Residue cover, soil structure, weed infestation and spring cereal yields as affected by tillage and straw management on three soils in Norway

ABSTRACT

Four field trials (spring wheat and oats) were conducted (one on clay soil, one on loam soil and two on silt soil) for three years in important cereal growing districts, to investigate the influence of tillage regimes (ploughing versus reduced tillage in either autumn or spring) and straw management (removed and retained) on plant residue amounts, weed populations, soil structural parameters and cereal yields. The effect of tillage on soil structure varied, mainly due to the short trial period. In general, the amount of small soil aggregates increased with tillage intensity. Reduced soil tillage, and in some cases spring ploughing, gave significantly higher aggregate stability than autumn ploughing, thus providing protection against erosion. However, decreasing tillage intensity increased the amounts of weeds, particularly of Poa annua on silt soil. Straw treatment only slightly affected yields, while effects of tillage varied between both year and location. Reduced tillage, compared to ploughing, gave only small yield differences on loam soil, while it was superior on clay soil and inferior on silt soil. Our results suggest that shallow spring ploughing is a good alternative to autumn ploughing, since it gave comparable yields, better protection against erosion and was nearly as effective against weeds.

KEYWORDS:

• Cereal yield
• straw cover
• straw treatment
• soil structure
• soil tillage
• weed infestation

Introduction

Soil tillage is important to loosen the soil, for the incorporation of plant residues and for the control of weeds (Håkansson et al. 1998), in order to obtain optimal plant emergence and high yields of good quality (Beyer et al. 2006; Tørresen et al. 2012). On the other hand, tillage, especially ploughing, is one of the most energy- and labour-intensive farming operations (Filipović et al. 2004; Saraukis et al. 2012). Approximately 50% of fuel consumption on farms is due to tillage (Kowalewsky 2009), making it a significant contributor to CO₂ emissions (Stajnko et al. 2009). There is a worldwide trend towards reduced factor input to improve the sustainability of crop production. One measure is the reduction of tillage intensity, the motivations being both economic (reduced energy and labour input), agronomic (improvement of soil structure and productivity), organizational (optimizing field operations, timeliness) and ecological (erosion control, reduced nutrient leaching), as described by various authors (Unger & Kaspar 1994; Soane & Vanouwerkerk 1995; Håkansson 2005; Riley et al. 2009; Saraukis et al. 2012).

Soil erosion from agricultural land has been of increasing concern in Norway, where it is mostly due to a high percentage of spring cereals in combination with autumn ploughing, which leaves the soil bare during rainy periods in autumn and snowmelt in spring (Øygarden 2000; Lundekvam 2007). Soil tillage influences the location of plant residues: in conventional systems they are mostly incorporated into the soil, whereas reduced tillage leads to an increased amount of residue on or near the surface. This is promoted as a management tool to reduce erosion (Guerif et al. 2001) by means of both residue cover and standing stubble (Papendick 2004). In areas with medium to high erosion risk, the Norwegian government supports farmers for delaying primary tillage operations until spring. This has led to a decrease in autumn ploughing from 82% in 1989 to 40% in 2011 (Gundersen et al. 2008), while the area not tilled at all in autumn increased to about 57% (Bye et al. 2012).

Although reduced tillage increases soil bearing capacity (Seehusen et al. 2014), it may decrease yields due to a compacted lower topsoil layer, which impedes root growth under certain circumstances (Riley et al. 2005; Hansen et al. 2007). Increased amounts of perennial and overwintering weeds (Ekeberg et al. 1985; Tørresen et al. 2003) make reduced tillage systems reliant upon the use of herbicides (Papendick 2004; Nail et al. 2007). Furthermore, plant debris on the soil surface may hamper sowing and plant establishment (Børresen 1999) and increase overwintering of fungal cereal diseases (Dill-Macky 2008; Pereyra & Dill-Macky 2008) that survive on crop residue and stubble or even on weeds (Parry et al. 1995; Paulitz 2006). Bateman et al. (1998) found an increase in plant diseases and a higher incidence of Fusarium in no-till systems compared to conventional tillage. Similar findings have been reported by Elen (2002) and Henriksen (2006) in Norway. This is of major concern to growers due to the increasing incidence of mycotoxins in grain in recent years (VKM 2013). Multidisciplinary field trials were therefore conducted in 2010–2012 on representative soils within three of the largest cereal growing districts in Norway, in order to investigate the influence of various tillage and straw coverage regimes on the overwintering of Fusarium spp. and development of mycotoxins in grain, as well as on other factors that influence both yield and quality. The object of this paper is to describe the effects of soil management on the amounts of plant residue, weed populations, soil structural parameters and cereal yields in these trials. A subsequent paper (Hofgaard et al. 2016) will present associated findings on Fusarium and mycotoxin incidence, and discuss these in relation to the results presented here. Preliminary results of this part of the study were published by Hofgaard et al. (2012) and Seehusen (2014).

Materials and methods

Sites and soil description

Four trials were established, two in autumn 2009 (on loam soil), the others in spring 2010. Two trials were located on silt soil in Solør (Stagnosol, medium erosion risk, poor natural drainage) near Kongsvinger (60.25°N, 12.08°E) with spring oats (Avena sativa L.) and spring wheat (Triticum aestivum L.), respectively. One trial (spring oats) was located on clay-loam in Østfold (Albeluvisol, medium erosion risk, imperfect natural drainage), near Sarpsborg (59.32°N, 11.03°E) and one trial (spring wheat) was on loam in Østre Toten (Cambisol, low erosion risk, moderate natural drainage), near Gjøvik (60.69°N, 10.86°E) (WRB 2006). Topsoil texture and organic matter contents at the three different locations are given in Table 1. Mean monthly air temperature and precipitation, recorded at nearby weather stations, are compared in Table 2 to 30-year normal values (1961–1990).

Table 1. Soil texture and organic carbon contents of the topsoil at all three locations.

Location	Soil texture	Depth (cm)	Gravel^a	C org^b	Sand^c	Silt^c	Clay^c
Toten	Loam	3.5–7.5	6	2.2	52	35	14
		15–19	7	2.3	50	36	15
Solør	Silt	3.5–7.5	0	1.3	12	82	6
		15–19	0	1.3	11	82	6
Østfold	Clay loam	3.5–7.5	1	2.0	29	41	30
		15–19	2	2.1	28	41	30

^aPer cent of whole sample.

^bPer cent of all material <2 mm.

^cPer cent of mineral material <2 mm

Table 2. Normal (1961–1990) air temperature (°C) and precipitation (mm) in the growing season at all three locations, and deviations from these values during the trial years.

