Biomass production, weed suppression, nitrogen and phosphorus uptake in white oat (Avena sativa L.) and grazing vetch (Vicia dasycarpa L.) cover crop bicultures under an irrigated no-till system

Abstract

Cultivation of a multipurpose cover crop is of interest to Eastern Cape farmers experiencing soil infertility and weed pressures. The objective of the study was to investigate effects of oat–vetch bicultures on biomass production, weed suppression, and nitrogen (N) and phosphorus (P) uptake. The study was conducted between 2009 and 2010. Treatments included 90% oat + 10% vetch, 70% oat + 30% vetch, 50% oat + 50% vetch, 100% vetch, 100% oat and a weedy fallow as control. Bicultured cover crops had significantly (p < 0.05) higher biomass compared to sole vetch in both seasons but reduced biomass compared to sole oat only in 2010. Weed suppression increased with an increase in biomass. Weedy fallow had the least weed suppression (p < 0.05) at all sampling times, whereas sole oat provided the best early weed control compared to the rest of treatments. Nitrogen uptake by sole vetch was comparable to 50% oat + 50% vetch and 70% oat + 30% vetch. Phosphorus uptake differed only in the 2010 season and 90% oat + 10% vetch, 70% oat + 30% vetch and sole vetch had the best uptake. Weedy fallow had the least N and P uptake during the two seasons. Bicultures can be used for improved biomass, weed suppression, and N and P uptake with potential of alleviating soil degradation effects in the Eastern Cape province.

Keywords:

• Conservation agriculture
• cover crop mixtures
• no-till farming
• soil cover
• weed species count

Introduction

The Eastern Cape (EC) is one of South Africa's provinces worst affected by soil degradation (Fox and Rowntree 2001, Fatunbi and Dube 2008). Poor agricultural practices leave the soil without cover, promoting erosion and loss of nutrient-rich soil particles leading to reduced soil productivity (Laker 2004). In order to increase and stabilise soil productivity, control of soil erosion is essential. One key principle of conservation agriculture (CA) is permanent soil cover, a potential solution to the problem of soil degradation observed in the EC. The proponents of CA suggest the use of cover crops, grown in rotation or intercropped with the main crop, as a way of providing permanent soil cover, increasing aggregate stability and eventually reducing erosion.

Cover crops provide several other advantages besides reduction of soil erosion. Some that are cited in the literature include weed suppression, mining of leached nutrients, improved soil organic matter, nitrogen (N) fixation and general improvement in soil fertility (Derpsch 2008). However, no single cover crop species can achieve all these benefits on its own. Production of two cover crop species together, usually a legume and a grass, known as biculturing, offers an option that can deliver diverse benefits compared to their monocultures (Sainju et al. 2005, Dabney et al. 2010).

Differences have been noted among cover-crop species regarding their growth rate, amount of biomass accumulation, weed suppression and nutrient uptake. Grasses germinate earlier and develop root systems at a faster rate than legumes (Ranells and Wagger 1996), hence may have more effective early season weed control. They also potentially contribute to increases in soil organic matter by supplying higher levels of carbon (C) (Odhiambo and Bomke 2001, Lithourgidis et al. 2006). However, grasses have been observed to provide little N for growth of the follow-on crop and are less economical as they require large amounts of N fertiliser to attain acceptable biomass compared to legumes (Murungu et al. 2010).

Conversely, legume species biologically fix N, increasing soil N levels and resulting in yield increases for subsequent crops when N is the limiting factor (Kuo et al. 1996, Crandall et al. 2005). In temperate and tropical environments, vetch (Vicia sativa L.) can accumulate 150–250 kg N ha⁻¹, replacing about two-thirds of N required for maize (Zea mays L.) production (Crandall et al. 2005). Although both grazing vetch and white oat (Avena sativa L.) can symbiotically interact with arbuscular mycorrhizal fungi, increasing phosphorus (P) uptake, legumes decompose faster, because of lower C:N ratios than grasses leading to improved availability of P for the follow-on crop (Xin et al. 2005). Hence, legume cover crops have the potential to improve yields of follow-on crops in the EC where low levels of N and P are a major constraint to improved crop productivity, particularly maize, the staple crop (Mandiringana et al. 2005, Fanadzo et al. 2009).

