Europ. J. Agronomy 30 (2009) 151–162

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European Journal of Agronomy

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The effect of wheat straw residue on the emergence and early growth of sugarbeet (Beta vulgaris) and oilseed rape (Brassica napus)

N.L. Morrisa,b,∗, P.C.H. Millera, J.H. Orsona, R.J. Froud-Williamsb

a The Arable Group, The Old Rectory, Morley St Botolph, Wymondham, Norfolk NR18 9DB, United Kingdomb School of Biological Sciences, The Harborne Building, University of Reading, Reading, Berkshire RG6 6AS, United Kingdom

a r t i c l e i n f o

Article history:Received 30 January 2008Received in revised form 6 September 2008Accepted 9 September 2008

Keywords:Straw residueCrop emergence and growthPhysical impedanceSeed–soil contactPhytotoxicity

a b s t r a c t

The management of straw residue can be a concern in non-inversion tillage systems where straw tendsto be incorporated at shallow depths or left on the soil surface. This can lead to poor crop establishmentbecause straw residue can impede or hinder crop emergence and growth. Small container-based experi-ments were undertaken using varying amounts of wheat straw residue either incorporated or placed onthe soil surface. The effects on days to seedling emergence, percentage emergence, seedling dry-weightand soil temperature using sugar beet and oilseed rape were investigated because these crops often followwheat in a cropping sequence.

The position of the straw residue was found to be the primary factor in reducing crop emergence andgrowth. Increasing the amount of straw residue (from 3.3 t ha−1 to 6.7 t ha−1) did not show any consistenttrends in reducing crop emergence or growth. However, in some instances, results indicated that aninteraction between the position and the amount of straw residue occurred particularly when the strawand seed was placed on the soil surface. Straw placed on the soil surface significantly reduced meanday-time soil temperature by approximately 2.5 ◦C compared to no residue. When the seed and straw wasplaced on the soil surface a lack of seed-to-soil contact caused a reduction in emergence by approximately30% because of the restriction in available moisture that limited the ability for seed imbibition. This trendwas reversed when the seed was placed in the soil, but with straw residue still on the soil surface, becausethe surface straw was likely to reduce moisture evaporation and improved seed-to-soil contact that ledto rapid emergence. In general, when straw was mixed in or placed on the soil surface along with theseed, sugar beet and oilseed rape emergence and early growth biomass was significantly restricted byapproximately 50% compared to no residue.

The consequences of placing seed with or near to straw residue have been shown to cause a restrictionin crop establishment. In both oilseed rape and sugar beet, this could lead to a reduction in final crop

densities, poor, uneven growth and potentially lower yields that could lower financial margins. Therefore,if farmers are planning to use non-inversion tillage methods for crop establishment, the managementand removal of straw residue from near or above the seed is considered important for successful crop

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establishment.

. Introduction

Recent adoption by farmers of non-inversion systems that

educe the costs of crop establishment including time and fuelsed has led to a reduction in the use of the mouldboard ploughhat inverts the soil, thus burying almost all crop residues. Manyon-inversion cultivation systems are based on disc and tine imple-

∗ Corresponding author at: The Arable Group, The Old Rectory, Morley St Boltolph,ymondham, Norfolk, NR18 9DB, United Kingdom. Tel.: +44 1953 713216;

ax: +44 1953 713234.E-mail address: nathan.morris@thearablegroup.com (N.L. Morris).

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161-0301/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.eja.2008.09.002

© 2008 Elsevier B.V. All rights reserved.

ents that incorporate some of the straw residue but leave varyinguantities (depending on the system used) lying on the soil surfaceGajri et al., 2002). In some crops such as oilseed rape, tillageystems that broadcast seed directly onto stubble at the same times the previous cereal is harvested (e.g. Autocast) or with minimalncorporation (e.g. Variocast seeders mounted to tine/disc cultiva-ors) leave a high proportion of straw residue on the soil surface.

An Autocast system works by mounting a seed hopper directly

nto the combine header and a trailing wheel provides the landrive for the seed metering unit via a chain and sprocket drive.eed is then broadcast directly on to the stubble before the straw ispread from the straw chopper to cover the seed. Results from usingutocast have shown that crop establishment and yield compared

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52 N.L. Morris et al. / Europ. J

avourably to a plough system under the right conditions although iftraw residue and chaff were not spread evenly it resulted in patchyrop emergence (HGCA, 2002). Traditionally, drilling sugar beet wasreceded by ploughing either in the autumn or spring dependingn soil type and therefore all crop stubble was buried (Ecclestone,004). Recent pressures on sugar beet establishment techniquesave shifted towards the use of non-inversion tillage techniqueshat retain straw residue on the soil surface for improved soiltructure and higher biological activity (Ecclestone, 2004). Recentrogress in the research to develop autumn-sown sugar beet vari-ties has allowed the isolation of genes that control flowering andolting of the sugar beet plant (Jaggard and Werker, 1999). Poten-ially this could lead to the autumn sowing of sugar beet that mayecessitate a change in thought into straw residue management

rom the preceding crop.There are many benefits of crop residues within non-inversion

illage systems including a reduction in soil erosion. However, cropesidues are often associated not only with implement blockages,ut to lower yields resulting from poor crop emergence and estab-

ishment (Elliott et al., 1976; Harper, 1989; Kaspar and Erbach,998). Therefore, straw residue needs to be managed effectively tonsure minimal disruption to seedling emergence but to maximiseoil protection (Kaspar and Erbach, 1998).