Temperature	Loam soil				Silt soil				Clay soil
	Normal	2010	2011	2012	Normal	2010	2011	2012	Normal	2010	2011	2012
April	2.3	+1.2	+6.0	+0.7	3.1	+1.5	+5.3	−0.1	4.6	+0.7	+3.4	− 0.4
May	9.0	−0.2	+0.7	+1.0	9.5	+0.0	+0.3	+0.9	10.4	− 0.6	+0.1	+1.0
June	13.7	−0.1	+0.9	−1.6	14.2	+0.2	+0.9	−2.1	14.6	− 0.5	+0.4	−2.2
July	14.8	+1.5	+1.0	− 0.3	15.3	+2.1	+1.0	− 0.5	16.7	+0.2	+0.4	−1.9
August	13.5	+0.8	+0.7	+0.6	13.9	+1.3	+0.7	+0.4	15.6	+0.1	− 0.3	−1.2
September	9.1	+0.1	+2.4	+0.6	9.5	+0.1	+2.1	+0.1	11.4	− 0.5	+1.3	− 0.6
Precipitation
April	32	−3	+8	+1	36	−23	−21	+14	42	− 2	+3	+45
May	44	−7	+28	+9	52	−8	+25	+25	58	−11	+23	+14
June	60	+22	+27	− 25	68	+65	+18	+6	72	−23	+45	+45
July	77	+37	+38	+67	77	+38	+40	+62	73	+40	+78	− 9
August	72	+36	+83	+11	80	+13	+42	+64	83	+95	+62	+18
September	66	+14	+43	−21	79	− 35	+40	− 7	94	+5	+142	+25

Experimental design

The trials had a randomized split-plot design with two replication blocks. Main treatments (plot size 42 x 15 m) comprised (i) straw removed leaving stubble only (s) and (ii) straw chopped and retained on the field (st). The plots were separated by borders of 8 m to allow correct operation of tillage implements. Within each main plot, five 6 x 15 m split-plots were established, with different tillage regimes (I–V), as follows:

SSH: Shallow spring harrowing (5–6 cm) without ploughing.

SSP: Shallow spring ploughing (12–15 cm) without any autumn tillage.

SAH: Shallow autumn harrowing (5–6 cm) without ploughing.

DAH: Deep autumn harrowing (10–12 cm) without ploughing.

DAP: Deep autumn ploughing (25 cm).

Agronomic practices

Cultural practices were relatively consistent during the study period with the aim of treating all four trials similarly. Times of seeding, fertilizing and soil tillage depended on local variations in weather (Table 3). All agronomic practices, types of machinery and dates for field operations were considered to be representative of good farming practice in Norway.

Table 3. Overview about dates (day.month) for fieldwork and measurements in the trials. The trials on silt soil were treated equally.

	Loam soil (wheat)			Silt soil (wheat and oats)			Clay soil(oats)
	2010	2011	2012	2010	2011	2012	2010	2011	2012
Soil tillage	7.5	29.4	1.5	26.5	25.5	1.6	26.4	29.4	–
Seeding (cereals)	12.5	30.4	8.5	29.5	25.5	1.6	27.4	29.4	22.5
Debris cover reg.	25.5	10.5	10.5	8.6	1.6	4.6	5.5	4.5	23.5
Germination	25.5	12.5	20.5	8.6	5.6	9.6	5.5	–	–
Herbicide (cereal)	17.6	16.6	19.6	16.6	15.6	21.6	2.6	3.6	15.6
Shooting	14.7	6.7	10.7	24.7	12.7	1.8	27.6	28.6	3.7
Weed assessment	–	11.8	14.8	–	18.8	21.8	–	9.8	–
Anthesis (approx.)	25.5	12.5	15.7	08./10.6	–	–	15./17.5	5.7	9.7
Harvesting	6.9	2.9	13.10	27.9	29.9	10.10	1.9	9.9	5.9
Cutting of the straw	–	–	–	14.10	20.10	–	–	13.10	–
Post-harvest herbicide	1.10	28.9	–	8.10	13.10	–	30.9	11.10	–
Soil tillage	11.10	11.10	–	20.10	24.10	–	20.9	25.10	–

Note: –, no data/not done.

Straw treatment

The method of cutting the straw varied slightly between both trials and years. All trials were harvested with a stubble height of 10–15 cm without cutting the straw. Plant residues were weighed at harvest. On plots with straw removal (s), the straw was baled and removed. On the other plots (st) the straw was cut to an average length of 6–7 cm, using a straw cutter mounted on the combine harvester or a stubble chopper, and spread evenly over their whole surface. The straw treatment in the trial on loam soil (Toten) was established already in autumn 2009. In the trial on clay soil (Østfold), the straw was cut, but not removed in autumn 2009. The relevant plots were autumn-tilled in that year. In the trials on silt soil (Solør), the plots were not established until spring 2010 due to late harvest and adverse weather conditions in 2009. In this case, all tillage and straw cutting was therefore done in spring 2010. The straw was not removed that spring. In that year, the term autumn tillage on silt soil relates therefore to working depth and not to the time of tillage.

Soil tillage

All soil tillage was performed in the length direction of the plots. The type of machinery used varied between locations, but at each location the same implements were used in most years. On loam soil shallow harrowing was done with a rotary harrow, DAH with a tine cultivator, SSP with a 1–3 furrow plough and DAP with a 3–4 furrow plough. On clay soil all shallow harrowing was done with a rotary harrow while deep harrowing was performed by a tine cultivator and all ploughing by a 2-furrow plough. On silt soil shallow harrowing was performed with a tine harrow (spring) and a rotary harrow (autumn), DAH was done with a rotavator and ploughing with a 2-furrow plough. Final seedbed preparation was done by harrowing to <5 cm, before sowing with a combined fertilizer and seed drill and rolling with a Cambridge roller.

Fertilizing and seeding

Cereals were fertilized with compound NPK fertilizer (Yara 22-3-10) which also contained magnesium (1.2%), sulphur (2.5%) and boron (0.02%). The fertilizer was row-placed at 25 cm row spacing and 8 cm depth at rates of 460–510 kg ha⁻¹ and the seed was sown at ca. 4 cm depth with coulters spaced at 13 cm. No fertilizer was applied later in the growing season. The same cereal varieties were grown in each trial without rotation during the period 2010–2012 (Bjarne wheat and Belinda oats) at seed rates of 220–240 kg ha⁻¹.

Plant protection

Annual weeds were controlled with herbicides applied at recommended dosages at growth stage BBCH 13 (three true leaves) (Lancashire et al. 1991) (Table 3). Most trials were treated with fluroxypyr + clopyralid + MCPA (Ariane S, Dow AgroSciences). Exceptions were Toten in 2010, where a mixture of fluroxypyr + clopyralid + MCPA and tribenuron-methyl (Express, DuPont) was used, the fields at Solør in 2012 treated with metsulfuron-methyl + carfentrazon-ethyl (Ally Class, DuPont), and the field at Østfold treated with mecoprop-p + tribenuron-methyl (Granstar Power, DuPont) in 2011 and 2012. Glyphosate, 720 g a.i ha⁻¹ (e.g. Glyfonova Pluss, Cheminova) was applied at all sites after harvesting to control perennial and overwintering weeds. All plant protection applications were made uniformly over the whole trial. No fungicides, insecticides or plant growth regulators were used.

