.
ST applied every 4th year in rotation with NT	ST-NT-NT-NT	Tillage rotation consisted of three consecutive years of no-tillage, followed by one year in which non-soil inversion tillage with a chisel plough was conducted to a depth of 75 mm.

Each tillage treatment had been conducted on specific plots for at least 22 years. All plots were sown with a no-till drill.

The continuous tillage treatments: MB, TT and NT were started in 1976, and the tillage rotations were introduced in 1996. The experiments described in this article were conducted from 2018 to 2020. The tillage rotation treatments and crops grown at this study site are summarised in Table 2. Wheat was planted in 2017 but the crop was abandoned before harvest, due to poor germination and dry conditions. Before 2018, the trial site received a standard application of synthetic agrochemicals as determined by the Langgewens Technical Committee according to best practices common in the Swartland region (ARC-Small Grain Institute, 2020; FERTASA, 2016). In this region, herbicides can be sprayed four or five times annually. Numerous applications of synthetic insecticides and fungicides are also made during a single crop growing season, largely depending on insect/pest severity.

Table 2. Summary of tillage rotation treatments and crops grown during the study period (2018–2020) at Langgewens research farm.

Treatment		Year
	2017	2018	2019	2020
Tillage rotation
ST-NT	NT	ST	NT	ST
ST-NT-NT	ST	NT	NT	ST
ST-NT-NT-NT	NT	NT	NT	ST
Crop rotation
Crop	Wheat (terminated crop)	Wheat	Canola	Wheat

ST: Non-soil inversion tillage with a chisel plough to a depth of 75 mm; NT: No-tillage.

In the period, 2018 to 2020, the tillage treatments remained unchanged, but the application of agrochemicals was changed. Twenty-eight of the 56 plots were randomly allocated to continue receiving a standard application of synthetic agrochemicals. The remaining 28 plots were changed in 2018 and allocated to receive a reduced application of synthetic agrochemicals. The term ‘reduced’ refers to the complete exclusion of synthetic insecticides and fungicides and only two applications of herbicides (instead of four or five) in a season, coupled with the application of biostimulants.

In the current study, biostimulants applied were Trichoderma, fulvic acid, silicic acid, and triacontanol and bull kelp (Nereocystis luetkeana) extracts. The specific biostimulants used in this study were selected based on ease of availability in the region and their rate of application was as advised by the supplier, RealIPM—South Africa (Pty) Ltd. Trichoderma asperellum, which can be used, both, as a biostimulant and biopesticide (Manzar et al., 2022; Saadaoui et al., 2023), was applied to the wheat and canola seeds as seed treatment before seeding in the plots that received the reduced application of synthetic agrochemicals. All other biostimulants were applied a day before planting wheat, then at growth stage 16 – 17 (Zadoks growth scale), which is characterised by the emergence of six or seven unfolded leaves, and lastly at growth stage 37 or flag leaf stage. For canola, biostimulants were applied before planting, then at the growth stage 16–17, and lastly at 20%–30% bloom. Biostimulants were mixed with water to deliver the desired application rates. A full list of agrochemicals applied during the trial period is listed in Tables 3–5.

Table 3. Quantity of fertilisers (NPK and S ratios) applied in the wheat and canola fields at Langgewens research farm.

Crop	Fertiliser component applied (kg ha^-1)
	N	P	K	S
Wheat	65	12.5	0	0
Canola	75	13	13	11

Table 4. Synthetic agrochemicals applied on the wheat crop, in the management system with standard use of synthetic agrochemicals at Langgewens research farm.

Agrochemicals applied	Active ingredient	Pure active ingredient content	Application rate
Standard synthetic agrochemicals applied in wheat
Herbicide	Glyphosate	360 g L^-1	2 L ha^-1
Herbicide	Pyroxasulfone	850 g kg^-1	125 g ha^-1
Herbicide	Pyrasulfotole; Bromoxynil; Mefenpyr di-ethyl	37.5 g L^-1; 210 g L^-1; 9.38 g L^-1	750 ml ha^-1
Herbicide	Carfentrazone – ethyl (Triazolinone)	500 g kg^-1	10 g ha^-1
Fungicide	Pyraclostrobin; epoxiconazole (triazole)	62,5 g L^-1; 62,5 g L^-1	1 L ha^-1
Fungicide	Epoxiconazole (triazole); Thiophanate-methyl (benzimidazole)	187 g L^-1; 310 g L^-1	550 ml ha^-1
Insecticide	Acetamiprid (Acetamidine)	222 g L^-1	50 g ha^-1
Standard synthetic agrochemicals applied in canola
Herbicide	Glyphosate	360 g L^-1	2 L ha^-1
Herbicide	Propyzamide	400 g L^-1	1.9 L ha^-1
Herbicide	Atrazine	500g L^-1	2 L ha^-1
Insecticide	Metaldehyde	15 g kg^-1	8 kg ha^-1
Insecticide	Acetamiprid (Acetamidine)	222 g L^-1	50 ml ha^-1