It is known that crop residues can have a detrimental effect onrop establishment and early growth because water-soluble toxinse.g. phenolic acids) are produced in anaerobic conditions eithery crop residues or microorganisms during decomposition (Alam,990; Cornish and Pratley, 1987; Kimber, 1967). Phytotoxic symp-oms of affected plants include reduced tillering, spindly stems,ellowing leaves and reduced yields (Elliott et al., 1976). Phyto-oxicity produced from the decomposition of wheat straw wasound to affect wheat germination, seedling root and shoot growthElliott et al., 1976). Further studies have shown that the posi-ion of crop residue in relation to the crop seed has an importantffect on the severity of phytotoxicity (Lovett and Jessop, 1982;uest et al., 2000). Wuest et al. (2000) found that positioning the

rop residue below seedling depth increased the phytotoxic effectecause the seedlings roots came into contact with the residue.eaving crop residues on the soil surface helped reduce phytotoxi-ity although a delay in emergence took place because the seedlinghoot (coleoptile) was prevented from growing straight to the soilurface because of mechanical impedance (Wuest et al., 2000). Con-equently, Elliott et al. (1976) and Wuest et al. (2000) concludedhat if residues were removed from the seed row then the risk ofelayed crop emergence and an impediment to seedling growthay be reduced.Concern has been raised by some scientists including Lyon et

l. (2004) who suggest that much of the research on phytotoxicityas been laboratory-based rather than examining the phenomenon

n the field. This has led to a debate on the concentration of suchxtracts used in the laboratory being up to 20 times higher thanhose collected from the field environment, suggesting some cau-ion should be used when concluding results from allelopathicffects based solely on laboratory procedures (Lyon et al., 2004).tudies have found that the greater the length of time betweenesidue decomposition and the sowing of the crop leads to a reduc-ion in the inhibitory effect of the toxins on the crop (Børresen,999).

Straw residue is also known to affect soil temperature throughchange in the soil heat flux and solar radiation adsorption

Flerchinger et al., 2003; Sauer et al., 1998; Shinners et al., 1994).esearch has found that the presence of crop residue can signifi-antly reduce crop emergence because of slow soil warming andrying. A study by Børresen and Njos (1990) found that straw covern the soil surface reduced soil temperature and slowed germina-

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nomy 30 (2009) 151–162

ion and development of a spring-sown crop. Mulching with strawas found to reduce the maximum soil temperature in May by 7.7 ◦C

nd raise the minimum soil temperature by 1.3 ◦C at 2 cm depthShinners et al., 1994). In a study undertaken in the Northern Cornelt of the U.S. it was found that creating residue-free strips had aignificant effect on soil temperature in the seed zone, but less effectn soil water content (Shinners et al., 1994). A residue-free band,00–300 mm wide would produce yields similar to no residue coverShinners et al., 1994). The study concluded that residue influencesoil temperature in three ways; residue acts as an insulating layern the soil surface, residue reflects more solar radiation than bareoil and residue reduces evaporation rates causing the soil to warmore slowly (Forbes and Watson, 1996).The objective of this research was to quantify the effect that

heat straw residue has on the emergence and growth of oilseedape and sugar beet using varying amounts of straw residue andarying positions in relation to the seed. Results from the studyould aid the knowledge and management of straw residues inon-inversion tillage systems and avoid the risks of delayed cropmergence and a reduction in seedling growth.

. Materials and methods

.1. Experimental design

The experiments were conducted over a 4–6 week periodither in the autumn or spring seasons (2005–2007). Wheat strawesidues were used because the area of wheat grown in the UK was.87 million hectares in 2005 (DEFRA, 2006), therefore wheat stub-le is likely to be used in many subsequent crop rotation sequences

ncluding oilseed rape and sugar beet. Sugar beet and oilseed rapeave both been grown in the United Kingdom using non-inversionillage techniques where straw residue has been a concern andherefore using these crops was considered appropriate for thistudy.