Screening cover crops for high biomass yield under conditions of the EC has shown that vetch and white oat are the most promising species under irrigation (Murungu et al. 2010). Sole crops of these two species do not adequately address high weed infestation and low soil fertility, both of which are challenges in the production of crops on many smallholder farms in the EC. Previous research has shown high biomass yields with optimum nutrient uptake from cover crop bicultures as a result of their efficient light utilisation (Clark et al. 2007) and transfer of symbiotically fixed N to grasses. However, the beneficial effects of cover crop bicultures may vary with the species ratios in the mixture.

Varying species ratios have been recommended in cover crop bicultures for high biomass and nutrient uptake, including 50% rye (Secale cereale) and 68% vetch (Clark et al. 1994), 66% rye + 33% vetch (Clark et al. 1997), 90% triticale (×Triticosecale) + 10% vetch (Sebahattin et al. 2004) and 70% oat + 30% vetch (Lithourgidis et al. 2006). This variation in seeding ratios could have been because of the species of grass and legume in the biculture as well as the different climatic conditions and soil characteristics in the respective study areas. Hence, this study was carried out to determine the effects of oat–vetch cover crop bicultures on biomass production, weed suppression and uptake of the most limiting nutrients, N and P, under the local conditions of the central region of Eastern Cape.

Materials and methods

Experimental site

The study was carried out at the University of Fort Hare Research Farm (32°47′ S, 27°50′ E). The farm is at an average altitude of 508 m and has a moderate climate with an average annual rainfall of about 575 mm and an annual mean temperature of 18 °C. The soil is of the Ritchie family of the Oakleaf form (Soil Classification Working Group 1991) and a Eutric Cambisol according to the World Reference Base for Soil Resources (WRB) system (IUSS Working Group WRB 2006). The soil at the initiation of the experiment had a pH (H₂O) of 5.9, a saturated paste electrical conductivity of 0.14 d Sm⁻¹, total C and N was 1.1% and 0.087%, respectively, and inorganic P (Bray 1) was 2.01 mg kg⁻¹. The soil type is sandy loam with 64.2% sand, 16.0% silt and 19.8% clay (Mandiringana et al. 2005).

Treatments and experimental design

Grazing vetch (Vicia dasycarpa cv. Max) and white oat (A. sativa cv. Pallinup) cover crops were grown as bicultures at three ratios and as monocultures in winter seasons of 2009 and 2010. Treatments were 90% oat 10% vetch, 70% oat + 30% vetch, 50% oat + 50% vetch, 100% oat and 100% vetch (where% refers to the percentage of the recommended seed rate used in the monoculture). The recommended rates were considered to be 100 and 50 kg ha⁻¹ for white oat and vetch, respectively (Murungu et al. 2010). A weedy fallow treatment was included as control. The experiment was laid out in a randomized complete block design with three replicates.

Agronomic practices

Land was ploughed, disked and rotovated to make a fine tilth before the initial cover crop establishment using a tractor-drawn plough and a harrow. At the second establishment of cover crops, no ploughing was done in line with the principles of CA. Cover crops were seeded on 30 April 2009 and 20 May 2010 in the first and second seasons, respectively. Weeds were controlled by spraying with glyphosate (360 g l⁻¹) at a rate of 3 l ha⁻¹ three days before cover crop planting. Cover crops were drilled into small furrows spaced 30 cm apart in 5.4 m × 6 m plots. Grazing vetch was inoculated with Rhizobium leguminosarium bv. viciae inoculant having 5 × 108 rhizobial cells g⁻¹ (Stimuplant CC, Zwavelpoort, South Africa) at planting. Only basal fertiliser was applied to all the cover crop treatments at planting at the rate of 200 kg ha⁻¹ of compound fertiliser with an N:P:K ratio of 2:3:4 (30) + 0.5% Zn, which supplied 13.33 kg N ha⁻¹, 20 kg P ha⁻¹ and 26.66 kg K ha⁻¹. Supplementary overhead irrigation water was applied to all treatments as summarised in Table 1, based on Class A evaporation pan readings. Neither weed nor pest control was done during the growth of the cover crop. Cover crop growth was terminated at the early flowering stage by tractor rolling and applying glyphosate herbicide on 5 September 2009 and 30 September 2010 in the first and second seasons, respectively.