Table 5. Synthetic chemicals and biostimulants applied on the wheat and canola crop, in the management system with reduced use of synthetic agrochemicals at Langgewens research farm.

Chemicals applied	Active ingredient	Pure active ingredient content	Application rate
Synthetic agrochemicals applied to wheat
Herbicide	Glyphosate	360 g L^-1	2 L ha^-1
Herbicide	Pyroxasulfone	850 g kg^-1	125 g ha^-1
Biostimulants applied in wheat (Reduced synthetic agrochemicals)
Biostimulant/ biopesticide	Trichoderma asperellum	1 x 10^9 colony forming spores ml^-1	100 ml ha^-1
Organic chelating agent	Fulvic acid	100 ml L^-1	1000 ml ha^-1
Biostimulant	Silicic acid	1 ml L^-1	500 ml ha^-1
Biostimulant	Triacontanol and bull kelp (Nereocystis luetkeana) extracts	10 ml L^-1	2000 ml ha^-1
Synthetic agrochemicals applied to canola
Herbicide	Glyphosate	360 g L^-1	2 L ha^-1
Herbicide	Propyzamide	400 g L^-1	1.9 L ha^-1
Biostimulants applied in canola (Reduced synthetic agrochemicals)
Biostimulant/ biopesticide	Trichoderma asperellum	1 x 10^9 colony forming spores ml^-1	100 ml ha^-1
Organic chelating agent	Fulvic acid	100 ml L^-1	1000 ml ha^-1
Biostimulant	Silicic acid	1 ml L^-1	500 ml ha^-1
Biostimulant	Triacontanol and bull kelp (Nereocystis luetkeana) extracts	10 ml L^-1	2000 ml ha^-1

The cultivars which were planted are listed in Table 6. Harvesting was conducted at the end of October of each year.

Table 6. The crop cultivars, their seeding rate and planting dates (2018–2020) at Langgewens research farm.

Crop	Cultivar	Seeding rate	Planting date
Wheat	SST 056	100 kg ha^-1	11^th May 2018
Canola	Alpha TT	2.5 kg ha^-1	30^th April 2019
Wheat	SST 0166	90 kg ha^-1	12^th May 2020

2.3. Assessments

2.3.1. Aboveground biomass and wheat ear-bearing tillers

The aboveground biomass of wheat and canola was determined at crop physiological maturity. Plant samples were cut to ground level from eight rows of 1 m length from each plot and dried in an oven at 60 °C for 72 hours.

The number of wheat ear-bearing tillers was determined by collecting samples from eight rows of 1 m length which were cut to ground level before harvesting.

2.3.2. Wheat grain and canola seed yield

In 2018, there was a severe windstorm, which led to crop damage just before harvesting that made conventional yield assessments impossible. Therefore, 50 visibly undamaged wheat spikes were randomly collected from each plot and used to determine the average weight of seeds per spike. The weight of seeds per spike and the number of wheat ear-bearing tillers per m² were used to estimate wheat grain yield (kg ha⁻¹). In 2019 and 2020, a Hege 140 plot combine was used to harvest a strip of 1.25 × 50 m along the centre of each plot where canola or wheat was grown to determine the seed yield.

2.3.3. Wheat grain and canola seed quality

A near Infrared (NIR) grain analyser (model IM 9500, Perten Instruments, Waltham, USA) was used to analyse the protein content (%), wet gluten (at 14% moisture content) and hectolitre mass (kg hL⁻¹) of wheat grains as well as the oil content of canola seeds (%). Both oil and seed protein values are reported on a dry matter basis. The thousand seed mass (g) was determined for both wheat and canola. Six samples of 1000 seeds for each treatment combination (for wheat and canola), were randomly drawn and weighed. The mean weight of the six samples was determined and used as the thousand seed mass.