All experiments used 24 plastic boxes measuring3 cm × 25 cm × 15 cm with twelve 5 mm holes in the bottomo aid drainage. All boxes were filled level to the rim with sterile,oam-based soil free from weed seeds and other organisms thatould affect the experiment. Wheat straw residue was preparedor the first season by cutting stems of straw into lengths betweencm and 4 cm and weighing out samples of 28 grams; equivalent

o 3.3 t ha−1. This produced straw residue that did not containhaff and therefore no volunteer seeds occurred during the firsteason. All subsequent seasons used chopped wheat straw col-ected directly from the straw chopper of a combine harvesterhat produced variable lengths of straw and chaff (includingn-threshed grains). Samples of chopped straw were weighedither at 28 grams; equivalent to 3.3 t ha−1 or 56 grams; equivalento 6.7 t ha−1. The straw residue was stored in sacks ready for usever the duration of the experiment.

The use of soil and chopped wheat straw in these experimentsnabled conditions to relate more closely to those associated witheld conditions compared to using a wheat straw extract in lab-ratory conditions. The containers were placed in blocks of eachreatment for the first season, in all subsequent seasons the treat-

ents were fully randomised. To stop the straw blowing away, weldesh was placed over the treatment with straw on the surface dur-

ng the first season although this could affect crop emergence, in

ll subsequent seasons, 13 mm nylon netting was placed over allreatments so that any effect this may have had would apply to allreatments.

For the spring and autumn 2005 season, three treatments repli-ated eight times with one amount of wheat straw residue were

N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162 153

Table 1Experimental design.

Season Crop Wheat straw Amount of residue applied to treatments Treatments

Spring 2005 Sugar beet Chopped straw of 2–4 cm lengths. 28 grams; equivalent to 3.3 t ha−1. No residue.Residue on surface.Residue mixed into top 5 cm.

Autumn 2005 Winter oilseed rape Chopped straw residue from strawchopper on combine.

28 grams; equivalent to 3.3 t ha−1. No residue.

Residue on surface.Residue mixed into top 5 cm.

Spring 2006 Sugar beet Chopped straw residue from strawchopper on combine.

28 grams; equivalent to 3.3 t ha−1. No residue.

Residue on surface with seedon surface.

Spring oilseed rape 56 grams; equivalent to 6.7 t ha−1. Residue on surface with seed insoil.Residue mixed into top 5 cm.

Autumn 2006 Winter oilseed rape Chopped straw residue from strawchopper on combine.

28 grams; equivalent to 3.3 t ha−1. No residue.

Residue on surface with seedon surface.

56 grams; equivalent to 6.7 t ha−1. Residue on surface with seed insoil.Residue mixed into top 5 cm.

Spring 2007 Sugar beet Chopped straw residue from strawchopper on combine.

28 grams; equivalent to 3.3 t ha−1. No residue.

Residue on surface with seed

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sed. In all subsequent seasons (spring 2006–spring 2007) eightreatments replicated three times were used with two amountsf wheat straw residue applied that would represent both a lownd high level of straw residue typically present on farms in the UKfter harvest (see Table 1). For all treatments where the residue wasncorporated, soil was removed from the top 5 cm of the containernd the required amount of residue evenly mixed before replacingoil into the container. Residue that was placed on the soil surfaceas applied after the seeds were sown so that a constant seed depth

ould be maintained.For most treatments the seed was sown into the soil at a specific

epth (2 cm for oilseed rape and 3 cm for sugar beet) using a dibberarked at the required depth. Only the treatments where the seedas sown on the soil surface were not sown into the soil to the

pecific depth. Twelve seeds, using a template, were sown in a 2 × 6rid in each of the 24 boxes to ensure accurate placement of the seedithin the boxes.

The experimental area for the first season consisted of a fruitage with a concrete base. All subsequent seasons were carriedut on a gravel base with a black polythene sheet placed overhe gravel to stop weeds emerging during the experiment. An alu-

inium frame with 13 mm nylon netting attached to the frame wassed to prevent birds from eating the seedlings during the autumn005 to spring 2007 seasons. To reduce the threat of vermin or mol-

usc damage, rodenticide and molluscicide bait points were placedround the experiment. In all seasons, the experiment was watered,hen required, to maintain soil moisture at the seed depth. Volun-

eer wheat that germinated in the chaff (occurred in all but the firstxperiment) was hand weeded to reduce crop competition.

Crop emergence was recorded every day, as soon after the first

eedlings had emerged (recorded as day 1); until 3 days had passedith no new seedlings emerged (between days 24 and 39 depend-

ng on season). Crop percentage emergence was calculated at 5nd 21 days that allowed differences to be related to a temporaryr lasting delay in emergence. On completion of the experiment

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on surface.56 grams; equivalent to 6.7 t ha−1. Residue on surface with seed in

soil.Residue mixed into top 5 cm.

ll seedlings from each container were cut at the hypocotyl andeighed before and after drying in an oven at 105 ◦C for 16 h.