Plant debris remaining, cereal yield and weed infestation measurements

Plant debris remaining on the soil surface was estimated using the line-transect method (Figure 1), which involves the use of a cord with 100 equally spaced knots (Morrison et al. 1993). This method is commonly used to assess erosion risk and in the second part of this study (Seehusen 2014; Hofgaard et al. 2016) also used to assess the possible effects of residues on disease inoculum. The cord was stretched diagonally across each plot and the coincidence of knots and plant debris was counted to give percentages of plant residue cover. To ensure accuracy, duplicate readings were made (Sloneker & Moldenauer 1977) and average values were used in analyses. These measurements were made after seeding in all years. In 2010 and 2011 the amount of plant residue was also measured after soil tillage in autumn in order to determine the erosion risk. In the trial at Toten, measurements were also made before soil tillage in spring 2010, to assess straw decomposition during the winter months. Weed infestation was assessed visually on each plot at or slightly before the yellow ripe stage of cereals (BBCH 87) in 2011 and 2012. The aboveground biomass of each weed species, including cereal volunteers, was estimated as the percentage of total aboveground biomass (weeds + crop = 100%). The percentage of lodging was estimated at each harvest. Grain yields were recorded on each tillage plot with a plot combine harvester on strips of 18–36 m². Grain moisture, crude protein concentration and grain bulk weights were measured in each sample using an Infratec 1241 (NIR) instrument. Grain yields are expressed at 15% moisture.

Figure 1. Measurement of plant debris on the soil surface using the line-transect method. Spots mark knots on cord.

Soil structure measurements

In order to determine soil bulk density (BD), air permeability, air-filled pore volume and soil moisture retention, undisturbed cylinder core samples (100 cm³) were taken in summer 2012 at all three locations, from the three most differing tillage treatments (SSH, SSP and DAP) at 3.5–7.5 cm (referred to as upper layer) and 15–19 cm (referred to as deeper layer). Samples were taken on all main plots, with two cylinders per subplot and depth (48 per location).

Air-filled porosity and soil water retention were measured by first saturating the samples from below, before draining them, on ceramic plates, at matric potentials of −2, −10 and −100 kPa, followed by weighing at each stage. BD was determined after drying (105°C, 24 h). Wilting point values (−1500 kPa) were calculated using pedotransfer functions, developed by Riley (1996) for each soil type. Proportions of pore size classes were estimated based on the water desorption characteristics. The equivalent pore sizes at these potentials are 160, 30, 3 and 0.2 μm, respectively. Pore function was classified as follows: transmission pores >30 μm (macropores), storage pores 0.2–30 μm (mesopores) and residual pores <0.2 µm (micropores) (modified after Greenland (1977)). Air permeability (A_perm) was measured at assumed field capacity (−10 kPa) by measuring air flow at a pressure of 2 kPa, as described in Green and Fordham (1975).

For aggregate size distribution, loose soil samples were taken from the upper 5 cm layer around the same area as the cylinder samples. After air-drying they were sieved over meshes of 2, 6, 10 and 20 mm. The mean weight diameter of aggregates was calculated according to van Bavel (1949), assuming a maximum aggregate size of 35 mm. To assess the stability of aggregates in the 2–6 mm and 6–10 mm size ranges, 40 g subsamples were exposed to 70 passes of simulated rainfall sprayed at 1 bar pressure for 2 minutes on 2 mm sieves, as described by Njøs (1967). Aggregate stability was expressed as the percentage of soil remaining on the sieves.

Statistical analyses

Data for straw residue, grain yield variables and results of soil sampling were analysed with Minitab 15 proc. Balanced ANOVA, using a split-plot model with straw treatment on main plots and tillage on sub-plots. Each trial was analysed separately. Where relevant (i.e. for crop variables, but not for soil variables), year was included as a split-split-plot factor. Error bars in figures show standard errors of the means. Least significant difference_{5 %} values were used to distinguish between significantly different treatments in some cases. Biomass of Poa annua L., grass weeds, dicot weeds and sum of all weeds (present in all fields) were analysed by an overall variance analyses using the SAS proc. ‘glm’ (SAS Institute Inc. 2002–2008) with similar split-plot models. In the overall analyses, trials were treated as replicates, averaged over replicates, but Elymus repens (L.) Gould, only present at two fields, was analysed within each trial, as above.

Results

Straw residue cover

The quantity of residue measured at harvest varied considerably, between both years and trials (Table 4). It was on average somewhat higher for oats than for wheat, but low values were measured in both oat trials in 2011, especially on clay soil, probably due to a high incidence of lodging after heavy rain. Lower amounts of straw cover were measured in autumn after straw removal, but this effect was only significant on loam soil (Table 6). Straw residue cover was in all cases reduced after harrowing and seeding in spring compared to the amounts present after soil tillage in autumn. A comparison between autumn and spring on loam soil showed that the reduction in plant residue during the winter period was mostly due to harrowing and seeding in spring (Figure 2).

Figure 2. Straw coverage (%) on loam soil in Toten in autumn 2009 and spring 2010. Measurements were made after soil tillage in autumn (a), before soil tillage in spring (b) and after seeding (c), n = 2.

Note: Tillage: SAH, shallow autumn harrowing, DAH, deep autumn harrowing, and DAP, deep autumn ploughing. Error bars show standard errors of the means

Table 4. Average quantities of total straw residue (Mg ha⁻¹) recorded at harvest in all trials and trial years.

	Loam soil		Silt soil				Clay soil
	Wheat	s.e.	Wheat	s.e.	Oats	s.e.	Oats	s.e.
2010	3.8	0.09	3.0	0.17	6.3	0.71	3.5	0.2
2011	4.0	0.1	4.8	0.24	2.8	0.23	2.4	0.36
2012	2.1	0.12	6.0	0.39	8.9	0.55	6.2	0.25

The straw cover measured after seeding in spring (Table 5), used as a basis to evaluate Fusarium risk (Seehusen 2014), showed significant differences between years (Loam p < .01, silt and clay p < .001), with the highest amount in 2010 and the lowest in 2012. In these years, oats left more residue on the surface than did wheat, but the opposite effect was seen in 2012. Although there was in all years a tendency towards higher amounts of straw cover following straw retention, the overall effect of straw treatment was significant only in the Toten (loam) and Østfold (clay) trials. Significant interaction effects between soil tillage and straw treatment (not shown) were found at Toten in 2010 (p < .01) and in 2012 (p < .05) and at Solør (oats) in 2011 and Solør (wheat) in 2012 (both p < .05). These interactions suggested that SSH gave higher cover after straw retention than after straw removal, but that the effect was much smaller with the other tillage methods.