2.4. Data analyses

The Variance Estimation, Precision and Comparison (VEPAC) package of STATISTICA^TM software version 13.5.0.17 (TIBCO Software Inc.) was used to analyse the data using the Restricted Maximum Likelihood (REML) procedure. Tillage treatment, agrochemical application, and their interactions were the fixed effects. Plot was specified as a random effect nested in block (to account for repeated measures). Growing season was also specified as a random effect.

For canola data, tillage treatment and agrochemical application were the fixed effects and block was the random factor. All parameters were subjected to a test of normality using the normal probability plots of raw residuals, and no data set required any transformation. Where the F-test was significant, the mean separation was performed using Fisher’s least significance difference (LSD) test at a 5% significance level.

3. Results

3.1. Response of wheat to tillage and agrochemical application

The interaction between the tillage treatment and agrochemical application was significant for the wheat grain yield (Table 7). Grain yield was generally higher in the system with standard synthetic agrochemicals, but there were no differences in grain yield between the systems with standard, and reduced synthetic agrochemicals in four (MB, ST, ST-NT, and ST-NT-NT-NT) of the seven tillage treatments (Figure 1). A major difference between the system with standard, and reduced synthetic agrochemicals was in the ST-NT-NT, TT and NT tillage treatments.

Figure 1. The wheat grain yield as influenced by the interactions between the crop management system and tillage treatment at Langgewens Research Farm. The different letters on top of the bars denote a significant difference (p < 0.05). Error bars denote the standard deviation of the mean. MB = Mouldboard at 200 mm depth; TT: Tine-tillage at 150 mm depth; ST: Shallow tine-tillage at 75 mm depth; NT: No-tillage. The underlined treatment in the sequence indicates the treatment for 2018 and 2020.

Table 7. The ANOVA for the wheat crop productivity parameters for 2018 and 2020 as affected by the tillage treatment, agrochemical application, and their interactions.

Parameter	Tillage treatment (T)	Agrochemical application (A)	T × A
	p-Value	p-Value	p-Value
Biomass (kg DM ha^-1)	0.184	0.093	0.384
Thousand seed mass (g)	0.146	0.568	0.375
Hectolitre mass (kg hL^-1)	0.053	0.839	0.295
Ear bearing tillers (m^-2)	0.028	0.011	0.026
Grain protein content (DB %)	0.002	0.309	0.659
Grain yield (kg ha^-1)	0.039	0.087	0.036

Bold p-values denote significant effects at p < 0.05.

DM: Dry Mass; DB: Dry Basis.

The wheat grain yield (3663 kg ha-1) under the ST-NT-NT-NT rotation system combined with reduced synthetic agrochemicals did not differ (p > 0.05) from the yield in the MB (3742 kg ha⁻¹) and NT (4205 kg ha⁻¹) tillage treatments from the system with standard synthetic agrochemicals (Figure 1). Also, grain yield in the tillage rotation ST-NT-NT-NT in the system with reduced synthetic agrochemicals was higher but did not differ (p > 0.05) from that in the MB and NT tillage treatments in the system with reduced synthetic agrochemicals. A comparison of the three tillage rotations in the system with reduced synthetic agrochemicals showed that the ST-NT-NT-NT treatment led to a higher (p < 0.05) grain yield than the ST-NT-NT treatment but did not differ (p > 0.05) from the ST-NT treatment (Figure 1).

The aboveground biomass of wheat, thousand seed mass and hectolitre mass of wheat grain were not affected (p > 0.05) by any treatment (Table 7). A combination of tillage rotations and biostimulants did not significantly affect the aboveground biomass production, thousand seed mass and hectolitre mass of wheat grain relative to other tillage treatments. The aboveground biomass of wheat was 6513 kg ha⁻¹ with a standard deviation (SD) of 1142. The thousand seed mass of the grains had a mean weight of 38.18 g (SD = 1.87) whilst the hectolitre mass of the grains was 82.19 kg hL⁻¹ (SD = 1.00).

The interaction between the tillage treatment and crop management systems was significant for the number of wheat ear-bearing tillers (Table 7). On comparing the crop management systems (standard vs. reduced synthetic agrochemicals) in individual tillage treatments, the only significant differences were in the ST-NT-NT (219 m⁻² vs. 152 m⁻²), and NT (236 m⁻² vs. 187 m⁻²) treatments (Table 8). The number of tillers in the tillage rotation ST-NT-NT-NT and NT treatment in the system with reduced synthetic agrochemicals did not differ (p > 0.05) from that in the MB treatment in the system with standard synthetic agrochemicals. The number of tillers was, however, significantly higher in the NT treatment of the system with standard synthetic agrochemicals.