Soil temperature at seed depth for oilseed rape (2 cm) and sugareet (3 cm) was recorded using a Campbell Scientific CR10 data log-er with 5 resin encapsulated thermistors during spring/autumn006. This enabled only one replication in five of the treatments

ncluding no residue, low/high residue mixed in and low/highesidue on the soil surface. Soil temperature was recorded every0 min with an hourly average stored to the data logger. Soil tem-erature was recorded over the period that the experiment ran fornd was split into 12-h day (0900–2000) and night (2100–0800)eriods. Calibration of the thermistors was completed by placinghe thermistors into a bucket of water at either, room temperature20 ◦C); or at or near to zero (3 ◦C) using cold water with ice. Mea-urements were recorded for 2 h allowing two averaged readingsor each thermistor. Accuracy at 20 ◦C was ±0.3 ◦C, at 3 ◦C it was0.6 ◦C.

.2. Statistical analysis

Data was analysed using GenStat, Release 8.1, Lawes Agricul-ural Trust. The data for days to emergence, percentage emergence,eedling dry-weight and soil temperature was analysed using anal-sis of variance (ANOVA), which was appropriate for a randomisedomplete block design. Data for days to emergence was transformedsing Log10 to normalise the distribution. Therefore, results are pre-ented using the transformed data so that the results, p-values and.e. are on the same scale for consistency and clarity. The analysisncluded every seed planted in all treatments and replicates (e.g.005: 12 seeds replicated 8 times equal to 96 seeds in each of three

reatments; 2006–2007: 12 seeds replicated 3 times equal to 36eeds in each of eight treatments) therefore the variability betweenreatments but also between replication could be analysed with allut one experiment showing no significant variation between repli-ations. Therefore the variations between replicates were smaller

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54 N.L. Morris et al. / Europ. J

han between treatments allowing for analysis and discussion ofhe main treatment effects.

Percentage emergence was analysed at days 5 and 21 to com-are differences between emergence a few days after emergencend then later at 3-weeks to assess if the percentage emergencead increased or whether the emergence was still poor. All data forays to emergence, percentage emergence and seedling dry-weightsed a factorial ANOVA to compare the interaction between strawosition and amount. Where significant differences were foundomparisons between treatments using a contingency table weresed. Soil temperature was analysed using ANOVA to analyse differ-nces in temperature between treatments during the day and nighteriods over the time period that the experiment ran for (either 27r 50 days). Repeated measures ANOVA could not be completedue to the limited number of thermistors reducing the replicationf the data within each treatment to one.

Statistical significance was evaluated at p-value ≤ 0.05 withppropriate pooled standard errors of the difference (s.e.d) ofreatment means. Results are displayed as the treatment meansith pooled standard errors (s.e.) of treatment means. The results

nd discussion that follow are divided between the main effectsstraw position or the amount of straw residue) and the interactionetween straw position and amount of straw residue.

. Results

.1. Days to emergence

It is important to note that the following results are presenteds Log transformed data so the s.e. values are also the s.e. of Logransformed data. Days to emergence for the first two experiments;pring 2005 and autumn 2005 (see Fig. 1) where straw was onlypplied at a low level, indicated that in sugar beet, straw residueoth mixed in and placed on the soil surface increased the dayso emergence significantly (p-value < 0.001, s.e.d 0.04) comparedo no residue. However, when the experiment used oilseed rapehe straw residue was found to improve the days to emergencearticularly between residue on the surface compared to no residue

p-value < 0.05, s.e.d. 0.04).

Further experiments were undertaken using two amounts oftraw residue, either equivalent to 3.3 t ha−1 or 6.7 t ha−1 (seeigs. 2 and 3). In all but one of the seasons there were no signif-

ig. 1. Days to emergence (sugar beet and winter oilseed rape 2005). Standard errorars of pooled mean values.

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nomy 30 (2009) 151–162

cant differences in days to emergence between the two residuemounts. The spring 2006 season with oilseed rape resulted in aignificant difference (see Fig. 2) between the amount of residuesed (p-value < 0.001; s.e.d. 0.04). The main effect of residue posi-ion significantly altered days to emergence in all seasons (seeigs. 2 and 3); spring 2006–spring 2007 (p-value < 0.001, s.e.d..06; p-value < 0.001, s.e.d. 0.06; p-value < 0.001, s.e.d. 0.07 and p-alue < 0.001, s.e.d. 0.04, respectively).

Straw residue mixed into the top 5 cm of soil significantlyncreased days to emergence compared to no residue (p-alue < 0.001, s.e.d. 0.06; p-value < 0.01, s.e.d. 0.04) in the spring006/2007 seasons, respectively (see Figs. 2 and 3) but not inhe autumn 2006 season. Placing the straw residue and the seedn the surface significantly increased the days to emergence (p-alue < 0.001, s.e.d. 0.07) only in the autumn 2006 season (seeig. 2). No difference in days to emergence was found between strawesidue placed on the soil surface with the seed in the soil comparedo no residue in any of the experiments in 2006 (see Fig. 2). Onlyn the spring 2007 season (see Fig. 3) did seedlings emerge signifi-antly earlier from buried seed where straw was placed on the soilurface than with no residue (p-value < 0.05, s.e.d. 0.04).

