Interactions of Straw Disposal Methods and Direct Drilling or Cultivations on Winter Wheat (Triticum aestivum) grown on a Clay Soil

J. Agric. Engng Res. (1999) 73, 297}309 Article No. jaer.1999.0423, available online at http://www.idealibrary.com on

Interactions of Straw Disposal Methods and Direct Drilling or Cultivations on Winter Wheat (¹riticum aestivum) grown on a Clay Soil D. G. Christian; E. T. G. Bacon; D. Brockie; D. Glen; R. J. Gutteridge; J. F. Jenkyn  Institute of Arable Crops Research, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK; e-mail: [email protected]  IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS18 9AF, UK; e-mail: [email protected] (Received 7 April 1998; accepted in revised form 16 February 1999)

A "eld experiment was started in 1979 on a clay soil to compare e!ects of baling to leave stubble only, chopping and spreading straw or burning on the yield of winter wheat (¹riticum aestivum). Results are reported for the period 1982}1988 with occasional references to earlier years. The crop was either direct drilled or sown after incorporation of straw or ash by cultivation to depths of 5, 15 or 25 cm. Tine cultivation was used until 1983 then replaced by ploughing. Averaged over a nine-year period, direct drilling into chopped straw caused a one-third reduction in yield compared to direct drilling where the straw was burnt. Over the same period, 5 cm deep straw incorporation resulted in one-"fth less yield compared to crops after burning. From 1985 to 1988, ploughing to 15 or 25 cm resulted in respectively 1 and 9% less yield if straw was incorporated rather than burnt. There was no evidence that these reductions were a consequence of consistent di!erences in pests or diseases. Temporary (1 year) burning after seven years of incorporating chopped straw increased average yield on tillage treatments by 17%. On this soil, ploughing at least to 15 cm is recommended to improve reliability of crop establishment. The average yields obtained from the straw returned treatments indicates that it was the presence of straw rather than the amount that most in#uenced yield.  1999 Silsoe Research Institute

1. Introduction Several "eld investigations on heavy soils in the UK have shown that leaving cereal straw on or near the soil surface can reduce the establishment and yield of a subsequent crop.1+3 In other "eld experiments where straw was removed by burning, the method of subsequent cultivation had little e!ect on yield.4+6 Ploughing of "netextured soils can be di$cult in both wet and dry seasons, and this can delay sowing and increase costs of production.7 Therefore, the adoption of simpli"ed methods of tillage, such as tine cultivation or direct drilling, can be an advantage in establishing crops. Ploughing techniques have developed over centuries of use. However, it is only relatively recently that simpli"ed methods of tillage, such as tine cultivation and direct drilling, have been introduced and needed evaluation over a succession of seasons in comparison with traditional ploughing and di!erent straw management methods. This was the main purpose of this experiment, 0021-8634/99/070297#13 $30.00/0

which was set up by the former Letcombe Laboratory in 1979. Continuous autumn cereals were grown to standardize the straw treatments. An interim report on crop growth and yield for the "rst period of the experiment (1980}81) has been published,8 but not the results of the later period between 1982 and 1988. This later part of the study (reported here) retained the same methods of straw disposal as the "rst period but included a greater range of cultivation methods and depths. The results are discussed in the context of the potential use of di!erent tillage systems where straw is retained in the "eld.

2. Materials and Methods 2.1. Site and soil The experiment was carried out on a non-calcareous clay soil (Lawford Series, verti-eutric gleysol) at a site in Oxfordshire (O.S. grid reference SU 394212). Particle size

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distribution was 21% sand (60 lm to 2 mm), 40% silt (2}60 lm) and 39% clay ((2 lm). Soil organic matter content (0}15 cm depth) was 2)3% w/w. A more detailed soil description has been reported elsewhere.4 The site was level and had secondary drainage provided by clay pipes laid 90 cm deep with a permeable back"ll and intersected by mole drains 2 m apart and 50 cm deep. The lateral spacing of drains is 44 m. Mole drains were redrawn in 1984.

2.2. ¹reatments and design Straw residues were either (i) burnt, or (ii) the cut straw was removed by baling, leaving only the stubble, or (iii) chopped and evenly spread over the plot. These methods of straw disposal were applied to the same areas of land throughout the experiment, as were the cultivation and direct drilling treatments, so that any cumulative e!ects could be observed. Cultivation methods and depths are given in Table 1. On the cultivated plots, ploughing replaced tine cultivation from autumn 1984. The reason for the introduction of greater depths of cultivation in autumn 1982 (15 cm) and 1984 (25 cm) was to see if a minimum depth of cultivation was necessary to overcome the adverse e!ects of straw on this soil type. The additional cultivation treatments were achieved by dividing areas already under cultivation. The new depth of tillage was always greater than that previously used. In autumn 1986, plots where chopped straw had been applied were split and the straw was burnt on one-half. After the harvest in 1987, these areas reverted to testing chopped straw.