Data collection

Two randomly thrown 0.35 m × 0.35 m quadrats were used for cover crop and weeds destructive sampling per plot. Cover crop and weed biomass sampling was done at 33, 47, 61, 77, 105 and 128 days after sowing (DAS) in 2009 and at 59, 84, 101, 117 and 132 DAS in 2010. All cover crop and weed biomass present within the quadrat were cut at ground level and separated into cover crop and weeds. Weed species counts were determined at cover crop termination using the same quadrats as above and weeds were identified following the guidelines by Bromilow (1995). The cover crop and weed samples were then oven-dried at 65 °C to constant mass for dry matter determination.

Cover crop and weed samples harvested at termination were ground to pass through a 1 mm sieve, and C and N content (%) were determined by dry combustion using a LECO Tru-Spec C/N analyser (LECO Corporation, St Joseph, Michigan, USA). Total P content was determined after wet digestion with H₂SO₄ and the concentration of P in the digesting solution was measured using antimony potassium tartrate solution as outlined by Okalebo et al. (2002). The C, N and P uptake by the cover crops was determined by multiplying the C, N and P contents by the cover crop dry matter.

Data analysis

Analysis of variance (ANOVA) was performed using the GenStat statistical package release 12.1 (Lawes Agricultural Trust 2009) on all cover crop, weed biomass and plant nutrient data. Data on weed species numbers were transformed using log (count + 0.5) to normalise the data before subjecting it to ANOVA. Cover crop growth rates (CCGRs) for bicultures was calculated using the combined dry matter of the two species in the mixture. The CCGRs were analysed by comparing the plotted slopes of cover crop dry matter accumulation against time (Fageria et al. 2006). This method was used because of the limited number of sampling points, which made it difficult to plot a sigmoid curve to analyse growth. To determine differences in CCGRs, methods described by Gomez and Gomez (1984) were used to test homogeneity of the regression coefficients for plots of dry matter accumulation against time. Treatment means were separated using the least significance difference (LSD) at 5% probability level. Where transformation was not required, means and LSD values are presented and where transformation was required, back-transformed means are shown, without presentation of the LSD values, as it is not appropriate (Gomez and Gomez 1984). Contrast analyses were performed to find the effectiveness of biculturing as a technology (i.e. the combined effect of all bicultured treatments) against either sole oat or vetch with regards to dry matter production. Correlation analyses were done to determine the relationship between cover crop and weed dry matter as well as the cover crop dry matter and nutrient uptake figures. In the 2010 season, stray cattle accidentally grazed the sole oat plots at 30 DAS and the dry matter data presented were not corrected for the grazing effect, as the other treatments were not affected. Comparisons of the other treatments with sole oat were based on the 2009 data only.

Results

Rainfall and irrigation

The temperature during the cover crop growth periods in the two seasons was comparable to the 30-year mean temperatures (Table 1). The cover crops received a total of 249 mm and 242 mm in both rain and irrigation in the 2009 and 2010 seasons, respectively

Table 1 : Mean monthly temperatures, rainfall and irrigation at the University of Fort Hare Research Farm from May to September in the 2009 and 2010 seasons

	Temperature (°C)				Rainfall (mm)				Irrigation (mm)
Month	2009	2010	30-year mean	CV (%)	2009	2010	30-year mean	CV (%)	2009	2010
May	16.3	17.2	15.73	6.2	14.9	5.6	20.7	114	20	15
June	13.6	13.2	12.99	6.2	20.5	48.3	21.4	98	40	30
July	13.5	14.23	12.97	8	53	16.8	17.8	105	15	45
August	15	16.17	14.31	7..3	15	12	32.1	109	45	50
September	16	17.49	16.05	5.8	25.3	9.1	34.2	95	0	10
Total					129	92			120	150

Cover crop dry matter accumulation

There was a significant interaction (p < 0.001) between season and cover crop treatment with respect to CCGRs (Table 2). In 2009, there were no differences in CCGRs among treatments, but higher CCGRs were observed in the bicultured treatments compared to the sole crops in 2010. In the 2010 season, 70% oat + 30% vetch treatment had the highest CCGR and differed significantly to that of the 50% oat + 50% vetch treatment but was the same as the 90% oat + 10% vetch treatment. Sole vetch and oat achieved comparable CCGRs.