As expected, ploughing was the most effective tillage method to bury straw and thereby reduce the amount of plant residue on the soil surface (Table 5). In spring, shallow ploughing (SSP) was more effective in burying plant residues than was shallow harrowing (SSH). In autumn, deep ploughing (DAP) was more effective than deep harrowing (DAH), while the latter was more effective than shallow harrowing (SAH). Comparisons between tillage methods SSH and SAH and between SSP and DAP show that autumn tillage was more effective than spring tillage in burying plant residue.

Soil structure

No overall effect of straw treatment was found on total porosity and pore size distribution, nor did tillage affect total porosity. In the upper layer (3.5–7 cm) tillage had a significant effect (p < .05) on the amount of transmission pores (>30 μm) on clay soil, where autumn ploughing (DAP) created the highest and spring harrowing (SSH) the lowest amount of such pores (Figure 3). In this trial, straw retention tended to increase the amount of these pores (p = .051) in the case of spring harrowing (SSH) and to decrease it in the case of ploughing (both SSP and DAP).

Figure 3. Pore size distribution (%) at two depths for all soils.

Note: Tillage: SSH, shallow spring harrowing, SSP, shallow spring ploughing, and DAP, deep autumn ploughing. Error bars are the standard error for total porosity

In the deeper soil layer (15–19 cm), DAP led to significantly more transmission pores than spring tillage (both SSH and SSP) on both loam soil (p = .001) and silt soil (p < .05). On loam soil an interaction effect between soil tillage and straw treatment was found (p < .05), where spring tillage (both SSH and SSP) in combination with straw retention increased the amount of transmission pores compared to straw removal. There was no difference in the amount of transmission pores at this depth following spring harrowing (SSH) and ploughing (SSP) on silt soil, but spring ploughing (SSP) decreased the amount of transmission pores on loam soil (p = .001). There were no significant effects of tillage on the amount of smaller pores.

In summary, there was a tendency that the amount of transmission pores increased with tillage intensity (DAP > SSP > SSH). Both total porosity and the amount of transmission pores were higher in the upper than in the deeper soil layer, while the reverse was found for the amount of residual pores. The highest amount of transmission pores was found in loam soil, and the lowest in silt soil. In the latter case, and sometimes also on clay soil, the macroporosity was below 10%, which is often quoted as a minimum level for good plant growth.

There was a tendency that BD in the upper soil layer was lower after autumn ploughing (DAP) than after spring harrowing (SSH) (Table 7) with the exception of silt soil (Solør). In most cases BD in the deeper soil layer was higher than that in the upper layer. There was no significant effect of straw treatment on BD. Tillage had a significant effect in the deeper soil layer of loam soil (p < .05) where SSP led to the highest and DAP to the lowest BD. At this depth, tillage also had a significant effect on silt soil (p < .05) where SSH led to the highest and DAP to the lowest BD. In summary, the results show a tendency towards reduced BD with increased tillage intensity at both soil depths (DAP > SSP > SSH).

Straw treatment had no overall significant effects on air permeability. In the deeper soil layer of the loam soil, autumn ploughing (DAP) created higher air permeability (p < .01) than either of the two spring tillage methods (Table 7). Differences were less marked in the other soils, but there was in most cases a tendency towards reduced air permeability after shallow harrowing (SSH) at both depths, compared to ploughing. There was also generally lower air permeability at 15–19 cm than at 3.5–7.5 cm depth.

Although not significant in all cases, the results for the aggregate size distribution (Figure 4) showed the biggest differences between tillage methods in the largest (>20 mm) and smallest (< 2 mm) aggregates (silt: >20 mm p < .05, 2–6 mm p < .05, <2 mm p < .01, loam: 10–20 mm p < .01, <2 mm p ≤   .01). Autumn ploughing (DAP) created the highest amount of small and the lowest amount of large aggregates on all three soils. The opposite effect was found for SSH on loam and silt and for SSP on clay.

Figure 4. Effects of tillage on aggregate size distribution for all soils.

Abbreviations: SSH, shallow spring harrowing; SSP, shallow spring ploughing; DAP, deep autumn ploughing

There were in most cases significant (p < .05) effects of tillage on aggregate stability in all three soils (Figure 5), with higher values after shallow harrowing (SSH) than after autumn ploughing (DAP). Furthermore, DAP gave in most cases lower aggregate stability than spring ploughing (SSP). The aggregate stability was the highest on loam soil and the lowest on silt soil. The differences between spring harrowing (SSH) and spring ploughing (SSP) were less distinctive on clay soil than on the other soils.

Figure 5. Effects of tillage on aggregate stability (2–6 mm, 6–10 mm) in all soils.

Abbreviations: SSP, shallow spring ploughing; DAP, deep autumn ploughing.

Weed infestation

Grass weeds dominated, and dicotyledonous weeds were less abundant (Figure 6). Within each field 9–18 weed species were detected. P. annua dominated in all trials, while E. repens and Alopecurus geniculatus L. also occurred in the trials on silt soil. The latter had greater weed biomass than the other trials (p < .001), and more weeds were observed in 2011 than in 2012 (p < .01).

Figure 6. Main effects of tillage treatments (N = 16), field (soil and crop type, N = 20), and year (N = 40) on biomass of P. annua, other grass weeds and broadleaved (dicot) weeds (% of total biomass, crop + weeds = 100%).

Abbreviations: SSH, shallow spring harrowing, SSP, shallow spring ploughing, SAH, shallow autumn harrowing, DAH, deep autumn harrowing; DAP, deep autumn ploughing.

The amount of weeds increased in general with decreasing tillage intensity, as follows: ploughing (SSP or DAP) < autumn harrowing (SAH or DAH) < spring harrowing (SSH). This was especially apparent for the sums of grass weeds (p < .01), while no significant effects were found in the case of dicotyledonous weeds. For P. annua (Figure 6, p < .01) and E. repens (Figure 7, wheat field: p ≤  .05 and oat field: p < .01) SSH gave significantly more weed biomass than other tillage treatments. Straw management had in general no significant effect on the weeds, but there was a tendency (p = .09) for more weed biomass when straw was removed than when it was retained. On silt soil, most E. repens was found when the straw was removed (Figure 7). In the oat trial on this soil, this tendency was seen in all tillage treatments (p = .09). In the wheat trial there was a significant straw × tillage interaction (p < .05), with most E. repens on unploughed plots (especially SSH and DAH) where straw had been removed.