In the 2006–2007 seasons an interaction occurred betweenhe amount and the position of the straw residue, and in partic-lar, when straw and seed were placed on the soil surface. This

nteraction was significant in the 2006 season (see Fig. 2) in allilseed rape and sugar beet experiments (p-value < 0.05, s.e.d. 0.08;-value < 0.001, s.e.d. 0.09 and p-value < 0.001, s.e.d. 0.07, respec-ively).

.2. Percentage emergence

Percentage emergence in spring 2005 (see Fig. 4) when strawas applied at a low level, indicated that straw residue placed on

he soil surface significantly reduced the percentage emergence onay 5 by 42% (p-value < 0.001, s.e.d. 6.47) compared to no residue.owever, by day 21 there was no significant difference betweenny of the treatments (p-value 0.447, s.e.d. 5.36). In autumn 2005ercentage emergence was not significantly different at day 5 (p-alue 0.116, s.e.d. 7.96) but by day 21 straw residue on the surfacead significantly reduced percentage emergence by 26% comparedo no residue (p-value < 0.01, s.e.d. 7.99).

Further experiments undertaken used two amounts of strawesidue, equivalent to 3.3 t ha−1 or 6.7 t ha−1 (see Figs. 5–8) withll but one showing no significant differences in percentage emer-ence at day 5 (p-value 0.613, s.e.d. 5.64; p-value 0.567, s.e.d. 7.97nd p-value 0.990, s.e.d. 6.62, respectively) or day 21 (p-value 0.460,.e.d. 8.00; p-value 0.647, s.e.d. 4.45 and p-value 0.440, s.e.d. 2.73,espectively) between the two residue amounts suggesting that dif-erences in percentage emergence were mainly affected by strawesidue position and not the amount of residue. Oilseed rape dur-ng spring 2006 (see Fig. 6) showed a significant effect of strawesidue amount at day 5 (p-value < 0.05, s.e.d. 8.31) and at day 21p-value < 0.05; s.e.d. 9.10) when straw and seed was placed on theurface.

Mixing the straw residue into the soil or placing the strawesidue and seed on the soil surface reduced the percentage emer-ence on day 5 in several seasons (see Figs. 5–7) by as much as 24%see Fig. 5) when the residue was mixed in (p-value < 0.01, s.e.d..98) or by 34% (see Fig. 7) when the straw and seed were placedn the soil surface (p-value < 0.001, s.e.d. 11.27). When straw was

laced on the soil surface and the seed was in the soil, percentagemergence on day 5 was similar or significantly improved in spring006 oilseed rape and spring 2007 sugar beet (p-value < 0.05, s.e.d.1.75 and p-value < 0.05, s.e.d. 9.36, respectively) compared to noesidue (see Figs. 6 and 8).

N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162 155

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Fig. 2. Days to emergence (sugar beet and spring/winter

In some instances, at day 21, percentage emergence (seeigs. 7 and 8) was not significantly different between straw residueosition (p-value 0.967, s.e.d. 6.29 and p-value 0.975, s.e.d. 3.86,espectively). During the spring 2006 season, seed and strawesidue placed on the surface resulted in a significantly lower per-

entage emergence in sugar beet emergence (p-value 0.007, s.e.d.1.31). Provided that the seed was in the soil and the straw waslaced on the soil surface, percentage emergence on day 21 wasignificantly increased compared to no residue (p-value < 0.01, s.e.d.2.87) in spring 2006 oilseed rape (see Fig. 6).

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Fig. 3. Days to emergence (sugar beet 2007). St

rape 2006). Standard error bars of pooled mean values.

The interaction between the position and the amount of strawesidue that occurred when recording days to emergence did notccur in any of the seasons (2006/2007) for percentage emergence.

.3. Seedling dry-weight

In all instances seedling dry-weight was significantly greatern no residue treatments compared to when residue was mixedn or when residue and seed were placed on the soil surface (p-alue < 0.001, s.e.d. 0.09; p-value < 0.05, s.e.d. 0.01; p-value < 0.05,

andard error bars of pooled mean values.

156 N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162

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Fig. 4. Percentage emergence at days 5 and 21 (sugar beet and w

.e.d. 0.13; p-value < 0.01, s.e.d. 0.67 and p-value < 0.001, s.e.d. 0.24,espectively) as shown in Figs. 9–11. In oilseed rape during spring006 and sugar beet during spring 2007, seedling dry-weight wasound to be 58% and 86% lower when the straw residue was mixed in

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Fig. 5. Percentage emergence at days 5 and 21 (sugar bee

oilseed rape 2005). Standard error bars of pooled mean values.

ompared to no residue (p-value < 0.05, s.e.d. 1.61; p-value < 0.001;.e.d. 0.24, respectively). In most seasons (see Figs. 10 and 11)eedling dry-weight between straw on the surface with seed in theoil and no residue showed little difference.

t 2006). Standard error bars of pooled mean values.