The experimental design was four randomized blocks of three plots, split into two and later split into four subplots. Straw disposal methods were tested on main plots, which measured 20 m;32 m. Each main plot was divided into subplots, along the 32 m axis, for the cultivation treatments or direct drilling. During two periods, harvest years 1983 and 1984 (nitrogen split) and again in 1987 (1 year burn of chopped straw plots), the subplots were divided across their 20 m length to make two 8 m;10 m subplots. Spray paths for the application of fertilizers and sprays were located in the same position each year, 10 m apart and at right angles to the length of the subplots. Cultivators used were either spring-tined (5 cm depth) or rigid tine (15 cm depth) and the ploughs were either a conventional 3-furrow right-hand mouldboard plough with 35 cm bodies or a six-furrow shallow plough with 25 cm bodies; both ploughs were "tted with mouldboard extensions. Each year, winter wheat (¹riticum aestivum) was sown using a conventional disc coulter drill on cultivated land and a triple-disc coulter drill for direct drilling. Fertilizer was applied in the spring at rates appropriate for this soil and crop sequence as recommended by the Ministry of Agriculture, Fisheries and Food.9 In the autumns of 1982 and 1983, subplots were split; 22 kg N on one half and no N on the other half was applied at the time of sowing, and the N sub-plots were used in 1983 and 1984 for disease assessments. After the harvest year of 1984, no further nitrogen was applied in the autumn. Crop protection chemicals were used according to good agricultural practice. Details of agronomy are presented in Table 2.

2.3. Measurements Table 1 Experimental treatments 1980+1988 Crop year

Straw disposal main plots

Cultivation subplots

Nominal depth, cm



Burnt (B) Baled (Ba) Chopped and spread (Cs)

Direct drill Rotary tine

0 5



B, Ba, Cs ''

Direct drill Tine

0 5, 15

B, Ba, Cs '' '' No straw treatment applied

Direct drill Plough

0 5, 15, 25

1980 1981 1982 1983 1984



1985 1986 1987  1988

 Cs plots included a one-year comparison of burnt versus not burnt.

Crop establishment was assessed in the autumn, normally about 6}8 weeks after the crop was sown. Plant populations were assessed by counting those dug from 6;2;1)5 m rows on each subplot. Crops were sampled again at the start of stem elongation, Zadoks GS 3010 to estimate plant and shoot number and dry matter. Grainbearing ears were counted after cutting stems from 6;2;1)5 m rows on each plot. Ears derived from volunteer cereals identi"ed by variety were counted after ear emergence (Zadoks GS 60) using a #oating quadrat (50 cm;50 cm) which rested on the top of the crop. Ten counts were made on each subplot; where subplots were divided, "ve counts were made on each sub-subplot. Volunteer ears are expressed as a percentage of total ears counted in the summed quadrats. The numbers of seeds and seedlings killed by slugs were estimated at Zadoks GS 11-12, by digging four 25 cm lengths of drill row from each subplot, placing

I N TE R AC TI O N S O F S TR A W DI S P OS A L M E TH O D S AN D D I RE CT D R I L LI N G O N W I N TE R W HE A T

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Table 2 Details of cultural practices during the crop years 1982 to 1988 Crop year Harvest date of previous crop Straw incorporation date Incorporation method Variety of wheat Sowing date No. of seeds, m\ Fertiliser N in spring, kg/ha NH NO , applied in two   dressings

1982

1983

1984

1985

1986

1987

1988

24.8.81 28.8.81 Rotary tine M. Huntsman 29.9.81 402 148

10.8.82 14.8.82 Tine Avalon 12.10.82 471 193

8.8.83 13.8.83 Tine Avalon 28.9.83 373 200

10.8.84 22.8.84 Plough Avalon 26.9.84 404 203

29.8.85 11.9.85 Plough Avalon 9.10.85 426 227

5.9.86 22.9.86 Plough Mission 17.10.86 401 200

21.8.87 2.9.87 Plough Avalon 23.10.87 406 194

Ground limestone, 5 t/ha, 52% neutralizing value, was applied on 11 September 1986.