Figure 7. Effect of tillage and straw management (removed or retained) on % biomass of E. repens in spring wheat (above) and spring oats (below) in Solør. Average over years and replicates (N = 4).

Notes: Error bars indicate + SE. Explanation of tillage is given in Figure 6.

Grain yields

Straw treatment had on average no significant effect on yields. The effects of soil tillage varied greatly between both years and soils. Averaged over all years (Table 8), tillage only had significant effect on clay soil (oats, p < .01), where yields were the highest following SAH and the lowest following SSP (+11% and –2%), respectively, compared to DAP. There was a significant interaction (p < .05) between year and tillage on clay soil (not shown), where autumn harrowing gave the highest yields (SAH in 2010 and 2011 and DAH in 2012) and ploughing the lowest yields (DAP in 2010 and SSP in both 2011 and 2012).

Grain quality

Straw treatment had no significant effect on the protein content, but tillage had significant effects in all trials, except that with oats on silt soil (Table 9). Autumn ploughing gave 0.5–1.6% higher average protein values than other treatments on loam soil and up to 0.5% higher average protein on clay, while autumn tillage gave ca. 0.8% higher average values than spring tillage on silt soil (wheat). Both on loam and silt soils, there were significant interaction effects between tillage and year (not shown). On loam soil, autumn ploughing (DAP) gave the highest protein content in all three years, whereas the effect of shallow harrowing (both SSH and SAH) was more variable. On silt soil, DAH gave highest protein contents in both 2010 and 2011 and SAH the highest value in 2012, while SSH gave the lowest values in both 2011 and 2012.

Straw treatment had a significant effect on the grain moisture content (not shown) on clay soil where the moisture content was reduced slightly by retaining the straw. Tillage only had a significant effect on moisture in oats on silt soil, where SAH gave the highest and SSP the lowest values. On silt (wheat) and clay soil significant interactions between tillage and year were found (not shown). On silt soil shallow harrowing gave the highest moisture content (SSH in 2010 and SAH in 2011 and 2012), while the lowest values were found after spring ploughing (SSP) in 2010 and after SSH in both 2011 and 2012. On clay soil ploughing gave the highest (DAP in 2011 and SSP in 2010 and 2012) and shallow harrowing the lowest moisture contents (SAH 2010 and 2012 and SSH in 2011).

Soil tillage had significant effects on grain test weight on loam and clay soils (not shown). In both trials spring harrowing (SSH) gave the highest values and ploughing the lowest (DAP on loam and SSP on clay soil). Differences due to straw treatment were generally small, but there was a significant interaction between year and straw treatment on clay soil, where removing the straw led to higher values in 2010 and 2012 and lower values in 2011. On loam soil a significant interaction effect between year and tillage was found (p < .001). In 2010 and 2011, shallow harrowing (SAH in 2010 SAH, SSH in 2011) and in 2012 autumn ploughing (DAP) gave the highest values, while autumn ploughing (DAP) in 2010 and 2011 and spring harrowing (SSH) gave the lowest.

Discussion

Differences in the straw cover between years and crops were expected, as the amount of straw is closely related to plant density, fertilizer intensity and climatic conditions. The percentages of plant residue cover found in our trials agree with American field trials, where wheat straw cover ranged from 9% after ploughing to 65% in the case of no-till (Dill-Macky & Jones 2000). Straw cover is difficult to measure under field conditions, as it depends on the height of the stubble. Small differences in adjustment or wear of the combine harvester may lead to variations in stubble height and the amount of straw left on the field. Furthermore, lodging, as occurred on loam soil, causes problems at harvest, influencing the amount of cut straw. Normal minimum cutting height for cereals is about 10 cm, which means that about 75–80% of the straw is cut. An increase in cutting height to 20 cm leads to a decrease of ca. 20% cut straw (Riley et al. 2012). In order to increase the speed and efficiency of harvesting, it is becoming common to cut straw at greater height, which makes it necessary to use a straw cutter after harvesting. This gives more even spreading.

Although straw retention left more plant residue on the soil surface in autumn than straw removal, this was not always reflected in our measurements of straw cover (Tables 5 and 6). This may be partly due to the measuring method, as the transect line passes through standing stubble with only parts of the plant material affecting the measurement, as shown by Morrison et al. (1993). Under practical conditions, even distribution of straw after harvesting may be difficult to obtain, leaving some plant material unspread in clumps. These are underestimated by the transect line method, since only the material at the top of the pile affects the measurement. Furthermore, as only knots covering plant material are counted as cover, independent of that between the knots, this may also cause underestimation. This may explain why, in some cases (as shown for 2011 in Table 6), DAH appeared to leave more residue on the surface than did SAH. A better technique to measure standing stubble and clumped straw may in future be photo-imaging (Morrison et al. 1989).

Table 5. Straw cover (%) in all trials measured after seeding in spring in all three years tillage method: SSH, shallow spring harrowing, SSP, shallow spring ploughing, SAH, shallow autumn harrowing, DAH, deep autumn harrowing and DAP, deep autumn ploughing.

	Spring wheat		Spring oats		Mean
	Loam	Silt	Silt	Clay
Straw treatment
Removed	8.0	10.0	12.0	19.9	9.9
Retained	10.7	13.3	13.7	16.6	13.6
P-level (straw)	<.05	ns	ns	<.05
Tillage method
SSH	18.8	26.8	31.2	21.7	24.6
SSP	1.4	2.3	2.3	2.2	2.1
SAH	14.2	11.6	15.7	23.8	16.3
DAH	10.4	16.8	12.3	17.2	14.2
DAP	1.3	1.3	2.3	1.2	1.5
P-level (tillage)	<.001	<.001	<.001	<.001
Tillage x Straw	ns	ns	ns	Ns
Year
2010	10.2	17.9	19.4	19.1	16.4
2011	7.4	11.0	12.3	18.2	12.2
2012	10.1	6.4	6.7	5.9	7.2
P-level (year)	<.01	<.001	<.001	<.001
Year x Straw	<0.05	ns	ns	<0.05
Year x Tillage	<0.01	<0.001	<0.001	<0.001
Three-way interaction	<0.05	ns	ns	<0.05

Table 6. Straw coverage (%) measured in autumn 2010 and 2011 on all trials.