N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162 157

Fig. 6. Percentage emergence at days 5 and 21 (spring oilseed rape 2006). Standard error bars of pooled mean values.

Fig. 7. Percentage emergence at days 5 and 21 (winter oilseed rape 2006). Standard error bars of pooled mean values.

158 N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162

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The interaction between the position and the amount of strawesidue that occurred when recording days to emergence did notccur in any of the seasons (2006/2007) for seedling dry-weight.

.4. Soil temperature

Soil temperature during the day-time, at both 2 and 3 cm soilepth, was significantly different for both sugar beet (spring 2006)nd oilseed rape (autumn 2006); p-value < 0.01, s.e.d. 1.04 and-value < 0.01, s.e.d. 0.75, respectively (see Tables 2 and 3). Mean

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Fig. 9. Seedling dry-weight (sugar beet and winter oilseed ra

t 2007). Standard error bars of pooled mean values.

ay-time soil temperatures in sugar beet (see Table 2) for residueixed in, either low or high amounts, and no residue of 20.63 ◦C,

1.10 ◦C and 21.07 ◦C, respectively, were not significantly differentp-value 0.826, s.e.d. 1.04).

Mean day-time soil temperature when straw residue was placed

n the soil surface, either at low or high amounts, and no residuef 18.77 ◦C, 18.22 ◦C and 21.07 ◦C, respectively, were found to beighly significant (p-value < 0.01, s.e.d. 1.04). Similar results werelso obtained in oilseed rape where the mean day-time soil temper-ture was highly significant when straw was placed on the surface

pe 2005). Standard error bars of pooled mean values.

N.L. Morris et al. / Europ. J. Agronomy 30 (2009) 151–162 159

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Fig. 10. Seedling dry-weight (sugar beet and spring/winter

p-value < 0.01, s.e.d. 0.75) but not when straw was mixed in (p-alue 0.746, s.e.d. 0.75) compared to no residue (see Table 3).

Mean night-time soil temperatures were found not to beignificantly different between any of the treatments, either inpring 2006 (sugar beet); or autumn 2006 (oilseed rape), see

Ttsra

Fig. 11. Seedling dry-weight (sugar beet 2007). S

ed rape 2006). Standard error bars of pooled mean values.

ables 2 and 3. Although not significantly different, mean night-ime temperatures did show that straw residue placed on the soilurface insulated the soil by roughly 1 ◦C compared to either noesidue or residue mixed in (see Tables 2 and 3). The low or highmounts of straw residue, equivalent to 3.3 t ha−1 or 6.7 t ha−1

tandard error bars of pooled mean values.

160 N.L. Morris et al. / Europ. J. Agro

Table 2Effect of straw residue on soil temperature (3 cm depth) during spring 2006 seasonin sugar beet.

Treatment Day temperature (◦C)a Night temperature (◦C)a

No residue 21.07ab 12.40Low residue mixed in 20.63a 12.44High residue mixed in 21.10a 12.26Low residue on soil surface 18.77b 13.13High residue on soil surface 18.22b 13.06

p-Value <0.01 n.s.

a Values are the mean day or night soil temperature during the length of theexperiment (50 days).

b Means identified by the same letter are not significantly different at the 5% levelusing comparisons in analysis of variance.

Table 3Effect of straw residue on soil temperature (2 cm depth) during autumn 2006 seasonin oilseed rape.

Treatment Day temperature (◦C)a Night temperature (◦C)a

No residue 21.49ab 11.52Low residue mixed in 22.68a 11.46High residue mixed in 21.30a 11.43Low residue on soil surface 18.60b 12.38High residue on soil surface 17.86b 12.99

p-Value <0.01 n.s.

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Values are the mean day or night soil temperature during the length of thexperiment (27 days).b Means identified by the same letter are not significantly different at the 5% level

sing comparisons in analysis of variance.

howed little effect on soil temperature, respective to the treat-ents.

. Discussion

.1. Days to emergence

Dry conditions during the autumn 2005 and spring 2007 experi-ents resulted in a reduced time to emergence when straw residueas placed on the soil surface with the seed in the soil compared too residue. Although the containers were watered throughout thexperiment to maintain moisture at the seed depth, soil moistureetention was likely to be improved when the straw was placedn the soil surface aiding germination. Soil moisture content haseen shown to be critical for increased germination and growth,ith minimum soil moisture contents that permit seed germina-

ion (Forbes and Watson, 1996).It was also possible that attempts to maintain soil moisture by

atering may have caused surface capping to the bare soil (noesidue) treatments. Water impact has been shown to lead to theormation of a seal during rainfall and a crust when the soil driesRamos et al., 2003). A study by Awadhwal and Thierstein (1985)ound that a soil crust would impede the emergence of young cropeedlings due to the physical resistance of a soil crust althoughtraw residue on the soil surface had prevented the formation of therust by dissipating the energy of the raindrops. This may explainhy in some of the experiments days to emergence was greater in

he no residue treatments compared to the treatments with strawither mixed in or placed on the soil surface.