each sample in a plastic box, then hand-sorting the soil to "nd seeds and seedlings. Two types of lethal slug damage were recorded: (i) characteristically hollowed grain in which slugs had eaten the embryo and sometimes part of the endosperm, and (ii) shoots eaten through below ground level, killing the growing point. Slug numbers were estimated at monthly intervals just before and just after sowing by digging soil samples [25 cm;25 cm (square);10 cm (deep)] on each subplot and extracting slugs by a slow #ooding process.11 This technique is a more sensitive method than bait-trapping for detecting di!erences in slug populations between straw disposal treatments. From 1983 to 1988, crops were sampled in summer to assess diseases a!ecting the stem bases and roots. Following the changes in the experimental treatments in 1985 (see Table 1), samples were taken in each year from plots where straw was burnt, baled or chopped and then ploughed to 5, 15 or 25 cm or direct drilled after burning. Crops direct drilled after baling or chopping were not routinely assessed because of the presence of volunteer plants. Sampling normally took place between anthesis (Zadoks GS 65) and the grain milky ripe stage (Zadoks GS 75). Ten 20 cm lengths of row were dug from each subplot; "ve from each half of each subplot. The data for the two halves were normally combined before statistical analysis, except for the chopped straw plots in 1987, when one-half of the subplot was burnt. Samples were used to determine percentages of straws with eyespot (Pseudocercosporella herpotrichoides), graded as slight, moderate or severe,12 and with sharp eyespot (Rhizoctonia cerealis) and brown foot root (Fusarium spp.). Take-all (Gaeumannomyces graminis var. tritici) on individual root systems was graded as slight, moderate or severe and the data used to calculate a take-all rating (TAR) for each plot [% plants with slight infection#2;(% moderate)#3;(% severe); maximum"300].13,14 For statistical analysis of data ex-

pressed as percentages, a mean value (P in %) was calculated for each plot and a logit transformation used to normalize distribution (e.g. 0)5 l [(P#0)05)/(100)05!P)],  a formula which avoids problems arising from P values of 0 or 100%. Symptoms of Cephalosporium stripe (Cephalosporium gramineum) were assessed in May or June when they were fully expressed. Four rows of 50 cm lengths were examined and shoots with visible symptoms counted. Harvestable grain yield was measured using a plot combine (Claas Compact 25). At least 42 m were harvested per plot, except in 1987 when 17 m were harvested on each half of the chopped straw plots divided for part burning. The moisture content of grain was measured and yields adjusted to 85% dry matter.

3. Results 3.1. Crop growth and development The longest period of continuous treatment was for crops established by direct drilling or after shallow tillage to 5 cm depth (1980}1988). In both of these treatments, the plant population was generally smaller where whole straw residues or stubble were present in the seedbed than where straw was burnt (Table 3). In 1988, the abundance of volunteers made it impossible to measure plant establishment accurately, so no data are presented. In 1987, establishment following cultivation where residues were present was generally more successful than after burning but some of the di!erence may have been due to fewer volunteers where straw had been burnt. Increasing the depth of cultivation did not signi"cantly a!ect plant establishment. There was no evidence that the time between straw incorporation and sowing date a!ected plant establishment. Between 1982 and 1985, crop establishment on the chopped straw treatment was substantially reduced by

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Table 3 Number of plants established per m2 for a combination of three straw treatments and three depths. B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Cultivation depth, cm (tine to 1984, ploughed afterwards)

Number of plants, m!2 Straw

1983

1984

1985

1986

1987

1988

0 

B Ba Cs

336 205 177

299 216 155

277 219 194

364 268 205

341 255 325

329 * 424

5

B Ba Cs

295 241 194

276 272 243

322 273 150

351 331 347

326 * *

395 * 508

15

B Ba Cs

* * *

306 267 273

306 286 183

363 348 373

288 368 395

383 * 427

25

B Ba Cs

* * * 25)4

* * * 21)7

* * * 28)8

349 393 375 20)0

306 354 356 37)0

347 * 417 50)2

SED

 Crop direct drilled 1988 data are not presented because volunteers made it di$cult to assess plant establishment accurately.

slugs, which killed both seeds and emerging seedlings. Crops sown by direct drilling on both the baled and chopped straw treatments showed the greatest damage (Table 4). In contrast, where the straw was burnt, there was little damage to direct-drilled crops or those sown after cultivation. In 1983 on the direct-drilled plots, the estimated kill by slugs on the chopped straw treatment plots, compared to the straw-burnt treatment, would

account for much of the di!erence in plant population between these treatments (Tables 3 and 4). In subsequent years, losses attributed to slug attack declined. Virtually, no slug damage was recorded in 1987, when few slugs were present. Slug damage on cultivated plots was less than that on direct-drilled plots, except on burnt plots in 1985, and the population of slugs was always lower on cultivated plots than on direct-drilled plots.