			Spring tillage					Autumn tillage						Straw treatment	Soil tillage	Interaction tillage/straw
			Straw	SSH		SSP		SAH		DAH		DAP
2010	Loam soil	Wheat	s	45	a	/		25	ab	22	bc	2	cd	P < .01	P < .01	P < .05
			st	79		/		37		27		1
	Silt soil	Wheat	s	38	a	/		12	ab	12	bc	0	cb	n.s.	P < .05	n.s.
			st	70		/		21		18		1
	Silt soil	Oats	s	32	a	/		19	abc	12	bc	1	bc	n.s.	P < .001	P < .01
			st	69		/		22		20		2
	Clay soil	Oats	s	39	a	/		29	a	23	b	0	B	n.s.	P < .01	n.s.
			st	44		/		42		21		1
2011	Loam soil	Wheat	s	20	a	17	a	19	a	10	b	0	B	P < .01	P < .001	n.s.
			st	52		40		34		14		1
	Silt soil	Wheat	s	27	a	23	ab	10	ab	15	ab	1	bc	n.s.	P < .01	n.s.
			st	47		45		14		18		4
	Silt soil	Oats	s	32	ab	33	a	12	ab	11	bc	1	bc	n.s.	P < .01	n.s.
			st	57		51		20		19		4
	Clay soil	Oats	s	32	a	40	ab	44	b	20	abc	3	abc	n.s.	P < .01	n.s.
			st	57		73		36		57		43

Note: Different letters within rows indicate significant differences between tillage treatments.

Unspread plant material may create problems during soil tillage, leading to uneven incorporation into the soil. Other authors (Riley 1983; Riley et al. 1994; Børresen 1999) showed that the presence of straw residue on the soil surface may cause poor germination due to mechanical problems as well as the toxins they may produce (Harper 1985). Germination and plant development may also be delayed due to lower soil temperature beneath straw cover (Børresen & Njøs 1990). The comparison between residue cover after soil tillage in autumn and before tillage in spring (Figure 2), illustrates that only minimal decomposition had occurred during autumn and winter. Pereyra et al. (2004) also showed that decomposition of plant residue during the winter months is minimal in northern regions, compared to that during the growing period. Stubble height, cutting length and even spreading of straw residue are crucial to avoid problems with soil tillage and impediment of plant growth and to ensure that all residue has contact with soil in order to start decomposition as soon as possible (Parr & Papendick 1978). Undecomposed plant residue, especially that which is recalcitrant to decomposition (stem nodes, standing stubble), may be a long-term inoculum source (Champeil et al. 2004; Pereyra et al. 2004; Pereyra & Dill-Macky 2008).

It is well known that ploughing is the most effective means to reduce surface residue cover, as it leaves soil bare (Njøs & Børresen 1991; Bateman et al. 1998; Guerif et al. 2001; Pereyra & Dill-Macky 2008). Reduced soil tillage, as recommended to reduce the risk of soil erosion especially during the winter, requires a coverage of more than 30% plant residue (Frielinghaus et al. 2002; Riley et al. 2009). Our results (Table 6) suggest that such a cover is most likely if soil is left untilled in autumn and straw is retained. Interest in the value of straw for production of bioenergy has recently increased in Norway, which may cause a conflict between retaining and removing the straw (Riley et al. 2012). Although the removal of straw may reduce cover to values lower than 30%, reduced soil tillage may nevertheless protect against erosion, both because it increases soil stability, as we found, and because standing stubble can itself be highly effective in terms of reducing erosion (Morrison et al. 1993).

Although straw was partly incorporated by soil tillage, we found no significant effects of straw treatment on soil parameters. This agrees with findings of Børresen (1999), who showed that straw treatment had no impact on soil BD, total porosity and pore size distribution within the first five years of his study.

In our trials, tillage had, as expected, significant effects on soil parameters in some cases (Table 7). Earlier studies show that changes take place relatively rapidly after the introduction of reduced tillage (Riley & Ekeberg 1998; Børresen 1999). Increased BD with reduced tillage has been reported by many authors (Dexter 1997; Gomez et al. 1999; Wiermann & Horn 2000). This may give higher bearing capacity and robustness against compaction (Seehusen et al. 2014), but it may also impede root growth (Dexter 1997), especially on sandy soils (Riley et al. 2005). This may be a major objection to direct drilling (Håkansson 2005) and is of growing concern in Denmark (Hansen et al. 2007). This was probably not the case in our study, since BD did not exceed its assumed threshold value for plant growth of 1.5–1.6 g cm⁻³ (Entrup & Oehmichen 2006).

Table 7. Effects of tillage on BD g/cm⁻³ and air permeability μm² at two depths in all soils.

	Depth cm	SSH		SSP		DAP
BD
Loam	3.5–7.5	1.37	A	1.28	a	1.25	a
	15–19	1.34	Ab	1.42	a	1.25	b
Silt	3.5–7.5	1.25	A	1.28	a	1.27	a
	15–19	1.42	A	1.33	b	1.30	b
Clay	3.5–7.5	1.28	A	1.21	a	1.14	a
	15–19	1.37	A	1.32	a	1.31	a
Air permeability
Loam	3.5–7.5	23.0	A	26.9	a	26.0	a
	15–19	18.7	a	19.9	a	28.7	b
Silt	3.5–7.5	20.7	a	20.7	a	20.5	a
	15–19	14.5	a	16.3	a	17.1	a
Clay	3.5–7.5	22.4	a	26.9	a	29.6	a
	15–19	22.6	a	25.3	a	25.0	a

Abbreviations: SSH, shallow spring harrowing; SSP, shallow spring ploughing; DAP, deep autumn ploughing.

Note: Different letters within rows indicate significant differences between tillage treatments.

Our data (Figure 3) show that increasing tillage intensity led to a higher amount of transmission pores (>30 μm) in the upper soil layer, especially on loam and clay soils. In the deeper soil layer the same effect was found at all three locations. In the case of spring ploughing, decreases in porosity and transmission pores compared to the other two tillage methods were found on loam and clay soils, as reflected in an increase in BD at this depth. This suggests the formation of a plough pan beneath the working depth of 15 cm, as also shown by Riley and Ekeberg (1998). Our findings do not agree with those of some other authors (Wiermann et al. 2000; Pagliai et al. 2004; Raper 2005), who found a higher amount of macropores and better pore continuity on reduced tilled soil. There may be several reasons for these conflicting results. Changes in macroporosity and pore continuity may take approximately 5–6 years to establish (Wiermann et al. 2000), while soil samples in our trial were taken after just three years of reduced soil tillage. Besides, the sampling depth in our study was limited to 19 cm, whereas with the above studies included deeper sampling.

The lower porosity and higher BD of the reduced tilled plots on both loam and clay soils led to decreased air permeability on harrowed plots compared to ploughing, especially in the upper soil layer (Table 7). Our values for k_sat calculated from air permeability (not shown) are much higher than those reported from loam soil by Riley (2014) and should not be limiting for infiltration in most situations in southeast Norway, where rainfall intensity seldom exceeds 10 mm h⁻¹ (Manen et al. 2011).