Most of the results on days to emergence indicated that the main

ffect relating to a delay in emergence was the position of the strawesidue and not the amount applied. Delayed emergence when thetraw and seed was placed on the soil surface would suggest pooreed-to-soil contact, which is important for transferring moisturerom the soil to seed (Forbes and Watson, 1996) This was considered

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he primary factor rather than phytotoxic or physical impedance asound by Lovett and Jessop (1982) and Wuest et al. (2000). Whentraw residue was mixed into the soil surface any release of toxinsrom the decomposition of the straw can come into direct contactith the seed or root and may increase days to emergence. It has

een suggested by Lovett and Jessop (1982) that the developing roots highly sensitive to the presence of phytotoxins released from thetraw during decomposition. During observations in this study, novidence of phytotoxicity, e.g. discoloured leaves or spindly stems,ere detected although root contact with the straw residue when

t was mixed in did generally increase days to emergence and mayuggest an allelopathic effect of the straw residue.

Results when straw was placed on the soil surface with the seedn the soil suggested similar or a reduced time for days to emergenceompared to no residue in the 2006 and 2007 seasons. This wasurprising as previous studies suggest that straw placed on the soilurface would prevent coleoptiles from growing straight to the soilurface and therefore increase the days to emergence (Wuest et al.,000). One possible explanation to this is likely to be related tohe earlier comment made to soil moisture levels. It is understoodhat straw residue placed on the soil surface can act as a mulchuring warm, sunny periods reducing vapour transfer between theoil surface and the overlying air layer thus reducing evaporationnd this leads to higher soil moisture levels (Sauer et al., 1998)ompared to bare soil thus improving water retention and aidingeed imbibition that accelerates emergence provided that the seeds in good contact with the soil.

.2. Percentage emergence

Although no consistent trends were found, percentage emer-ence reduced at day 5 when straw and seed were placed on theurface and was likely to be caused by poor seed-to-soil contactith low soil moisture levels. Poor percentage emergence, par-

icularly when the seed was placed on the soil surface with aow amount of residue showed the effect of moisture evaporationn high temperature conditions compared to high levels of strawesidue where increasing amounts of straw residues are known tonsulate the soil and retain moisture (Sauer et al., 1998; Wuest et al.,000). Using a high amount of straw residue placed on the surfaceith the seed improved percentage emergence in the majority of

he experiments reported in this paper.Placing the straw on the soil surface and the seed in the soil, at

imes, improved early percentage emergence compared to bare soiluggesting that during periods of higher temperatures (particularlyuring the day-time) the straw retains soil moisture and reduceshe fluctuations in soil temperature leading to improved percentagemergence at day 5.

It would appear that when the seed is within close proximity tohe straw, toxins produced from the straw may significantly lowerercentage emergence although this hypothesis was not tested

n this study but were suggested in previous studies (Lovett andessop, 1982; Wuest et al., 2000). Previous studies have suggestedhat the possibility of toxin release are affected by the decomposi-ion rate of the straw residue and this was found to be more rapidhen the straw residue was incorporated compared to when left

n the soil surface. This potentially may lead to poor establishmentn the field leading to uneven crop growth and crop maturity andllow weed competition to increase within the crop leading to aecline in crop yields.

In some instances, no difference in percentage emergence wasound at day 21 and would suggest that any early delay in emer-ence at day 5 because of the straw residue physically impeding theoleoptile from growing straight to the soil surface as reported by

uest et al. (2000) was rectified by day 21. Physical impedance was

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ig. 12. An example of oilseed rape plant size between no residue (a) and residueixed in (b) treatments.

hought to occur where a high amount of straw residue was placedn the soil surface that would cause the growth of the coleoptileo be mechanically impeded by the straw causing the coleoptile tond a more indirect route to the surface thus increasing the days tomergence and so reducing percentage emergence at day 5. Over-ll the mechanical impedance would delay rather than prohibitercentage emergence.

.3. Seedling dry-weight

It would appear that straw mixed in or placed on the surface withhe seed on the surface delayed emergence and subsequent growthf the crop and may have been caused by a number of factors includ-ng mechanical impedance, sub or super-optimal soil temperatures,oor seed-to-soil contact or direct contact with the straw residueausing phytotoxicity. Because of the complex interactions betweenlant and soil it would be difficult to define exactly which of theseas the principal factor involved. These factors resulted in reduced

mergence and low plant vigour resulting in the majority of smallereedlings compared to no residue (see Fig. 12) on completion of thexperiments.