Table 2 : Effect of cover crop treatments and season on cover crop growth rates (CCGRs). Means followed by different letters in the same column differ significantly at p < 0.05

	2009 season		2010 season
Treatment	Slope	R ^2	Slope	R ^2
100% oat	68.5	0.96	55.4^c	0.93
90% oat + 10% vetch	65.0	0.96	84.3^ab	0.97
70% oat + 30% vetch	64.1	0.95	93.7^a	0.95
50% oat + 50% vetch	65.9	0.91	76.3^b	0.94
100% vetch	70.9	0.92	61.6^c	0.96
LSD0.05 (Interaction)	14.07
LSD0.05 (Season)	6.29
LSD0.05 (Treatment)^1	9.95
^1 Treatment LSD0.05 used for the mean separation of the CCGRs

The season × cover crop treatment interaction was significant (p < 0.001) with respect to cover crop dry matter measured at termination (Figure 1). Cover crop treatment main effect was significant (p < 0.01), whereas season main effect was not (p > 0.05). In 2009, cover crop treatments did not differ in the dry matter achieved. However, in 2010 season, the 90% oat + 10% vetch and 70% oat + 30% vetch treatments had the highest dry matter that differed significantly with the rest of the treatments. The 50% oat + 50% vetch and the 100% vetch treatments had comparable and intermediate dry matter yields that were higher than 100% oat (which had been grazed). Contrast analyses showed that biculturing technology significantly improved cover crop dry matter compared to sole vetch only in the 2010 (p < 0.001) season.

Figure 1:  Effect of cover crop treatments on final cover crop dry matter at termination in the 2009 and 2010 winter seasons

Weed dry matter and species counts

The season × cover crop treatment interaction was not significant (p > 0.05) with respect to weed dry matter at cover crop termination. However, both cover crop treatment and season main effects were significant (p < 0.001). In 2009, the weedy fallow plots (control) consistently gave higher weed dry matter at all sampling dates than the cover crop treatments, which were comparable at 47, 61 and 77 DAS (Table 3). However, the cover crop treatments only differed at 33, 105 and 128 DAS, with the sole oat consistently having the least weed dry matter, whereas bicultures and sole vetch were comparable. In the 2010 season, plots under cover crops had comparable weed dry matter at 117 and 132 DAS and were only different at 59, 84 and 101 DAS (Table 4).

Table 3 : Effect of cover crop treatments on weed dry matter (kg DM ha⁻¹) at various sampling times during the 2009 winter season. Means followed by different letters in the same column differ significantly at p < 0.05

Cover crop treatment	33 DAS	47 DAS	61 DAS	77 DAS	105 DAS	128 DAS
100% oat	98^c	321^b	682^b	810^b	917^c	961^c
90% oat + 10% vetch	150^bc	340^b	769^b	819^b	1 093^b	1 109^bc
70% oat + 30% vetch	176^bc	360^b	735^b	782^b	1 056^b	1 195^bc
50% oat + 50% vetch	236^b	355^b	743^b	847^b	1 121^b	1 213^bc
100% vetch	250^b	266^b	670^b	820^b	1 094^b	1 419^b
Weedy fallow	747^a	1 614^a	2 620^a	3 084^a	3 431^a	3 916^a
Significance	***	***	***	***	***	***
LSD0.05	136	244	408	502	130	406
CV (%)	27.2	24.8	21.5	23.1	4.9	13.6

Table 4 : Effect of cover crop treatments on weed dry matter (kg DM ha^-1) at various sampling times during the 2010 winter season. Means followed by different letters in the same column differ significantly at p < 0.05