Table 8. The number of wheat ear-bearing tillers and grain protein content as influenced by the interactions between the crop management system and tillage treatment at Langgewens research farm.

Tillage treatment	Synthetic agrochemical application
	Standard	Reduced
Wheat ear-bearing tillers (m^-2)
MB	204 ± 28.57^bc	192 ± 29.55^bc
TT	195 ± 20.06^bc	177 ± 19.34^cd
ST	202 ± 42.04^bc	182 ± 46.80^c
ST-NT	188 ± 13.81^c	183 ± 22.25^c
ST-NT-NT	219 ± 26.17^ab	152 ± 22.38^d
ST-NT-NT-NT	217 ± 25.76^ab	200 ± 42.73^bc
NT	236 ± 26.65^a	187 ± 30.31^bc
Grain protein content (dry basis %)
MB	14.83 ± 1.28^a	14.06 ± 1.36^abc
TT	13.81 ± 1.72^bcd	13.74 ± 1.67^bcd
ST	14.13 ± 1.38^abc	13.56 ± 1.57^bcd
ST-NT	13.74 ± 1.88^bcd	13.48 ± 1.47^bcd
ST-NT-NT	13.95 ± 1.57^bcd	13.23 ± 1.67^cd
ST-NT-NT-NT	14.09 ± 1.81^bc	13.70 ± 1.44^bcd
NT	13.73 ± 1.79^bcd	12.89 ± 1.76^d

Values are the mean ± standard deviation of the mean. The different superscripts in each row and column denote a significant difference (p < 0.05).

MB: Mouldboard at 200 mm depth; TT: Tine-tillage at 150 mm depth; ST: Shallow tine-tillage at 75 mm depth; NT: No-tillage.

The protein content of wheat grains was affected by the tillage treatments (Table 7). Protein content was higher (p > 0.05) in the MB treatment (standard synthetic agrochemicals) than in the NT treatment, and tillage rotation ST-NT-NT-NT of the system with reduced synthetic agrochemicals (Table 8). Concerning individual tillage treatments, there were no significant differences between the system with standard and reduced synthetic agrochemicals. The grains from the tillage rotation ST-NT-NT-NT in the system with reduced synthetic agrochemicals had higher protein content (p > 0.05) than that in the NT treatment of the same system. No significant differences in grain protein content were found among the three tillage rotation treatments.

3.2. Response of canola to tillage and agrochemical application

The crop management system affected (p < 0.05) aboveground biomass production (Table 9). Relative to the system with reduced synthetic agrochemicals, the system with standard synthetic agrochemicals led to a significantly higher aboveground biomass production in the MB and TT treatments only (Table 10). Reduced synthetic agrochemical application led to similar (p > 0.05) biomass production across all seven tillage treatments (Table 10). In addition, no significant differences in the thousand seed mass were observed (Table 9). The thousand seed mass of canola narrowly ranged between 2.55 and 2.75 g across all treatments.

Table 9. The effect of tillage treatment, crop management system and their interactions on canola productivity.

Parameter	Tillage treatment (T)	Agrochemical application (A)	T × A
	p-Value	p-Value	p-Value
Biomass (kg DM ha^-1)	0.169	0.050	0.222
Thousand seed mass (g)	0.317	0.507	0.734
Seed oil content (DB %)	<0.001	0.629	0.610
Seed yield (kg ha^-1)	0.003	0.259	0.143

Bold p-values denote significant effects at p < 0.05.

DM: Dry Mass; DB: Dry Basis.

Table 10. Canola aboveground biomass productivity as affected by the crop management system; and the seed oil content as affected by different tillage treatments at Langgewens research farm.