Seedling dry-weight was generally similar when straw waslaced on the soil surface with the seed in the soil and no residueecause crop emergence was similar between both and appeared toave had little effect on the vigour of the seedlings, either by phys-

cal impedance or as a result of an inhibitory effect as the strawecomposes.

.4. Soil temperature

Mean night-time soil temperatures were found not to be sig-ificantly different between any of the treatments, either in sugar

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eet or oilseed rape. This would suggest that the differences in soilemperature found during the day-time were when air tempera-ures were typically increasing. Straw placed on the soil surface haseen shown to lower thermal conductivity inhibiting heat trans-er between the soil and the atmosphere (Sauer et al., 1998). Theesults in this study support the assumption that straw residueseflect more radiation energy than bare soil and that straw residueeduces the evaporation rates which can cause the soil to warmlowly (Shinners et al., 1994).

Although not significantly different, mean night-time temper-tures did show that straw residue placed on the soil surfacensulated the soil by roughly 1 ◦C compared to either no residue oresidue mixed in, therefore, retaining straw residue on the surfaceeduced soil temperature by reflecting solar radiation and insulat-ng the soil surface (Licht and Al-Kaisi, 2005). Varying the amountsf straw residue, equivalent to 3.3 t ha−1 or 6.7 t ha−1 showed littleffect on soil temperature, respective to the treatments. This woulduggest that the differences in soil temperature were primarily dueo residue position and not the amount of straw residue.

Soil temperature is an important edaphic factor for seedlingstablishment with an optimum temperature range for germi-ation being dependant on crop species (Gajri et al., 2002). The

ncrease in percentage emergence found in this study when strawas placed on the soil surface with seed in the soil suggests

hat the smaller diurnal fluctuations measured in this treatmentould reduce moisture evaporation. Gajri et al. (2002) reported

hat percentage emergence was greatly reduced when soil tem-eratures ranged from 18 ◦C to 33 ◦C in moist conditions comparedo wet conditions. Therefore, unfavourable soil temperature wasikely to aggravate low soil moisture in the seed zone. Sub-optimalemperatures have been shown to retard emergence throughecreased metabolic activity and super-optimal temperatures thatan increase respiration rates and lower emergence caused byetabolic failures (Gajri et al., 2002). It is therefore possible that

ome of the reduced emergence and dry-weights found in thistudy would be strongly influenced by soil temperature and mois-ure and not just by the physical or chemical effects of the strawesidue.

. Conclusion

Results from this study suggest that crop establishment wasrimarily affected by the position of the wheat straw residue inelation to the crop seed. No consistent trend was found betweenhe amounts of wheat straw residue applied, equivalent to either.3 t ha−1 or 6.7 t ha−1. Therefore, it was considered that the methodf straw incorporation should be of greater concern than themount of straw produced from the previous crop.

The position of the straw residue either mixed in or whenesidue and seed was placed on the soil surface was likely to reducerop percentage emergence by up to 40% compared to no residue. Inost situations seedling dry-weight was up to 80% lower in these

reatments compared to no residue. It is assumed that physicalmpedance or poor seed-to-soil contact resulted in lower emer-ence and poor crop vigour that would indicate that removing strawesidue from the seed-row desirable.

When straw residue was placed on the surface it would appearhat the thermal insulating properties of the straw reduced diur-al fluctuations in soil temperature between day and night periodshat allowed for crop emergence and growth to be similar to, or in

ome cases, better than no residue provided that water availabilityor seed imbibition was not restricted. Maintaining adequate seedlacement to a depth below that of the straw would be imperativeo ensure that seed-to-soil contact was good, otherwise the reducedrop performance on emergence and growth could clearly be seen

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62 N.L. Morris et al. / Europ. J

hen the seed was placed on the soil surface and therefore wasusceptible to poor soil moisture availability.

Using a system such as Autocast could potentially produce goodrop establishment compared to a conventional plough system ifesults reported in this study were found in the field. However,otential difficulties working in field situations with high lev-ls of straw residue on the surface can increase slug populationshat would significantly reduce plant populations. Placing the seedirectly onto the soil surface also risks low levels of soil moisturevailability. This can lead to seeds being placed on the surface whererop establishment has been shown to be much more variable inhis study. Therefore, it should be considered necessary to remove,ury or move away the straw residue from the seed row so that theeed can be drilled into residue-free soil.

cknowledgements

We are most grateful to The Morley Agricultural Foundation andhe Chadacre Trust for funding the PhD of the first author. Thanksre also due to Mr. Pete Richards of Solutions for Research Ltd. forhe loan of the data logging equipment and The Arable Group forroviding the trial sites.

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