Cover crop treatment	59 DAS	84 DAS	101 DAS	117 DAS	132 DAS
100% oat	455^b	633^b	699^b	317^b	303^c
90% oat + 10% vetch	321^c	496^c	357^c	234^b	221^b
70% oat + 30% vetch	382^bc	400^c	294^c	218^b	213^b
50% oat + 50% vetch	310^cd	418^c	318^c	226^b	226^b
100% vetch	188^d	395^c	299^c	252^b	202^b
Weedy fallow	2 010^a	2 156^a	2 591^a	2 955^a	3 292^a
Significance	***	***	***	***	***
LSD0.05	124	132	174	161	205
CV (%)	11.2	9.7	12.6	12.6	15.2

The season × cover crop treatment interaction was significant (p < 0.05) with regard to weed species counts at cover crop termination (Figure 2). In the 2009 season, weedy fallow had significantly (p < 0.05) higher weed species counts in comparison to the rest of the cover crop treatments, which performed the same. However, in the 2010 season, significant differences were observed among the cover crop treatments as well as the weedy fallow. Sole vetch treatment had the least weed species count but was similar to 70% oat + 30% vetch. The three bicultured treatments and 90% oat + 10% vetch treatment had comparable weed species counts, and these counts were significantly (p < 0.05) lower compared to that with 100% oat treatment.

Figure 2:  Effect of cover crop treatments on weed species counts at cover crop termination in the 2009 and 2010 winter seasons

Nitrogen and phosphorus uptake

The season × cover crop treatment interaction was significant (p < 0.05) with respect to cover crop N uptake at termination (Figure 3). During the two seasons, sole vetch consistently had the greatest N uptake whereas weedy fallow had the least. In the 2009 season, the three bicultured treatments together with sole oat had similar and intermediate N uptake values compared to sole vetch and weedy fallow. Although the bicultured treatments had comparable N uptake values among the treatments in the 2010 season, their values were significantly higher (p < 0.05) than that of sole oat. Treatment 70% oat + 30% vetch had comparable N uptake to that of the highest yielder (sole vetch), whereas the sole oat had comparable N uptake values to that of the lowest yielder (weedy fallow), with respect to N uptake.

Figure 3:  Effect of cover crop treatments on nitrogen uptake at cover crop termination in the 2009 and 2010 winter seasons

The season × cover crop treatment interaction was significant (p < 0.05) with respect to P uptake by cover crop at termination (Figure 4). In 2009, all cover crop treatments had similar and significantly higher (p < 0.05) P uptake values than weedy fallow. In 2010, bicultured treatments had comparable P uptake to 100% vetch, but they significantly differed (p < 0.05) among the treatments. Treatment 90% oat + 10% vetch and 70% oat + 30% vetch had similar but higher P uptake than 50% oat + 50% vetch. The 100% oat had the least P uptake and was comparable to weedy fallow. A significant, positive and linear correlation (p < 0.01, r = 0.99) was observed between P uptake and cover crop dry matter at termination.

Figure 4:  Effect of cover crop treatments on phosphorus uptake at cover crop termination in the 2009 and 2010 winter seasons

Discussion

The study demonstrated the possibility of increasing cover crop dry matter by use of biculturing technology, particularly when compared to the legume monoculture. The results echo findings by Sebahattin et al. (2004), Sainju et al. (2005) and Lithourgidis et al. (2006), who observed increased cover crop dry weights by combining vetch with the grasses triticale, rye and white oat, respectively. The observed increase in cover crop dry weights with an increase in the white oat component, as well as the reduction of dry matter with an increase in the legume component, was also in agreement with several researchers (Sebahattin et al.2004, Sainju et al. 2005, Lithourgidis et al. 2006), which further verified the effectiveness of the grass component in providing for the biomass compared to the legume component.

The reduction in interspecific competition could explain the increase in the cover crop CCGRs with an increase in white oat proportion. At 50% oat + 50% vetch, more inter-specific competition is expected compared to the other biculture treatments tested, which had higher white oat and less vetch components. Although legumes are naturally slower to accumulate biomass compared to white oat (Ranells and Wagger 1996), very low temperatures normally experienced after cover crop planting in winter could have further reduced growth rates by vetch. Quick accumulation of biomass is an important factor in the management of early weeds.