Tillage treatment	Synthetic agrochemical application
	Standard	Reduced
Aboveground biomass (kg ha^-1)
MB	8542 ± 1758^a	6563 ± 1018^c
TT	8188 ± 797^ab	5813 ± 954^c
ST	7146 ± 1062^abc	5563 ± 844^c
ST-NT	6000 ± 620^c	6365 ± 1057^c
ST-NT-NT	6646 ± 1240^bc	5938 ± 691^c
ST-NT-NT-NT	7052 ± 232^abc	5677 ± 513^c
NT	7146 ± 644^abc	6552 ± 1086^c
Seed oil content (dry basis %)
MB	37.53 ± 0.88^cd	37.13 ± 0.63^d
TT	40.85 ± 2.30^ab	39.48 ± 1.24^bc
ST	41.10 ± 0.70^ab	40.75 ± 2.16^ab
ST-NT	42.33 ± 0.93^ab	41.18 ± 0.70^ab
ST-NT-NT	40.85 ± 0.87^ab	41.78 ± 1.03^ab
ST-NT-NT-NT	40.35 ± 1.54^ab	39.38 ± 0.42^bc
NT	42.25 ± 1.94^ab	42.47 ± 1.84^a

Different superscripts in each row and column denote a significant difference (p < 0.05). Values are the mean ± standard deviation of the mean. The underlined treatment in the sequence indicates the treatment for 2019.

MB: Mouldboard at 200 mm depth; TT: Tine-tillage at 150 mm depth; ST: Shallow tine-tillage at 75 mm depth; NT: No-tillage.

The oil content of the canola seeds was affected by the tillage treatments (Table 9). The MB treatment generally led to the lowest (p < 0.05) seed oil content in both systems where synthetic agrochemicals were applied at standard and reduced rates (Table 10). Except for the tillage rotation ST-NT-NT-NT (in both standard and reduced synthetic agrochemicals) and NT (standard synthetic agrochemicals), the seed oil content generally increased (p < 0.05) with a reduction in tillage frequency. For each tillage treatment, the application of reduced synthetic agrochemicals did not affect (p > 0.05) the seed oil content in both systems with standard and reduced synthetic agrochemicals. A combination of reduced synthetic agrochemical application and the ST-NT-NT-NT tillage rotation resulted in reduced, and higher (p < 0.05) seed oil content compared to the NT and MB treatments, respectively (Table 10).

The tillage treatments affected (p < 0.05) the canola seed yield (Table 9). The ST treatment was the only tillage treatment that significantly affected (p < 0.05) canola seed yield under the crop management systems with standard, and reduced application of synthetic agrochemicals (Figure 2). In the other six tillage treatments (MB, TT, ST-NT, ST-NT-NT, ST-NT-NT-NT, and NT), the seed yield in the system with standard synthetic agrochemicals did not differ (p > 0.05) from that in the system with reduced synthetic agrochemicals. Regarding the system with reduced application of synthetic agrochemicals, the NT treatment resulted in a significantly higher seed yield than the MB, and ST-NT-NT-NT tillage rotation. Furthermore, the tillage rotation ST-NT-NT generally had higher seed yield than the ST-NT-NT-NT tillage rotation.

Figure 2. Canola seed yield as affected by different tillage treatments at Langgewens Research Farm. Different letters on top of the bars denote significant differences (p < 0.05). Error bars denote the standard deviation of the mean. MB = Mouldboard at 200 mm depth; TT = Tine-tillage at 150 mm depth; ST = Shallow tine-tillage at 75 mm depth; NT = No-tillage. The underlined treatment in the sequence indicates the treatment for 2019.

4. Discussion

Our findings indicate that it can be feasible to decrease both tillage intensity and the quantity of synthetic agrochemicals that are applied in wheat and canola production without incurring significant yield losses. For wheat production, there were no significant differences in grain yield between the systems with standard and reduced synthetic agrochemicals in four of the seven tillage treatments. For canola production, six of the tillage treatments resulted in no differences in seed yield between the systems with standard and reduced use of synthetic agrochemicals. Although we could not find any study similar to ours regarding a focus on the combined effects of reduced synthetic agrochemicals (with biostimulants) and different tillage treatments, some studies have shown that the use biostimulants or biofertilisers can be an effective means of cutting down on synthetic agrochemicals such as N fertiliser in wheat production (Maksoud et al., 2023; Saadaoui et al., 2023; Sleighter et al., 2023; Youseif et al., 2023). Maksoud et al. (2023) used henna leaf (Lawsonia inermis) extract as the biostimulant whilst Youseif et al. (2023) used Streptomyces as biostimulants for wheat production. Sleighter et al. (2023) used natural organic matter-based biostimulants for stress mitigation in wheat production. Similar to our study, Saadaoui et al. (2023) investigated the use of Trichoderma spp. as a biocontrol agent in substitution of synthetic inputs. They reported that Trichoderma was effective in mitigating some pathogens and aided in wheat growth. In our study, seeds were treated with Trichoderma asperellum, and the biostimulants; silicic acid, and Nereocystis luetkeana extracts, were applied as foliar spray to the plants. Although we did not monitor the pest and plant diseases, we did not encounter any noticeable pest or disease attack within any part of the trial site during the entire three years of the study.