Data for both loam and silt soils show that the more intensive the soil tillage, the higher was the proportion of amount of small aggregates (<2 mm) and the lower was that of larger aggregates (>20 mm) (Figure 4). This is not in agreement with Riley et al. (2008), who showed reduced soil tillage to give a greater amount of small aggregates than annual ploughing. A high amount of small aggregates is expected to be positive for optimal plant emergence. Håkansson et al. (2002) showed that a good seedbed contains >50% of small aggregates (<5 mm). In our case, this was achieved on loam and silt soils, independent of soil tillage, while on clay soil neither of the spring tillage methods did so. On the other hand, a high amount of small aggregates, as we found with autumn ploughing, may be negative because it is these that are most likely to erode if exposed to rain and wind. Stability of the aggregate size 2–6 mm, which normally dominates in the seedbed, is of special importance, because unstable, small aggregates may cause surface sealing, which might decrease infiltration and impede plant establishment. Increased stability with reduced tillage has in Norway previously been found by Marti (1984), Børresen (1993), Børresen and Njøs (1993), Riley et al. (2008) and Riley (2014). Reduced tillage gave the highest stability in all our trials, and our results also show that ploughing in spring improved stability relative to ploughing in autumn (Figure 5).

Despite herbicide use, weed infestation increased in our trials with less intensive tillage (Figure 6). It is well known that ploughless tillage leads to an increase in the amount of grass weeds, as well as perennial and biennial weeds (Froud-Williams et al. 1983; Ekeberg et al. 1985; Tørresen et al. 2003). Weed infestation increases competition with cultural plants and the possibility for survival of pathogens (Jenkinson & Parry 1994; Parry et al. 1995). Consequently, reduced soil tillage is commonly based on the use of herbicides, especially glyphosate (Nail et al. 2007; Yamada et al. 2009; Tørresen et al. 2014). However, intensive use of glyphosate may have negative consequences for human and plant health (Heu et al. 2012; Koller et al. 2012), (Fernandez et al. 2009; Johal & Huber 2009; Yamada et al. 2009). Possible future restrictions on its use may decrease the viability of reduced soil tillage.

In our study the dicotyledonous weeds were well controlled, while the control of grass weeds like P. annua was poor in all trials. The use of glyphosate controlled P. annua in autumn, but its germination from seeds in spring was not controlled by the post-emergence herbicides used. The reason for the large infestation of E. repens on silt soil was that its control by glyphosate was poor due to late harvesting. This gave too little time to produce enough biomass before treatment. This problem has been observed previously at the same location, while in areas with earlier harvest, the control of E. repens by glyphosate is good (Tørresen et al. 2003). In Denmark and Finland the infestation of E. repens has decreased with ploughless tillage, due to long-term glyphosate use (Andreasen & Stryhn 2008; Salonen et al. 2013). In our case, the increase in E. repens on plots with straw removal may have been due to less shading (Figure 7). On the other hand, the presence of straw may hinder contact between glyphosate and weeds.

The average cereal yields in Norway in the trial period (2010–2012) were 3.5 Mg ha⁻¹ for oats (32% lower than our results) and 4.18 Mg ha⁻¹ for wheat (22% higher than in our results) (SSB 2014). Our results show that straw treatment hardly affected yields (Table 8). By contrast, Riley et al. (2009) reported yield declines on a clay soil when straw was retained, especially with direct drilling, while Børresen (1999) found higher yields after straw retention in some cases. Riley et al. (2012) suggested that straw residues have been one of the most limiting factors for reduced soil tillage in Norway. Reduced tillage appears to decrease yields most in years with high rainfall in May and June (Børresen 1999; Riley et al. 2009). Although rainfall was above average in nearly all cases during our trial period (Table 2), straw residue on the soil surface did not necessarily decrease yields. Studies by Børresen and Riley (2003) show that reduced tillage systems have proved to be generally successful on well-drained loam and clay soils under relatively dry conditions in southeast Norway, but may be more problematic under wetter conditions, especially on silty and sandy soils. This generally agrees with our results, as there was no evidence that ploughing was superior to reduced tillage on either loam or clay soil (Table 8). On clay soil ploughing gave the lowest average yields. This agrees with the findings of Riley et al. (2009), who reported that reduced tillage gave a 6% yield increase in spring oats relative to ploughing on such a clay soil. The low yield on the ploughed plots on clay soil in 2011 may be explained, however, by the high degree of lodging on these plots, possibly due to their having better initial growing conditions. On loam soil, both spring ploughing and DAH gave slightly higher yields than autumn ploughing. Riley (2014) found little yield difference between ploughing and reduced tillage on comparable loam soil.

Table 8. Mean effects of straw treatment, tillage method and year on grain yields (Mg ha⁻¹).

	Spring wheat		Spring Oats		Mean
	Loam	Silt	Silt	Clay
Straw treatment
Removed	3.6	3.0	3. 8	5.4	3.9
Retained	3.6	2.9	3.9	5.4	3.9
P-level (straw)	ns	ns	ns	Ns
Tillage method
SSH	3.6	2.8	3.3	5.4	3.8
SSP	3.6	3.0	4.0	5.1	3.9
SAH	3.5	3.0	3.8	5.7	4.0
DAH	3.7	2.9	4.0	5.5	4.0
DAP	3.6	3.0	4.1	5.3	4.0
P-level (tillage)	ns	ns	ns	<.01
Tillage x Straw	0.093	ns	ns	Ns
Year
2010	3.5	3.7	4.1	6.1	4.3
2011	3.3	2.4	3.3	4.5	3.4
2012	3.9	2.8	4.1	5.6	4.1
P-level (year)	<.001	<.001	<.001	<.001
Year x Straw	ns	ns	ns	Ns
Year x Tillage	ns	ns	ns	<0.05
Three-way interaction	ns	ns	ns	Ns

Note: Tillage methods as in Table 5.

On silt soil harrowing in spring gave approximately 20% (oats) and 8% (wheat) lower yields than autumn ploughing, probably due in part to more weediness. Of the two spring tillage methods, yields on silt soil were higher after spring ploughing than after SSH (oats + 19%, wheat + 5). Our findings thus confirm that the success of reduced tillage depends not only on the climatic conditions but also on the soil type, as shown by Riley et al. (1994) and Børresen (1999).

Grain quality parameters are strongly dependent upon management (tillage, time of seeding, time, amount and type of fertilizer) and climatic factors, as shown for protein by Johnson and Mattern (1987). Our data show that the quality parameters such as grain moisture and test weight were generally little affected by soil tillage (not shown), as reflected also in other trials (Riley 2014). The grain moisture content was only significantly increased by straw removal on clay soil over the three-year period. On average straw retention on silt soil led to slightly higher protein content than straw removal, while the opposite effect was found on both loam and silt soils and was in all cases absent on autumn-ploughed plots. The protein content in oats on clay soil showed the same trend of higher protein content following autumn ploughing than after spring harrowing as that found at this location by Riley et al. (2009). In general, our data illustrate that tillage only in spring led to lower protein content relative to autumn ploughing (Table 9).