The final biomass yields obtained from the bicultured treatments were above 6 t ha⁻¹, a value considered to be above the minimum required for effective weed suppression, reduction in erosion and nutrient addition (Fourie et al. 2001, Toure et al. 2011). The high cover crop biomass observed in this study could have smothered weeds, explaining the lower weed dry matter and weed species counts observed in the cover crop treatments in comparison to high weed variables in the weedy fallow. High biomass cover crops are known to suppress weed growth by altering light and temperature to the ground (Teasdale et al. 2007), as well as providing a barrier to emerging weed seedlings. Researchers have also reported vetch's allelopathic weed suppression of species such as yellow foxtail (Setaria glauca) and yellow nutsedge (Cyperus esculentus), but the effects are said to be more apparent after termination and incorporation of residues into the soil (Teasdale et al. 2007). Sole oat treatment provided the best weed suppression during the early cover crop growth as observed with lower weed dry weights during the early samplings. This could have been because of the fast accumulation of dry weight by white oat compared to the slower rate by vetch.

The lower weed dry matter and weed species observed at cover crop termination in the second season of the experiment could have been because of possible reduction of the weed seed bank as an effect of the follow-on maize crop after the first season of cover cropping (not reported in this study). The weed suppression by the cover crop treatments ranged between 64–76% in 2009 and above 90% in 2010, agreeing with the findings by Teasdale (1996), who observed weed suppression of 80–100% by cover cropping. The similarity of weed suppression by the bicultures and monocultures suggest that biculturing technology does not compromise weed control when compared to sole vetch and non-grazed oat.

Vetches are known to fix atmospheric N, hence their inclusion into white oat managed to improve the N uptake by the bicultured treatments to above that of sole oat (Kuo et al. 1996, Sainju et al. 2005). However, N uptake by these cover crops is a function of the species and amount of biomass accumulated (O'Reilly 2009). This could explain the higher N uptake by the treatment with 70% oat + 30% vetch compared to the sole vetch, which is an efficient N fixer. However, P uptake by the cover crop treatment was largely influenced by the amount of biomass accumulated as shown by increased P with increase in biomass accumulated.

Nitrogen and P uptake by the bicultured treatments, which ranged from 188–220 kg ha⁻¹ and 20–28 kg ha⁻¹, respectively, emphasise the potential of biculturing technology to improve soil N and P availability compared to sole oat, and potentially reduce fertiliser requirements of the follow-on crop. However, the overall success of biculturing depends on the ratio of the legume and grass, often guided by the principal objective of cover cropping. This study has shown that increasing the grass composition in a mixture may result in increased biomass accumulation for weed control. Similarly, if a cover crop's objective is improving fertility, increasing the vetch (legume) component in the mixture is recommended as it results in N fixation and uptake, for the benefit of a follow-on crop.

Conclusions

Bicultures of white oat and vetch produced comparable biomass to that of the sole cover crop species in the first season, but continued use of bicultures resulted in increased biomass compared to that of sole vetch. Increasing the white oat component in the mixture resulted in increased CCGRs and dry matter, whereas increasing the vetch component resulted in reduction of the mentioned variables. Better weed suppression was observed with an increase in biomass, but sole oat can be used for early weed control. Biculturing resulted in improved N uptake when compared to the sole oat. The high biomass-yielding biculture treatment, 70% oat + 30% vetch, resulted in similar N uptake to that of sole vetch. Increase in dry matter accumulation resulted in increased P uptake in both sole and bicultured cover crops. Among the biculture ratios, 70% oat + 30% vetch performed better than the rest with respect to cover crop biomass yield, weed suppression, and N and P uptake. Future studies may need to be sited on a farm with depleted soils to allow conclusive results on performance of these cover crops in conditions representative of the majority of EC soils.

Acknowledgements

This document is an output from a project funded by the Govan Mbeki Research and Development Centre, University of Fort Hare and the National Research Foundation, South Africa.