Other studies involving various tillage rotation systems in wheat production generally found increased productivity relative to continuous tillage systems (Yin et al., 2022; Zhang et al., 2021a; 2021b; 2022). Zhang et al. (2022) described their tillage rotation as no-tillage–no-tillage–plough. The tillage rotation described by Zhang et al. (2021b) involved no-tillage, conventional tillage and subsoiling rotated annually. Zhang et al. (2021a) indicated that their tillage rotations involved alternating periods of no-tillage and subsoiling; and also subsoiling and ploughing. In our trial, the tillage rotations involved a combination of shallow tine-tillage to a depth of 75 mm in rotation with one, two or three years of no-tillage. One major difference between the other trials and ours is that our trial involved the use of reduced synthetic agrochemicals, with partial substitution with biostimulants. In our study, the tillage rotations in combination with reduced synthetic agrochemicals gave contrasting yield results, for example, in wheat production, the ST-NT-NT-NT treatment led to the highest grain yields but not in canola production. The ST-NT-NT treatment led to a high canola yield. This difference could be attributed to crop-specific requirements in terms of soil disturbance. However, further studies on tillage rotations and canola production are recommended to get conclusive yield results.

Regarding wheat production in a system with standard synthetic agrochemicals, our results were in agreement with the results of Agenbag (2012) who found no differences in grain yield between the NT and tillage rotation treatments. In other studies involving once-off tillage practices (occasional or strategic tillage), crop yield neither improved nor decreased significantly from that of the NT treatment (Blanco-Canqui & Wortmann, 2020; Conyers & Dang, 2014; Dang et al., 2018; Kirkegaard et al., 2014). These results seem to suggest that tillage rotation practices do not negatively affect crop yield.

The analysis of wheat grain quality revealed that the thousand kernel mass and hectolitre mass were not affected by the tillage treatments. Our findings on hectolitre mass were similar to the results by Taner et al. (2015) and Seepamore et al. (2020). A four-year tillage study by Taner et al. (2015) in the Central Anatolia region of Turkey found that tillage treatments did not significantly affect the wheat grain hectolitre mass. Similarly, a long-term (37 years) study that evaluated tillage treatments and other management practices such as residue management, and weed control methods in the Eastern Free State province of South Africa, found that management practices did not significantly affect the wheat grain hectolitre mass (Seepamore et al., 2020). Contrary to our results on the thousand kernel mass, Taner et al. (2015) noted that the kernel mass was affected by the treatments. The differences between the studies could be a result of various factors such as the environmental conditions (rainfall distribution, temperature and light) during the grain-filling phase.

The highest and lowest wheat grain protein content was in the MB and NT treatments, respectively, possibly due to increased nutrient mineralisation in the MB-ploughed field. Other studies (Colecchia et al., 2015; Labuschagne et al., 2020; Seepamore et al., 2020; Woźniak & Rachoń, 2020) concur with our findings that tillage effects can affect the grain protein content. Nonetheless, tillage rotations in combination with reduced synthetic agrochemicals could not significantly improve the seed or grain quality attributes relative to the NT or MB treatments in the system with standard synthetic agrochemicals.

The economic feasibility of using biostimulants in arable farming was beyond the scope of this research but would need to be evaluated to encourage producers to reduce the quantity of synthetic agrochemicals they apply. In addition, the farming problems cannot be overcome by the simple substitution of individual inputs. Therefore, to reduce reliance on synthetic agrochemicals, it may be beneficial to change the management strategies to include livestock, competitive crops, cover crops, plant density and row spacing (MacLaren et al., 2021). The inclusion of livestock or conducting additional cultivation may be necessary for weed control in the systems with reduced synthetic agrochemicals. Long-term studies are needed to evaluate the trade-offs between the direct cost of biostimulants and environmental benefits so that more producers can be motivated to use fewer synthetic agrochemicals in field crop production.