Table 9. Mean effects of straw treatment, tillage method and year on protein content (%).

	Spring wheat		Spring oats		Mean
	Loam	Silt	Silt	Clay
Straw treatment
Removed	11.7	13.0	10.7	11.3	11.7
Retained	11.5	13.3	11.0	10.9	11.7
P-level (straw)	ns	ns	.086	Ns
Tillage method
SSH	11.1	12.6	10.7	10.9	11.3
SSP	11.9	12.8	10.6	11.2	11.6
SAH	10.8	13.4	11.0	10.9	11.5
DAH	11.9	13.6	10.9	11.1	11.9
DAP	12.4	13.4	10.9	11.4	12.0
P-level (tillage)	<.05	<.05	ns	<.05
Tillage x Straw	ns	ns	ns	Ns
Year
2010	11.6	12.3	10.9	11.9	11.7
2011	11.8	13.6	10.9	11.2	11.9
2012	11.4	13.5	10.7	10.2	11.5
P-level (year)	<.01	<.001	ns	<.001
Year x Straw	ns	ns	ns	Ns
Year x Tillage	<.05	<.05	ns	Ns
Three-way interaction	ns	ns	ns	Ns

Note: Tillage methods as in Table 5.

In conclusion, although it was wetter than the 30-year normal values in all our trial seasons, reduced tillage had no overall negative yield effect on either loam or clay soils. Reduced tillage, especially when the straw was retained, gave good protection against erosion by providing a residue cover throughout the winter period. At the same time, significant reductions in fuel consumption (Saraukis et al. 2012) and CO₂ emissions (Koga et al. 2003) might be expected, compared to ploughing. Reduced soil tillage also reduces labour requirements (Filipović et al. 2004). The time saved could be used to perform other work (Saraukis et al. 2012) or allow postponement of tilling until the soil moisture content is suitable. Our results showed only a few positive effects of reduced tillage on soil structure, maybe due to the shortness of the trial period.

An absolute prerequisite for the success of reduced tillage is the quality of the straw management, seeding and especially weed control. Our data confirm that weeds may be a major problem when soil is not ploughed. In cases where ploughing is necessary, SSP may be a good alternative to autumn ploughing because it often gives comparable yields (Riley & Ekeberg 1998). Our data suggest that spring ploughing, even with reduced working depth, is almost as effective as autumn ploughing in burying plant residue, while it gives better protection against erosion in late autumn and early spring. It therefore results in less need for spraying than reduced tillage (Tørresen et al. 2003).

Spring ploughing requires a higher amount of labour in spring, thus competing for time in a busy season. It may also be more demanding in relation to timeliness due to higher water content in untilled soil in spring, which may lead to a delay in seeding. If spring tillage is performed before the soil is dry enough, this also increases the risk of severe subsoil compaction (Seehusen et al. 2014), whereas if the soil becomes too dry it may result in a cloddy seedbed. Even on silt soil, where reduced tillage gave yield reductions in our trials, spring ploughing gave nearly as good a result as autumn ploughing.

Especially against the background of climate change, with higher temperatures in autumn (Hansen-Bauer et al. 2015) further research should be done on the effect of alternative straw treatment in autumn.

Acknowledgements

We thank the extension service groups at Østfold and Solør for conducting the experimental treatments on their research farms, as well as the technical staff at the research station at Apelsvoll.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Dr. Ingerd Skow Hofgaard has worked at Norwegian Institute of Bioeconomy Research (NIBIO) for 19 years. In 2003 she accomplished her PhD in Plant Pathology at NIBIO and the Norwegian University of Life Sciences (NMBU). Her main research area is fungal diseases in grasses and cereals, mainly focusing on Fusarium and Microdochium spp. Since 2012 Dr. Hofgaard has coordinated all activities within research on Fusarium and mycotoxins in NIBIO. She is involved in several projects focusing on Fusarium and mycotoxins in Norwegian cereals, including influence of agricultural practices on development of Fusarium spp, disease resistance, and disease forecasting. Dr. Hofgaard is the associate chair of the Department of Fungal Plant Pathology at NIBIO, and teaches graduate and undergraduate students in plant pathology at NMBU. Dr. Hofgaard has served at the scientific committee for the Nordic Baltic Fusarium seminars since 2010.

Dr. Hugh Riley has worked at Norwegian Institute of Bioeconomy Research (NIBIO) for 39 years. He achieved his PhD in Soil Science at Aberdeen University (Scotland) in 1977. His area of expertise and research has covered a wide variety of topics within applied soil science and agronomy, including the irrigation and fertilization of arable crops and field vegetables, as well as many years research into the effects of reduced tillage on crop yields and soil structure. He has been project manager of many national projects and has taken part in several international projects, also serving on the editorial board of three scientific journals. He has taught courses in Soil Physics and Tillage at undergraduate level and has supervised numerous postgraduate students within his field of expertise.

Dr. Kirsten Semb Tørresen has worked at Norwegian Institute of Bioeconomy Research (NIBIO) for 27 years. She achieved her PhD in Weed Science at NIBIO and the Norwegian University of Life Sciences (NMBU) in 1995. Her area of expertise and research is weed biology and integrated weed management in arable crops and in grasslands. This includes effect of climate change and conservation tillage on weeds and efficacy evaluation of herbicides. She has been and is project manager of many national projects on integrated pest management. She has served at the European and Mediterranean Plant Protection Organization (EPPO) panel on Efficacy Evaluation of Herbicides and Plant Growth Regulators since 2003 and at the scientific committee of European Weed Research Society (EWRS) since 2013.

Dr. Till Seehusen has worked at the Norwegian Institute of Bioeconomy Research (NIBIO) since 2009. He achieved his PhD in Soil Science at NIBIO and the Norwegian University of Life Sciences (NMBU) in 2014. His area of expertise and research is general cereal production (cereals), soil science, soil structure, soil compaction. This includes work on questions related to yield gap and the efficient use of farm machinery. Within his field of expertise, Till Seehusen is involved in several national and international projects and active with within teaching and publishing.

Additional information

Funding

This study was financed by The Foundation for Research Levy on Agricultural Products/Agricultural Agreement Research Fund/Research Council of Norway [research grant 199412/E50], Animalia, Bayer Crop Science, Braskereidfoss kornsilo, Felleskjøpet Agri, Felleskjøpet Rogaland og Agder, Fiskå Mølle, Flisa Mølle og Kornsilo, Graminor, Lantmännen Cerealia, Norgesfôr, Norgesmøllene and Norkorn.