5. Conclusion

A combination of reduced synthetic agrochemicals and tillage rotation practices that involve shallow tillage maintained but did not significantly improve crop yield and quality, relative to the NT and MB treatments in dryland farming conditions. The results also show that it is unnecessary to conduct intensive tillage with the MB plough as it did not result in significantly higher yields, relative to the tillage rotations and NT treatments in both systems with standard and reduced application of synthetic agrochemicals. In addition, it is possible to reduce the quantity of synthetic agrochemicals that are applied to crops by replacing them with biostimulants without significant changes in wheat grain or canola seed yields and quality. For environmental and farming sustainability, we suggest that producers avoid frequent, intensive tillage and opt for NT or tillage rotation practices during the process of converting from standard to reduced use of synthetic agrochemicals.

Authors’ contributions

PS, JL, FR, and JB contributed to the study conceptualisation, and methodology, and edited the manuscript. FT conducted the research and drafted the initial manuscript under the guidance of PS, JL, FR, and JB. FT also conducted the statistical analysis. All authors read and approved the final manuscript.

Acknowledgements

The authors thank the Western Cape Agricultural Research Trust for providing the research bursary and the Western Cape Department of Agriculture staff for their work in maintaining these trials and assisting with data collection. They also thank Real IPM for their technical support and for supplying the biostimulants.

Disclosure statement

The authors declare that they have no conflict of interest.

Additional information

Funding

The Western Cape Department of Agriculture, Coventry University and Stellenbosch University are acknowledged for funding the long-term trials within which this study was conducted.

Notes on contributors

Flackson Tshuma

Dr Flackson Tshuma is a researcher within the Department of Agronomy at Stellenbosch University, where his work is marked by a commitment to sustainable agricultural practices. He holds a dual PhD in Agronomy from Stellenbosch University in South Africa and Coventry University in the UK. His research interests primarily revolve around the utilisation of biostimulants to minimise the reliance on synthetic agrochemicals in cereal crop cultivation. Furthermore, he has interests in the impact of various tillage methodologies (particularly rotational tillage systems that involve no-tillage and deep ripping), on dryland cereal production and soil nutrient stratification.

Pieter Andreas Swanepoel

Prof Pieter Swanepoel is the Chairperson of the Department of Agronomy at Stellenbosch University. As an Associate Professor, Prof Swanepoel directs an active research program focused on conservation agriculture and pastures, providing mentorship to MSc and PhD students. His research is grounded in agroecosystem principles, which emphasize the integration of sustainable farm management techniques to support the intensification of grain production systems. With a focus on responsible and ethical resource use, his research encompasses soil, water, nutrient, crop, and animal resources. Swanepoel contributes to the Protein Research Foundation (PRF), which promotes sustainable local production of protein to meet the increasing demand for animal production purposes, with a current focus on canola and soybean production.

Johan Labuschagne

Dr Johan Labuschagne is a Senior Lecturer at the Department of Agronomy, Stellenbosch University, South Africa. He obtained his PhD in Agronomy at Stellenbosch University with the thesis titled: Nitrogen Management Strategies on Perennial Ryegrass-White Clover Pastures in the Western Cape Province. He is involved in lecturing undergraduate agronomy students as well as supervising various PhD and MSc Agric students. Dr Labuschagne has 25 years of experience in various aspects related to long-term studies, with a focus on soil disturbance and crop rotation systems. He also has extensive experience in nitrogen fertilisation of wheat and canola grown under conservation agriculture systems. He is registered as a Professional Natural Scientist with the South African Council for Natural Scientific Professions.

James Bennett

Dr James Bennett has a BSc and MSc in Biological Sciences from the University of Warwick and a PhD in Smallholder Farming Systems in South Africa from Coventry University. He is currently an Associate Professor in Environmental Studies at the Centre for Agroecology, Water and Resilience at Coventry University. His primary research interests are in the management of communal rangelands for livestock production in Africa and the integration of sustainable practices into smallholder agricultural systems in Africa.

Francis Rayns

Dr Francis Rayns has degrees in biological sciences from Bangor University and De Montfort University, both in the UK. He currently works in the Centre for Agroecology, Water and Resilience at Coventry University. His research interests focus on sustainable management of soil fertility (particularly the use of green manure crops and the utilisation of waste materials in the form of soil amendments such as compost, anaerobic digestate and biochar) and also has interests in intercropping, the use of heritage vegetable varieties, alternatives to the use of plastics in agriculture and citizen science as a method for improving agricultural practice.