Table 4 Estimated percentage of seed and seedlings killed by slugs for di4erent methods of straw disposal and depth of cultivation. (Figures in parentheses are numbers of slugs mⴚ2 estimated from the mean number on two sampling dates just before or just after sowing). Dashes indicate where no data are available Slug damage to seed and seedlings, %; (no. of slugs, m!2)

Crop year Straw burnt

Baled straw

Chopped straw

0 cm(a)

5 cm

15 cm

25 cm

0 cm(a)

5 cm

15 cm

25 cm

0 cm(a)

5 cm

1983

5)9 (7)

*

*

*

31)4 (86)

*

*

*

46)8 (33)

*

*

*

18)8

1984

6)0 (5)

4)6 (2)

4)6 (3)

*

25)4 (63)

12)1 (!)

4)3 (!)

*

36)2 (77)

9)7 (59)

13)5 (52)

*

12)2

1985

0)0 (12)

1)4 (0)

0)4 (8)

0)4 (8)

12)5 (52)

1)0 (28)

0 (32)

1)3 (40)

26)4 (140)

6)5 (36)

7)3 (84)

3)9 (36)

4)9

1986

0)8 (81)

0)5 (5)

0 (6)

1)0 (2)

8)4 (89)

0 (17)

0 (40)

0 (9)

14)1 (120)

3)5 (14)

1)4 (30)

0)7 (10)

5)3

1987

0 (7)

0 (1)

0 (0)

0 (0)

0)8 (8)

0 (!)

0 (!)

0 (8)

0)9 (14)

0 (!)

0 (8)

0)7 (6)

1)9

 Crop direct drilled.

15 cm 25 cm L.S.D.

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Table 5 Number of shoots mⴚ2 at stem elongation. B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Cultivation depth, cm

Number of shoots, m!2 Straw disposal 1982

1983

1984

1985

1986

1987

0 

B Ba Cs

1056 645 507

970 669 207

1080 795 727

1536 1520 1307

889 * *

869 * 1077

5

B Ba Cs

1167 854 841

1156 882 795

1069 784 617

1597 1667 1720

1036 1337 1194

845 * 743

15

B Ba Cs

* * *

1075 723 791

1123 752 523

1413 1449 1452

750 849 1058

757 * 968

25

B Ba Cs

* * * 134)0

SED

* * * 57)8

* * * 164)8

1365 1208 1310 109)1

773 785 919 114)6

708 * 829 137)4

 Crop direct drilled.

Between 1982 and 1984, shoot population (Table 5) and dry matter yield (Table 6) at stem elongation (Zadoks GS30) were less after baling or chopping and spreading straw than after burning. However, individual shoot weights were heavier than after burning (data not presented). In 1986 and 1987, shoot number tended to be greater on chopped straw plots than on burnt straw plots at each depth of cultivation. In 1982 and 1983, and

non-signi"cantly in 1984, ear populations (Table 7) were greatest in crops established after burning straw, re#ecting higher shoot numbers recorded at stem elongation (Table 6). From 1985 to 1988, di!erences in ear number between treatments were generally less than in previous years, probably due to the presence of volunteer cereals, although in all years, except 1987, these were di$cult to identify with certainty.

Table 6 Dry weight of shoots in g mⴚ2 at stem elongation. B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Cultivation depth, cm

Dry weight of shoots, g/m2 Straw disposal 1982

1983

1984

1985

1986

1987

0 

B Ba Cs

234 124 70

162 74 17

130 81 59

135 82 68

70 * *

56 * 60

5

B Ba Cs

239 144 122

209 152 115

154 68 47

128 151 162

77 128 110

60 * 51

15

B Ba Cs

* * *

210 98 122

170 93 66

117 134 135

61 82 82

58 * 68

25

B Ba Cs

* * * 28)4

* * * 12)7

* * * 13)1

100 103 108 11)2

58 69 76 11)0

53 * 56 8)4

SED  Crop direct drilled.

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Table 7 Number of fertile ears mⴚ2. B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Cultivation depth,cm

Number of fertile ears, m!2 Straw disposal 1982

1983

1984

1985

1986

1987

1988

0 

B Ba Cs

443 357 316

512 388 273

471 465 457

526 520 470

468 * *

579 632 561

414 * 195

5

B Ba Cs

423 339 344

540 510 416

507 481 448

533 507 492

496 665 659

569 570 553

454 * 406

15

B Ba Cs

* * *

546 473 457

502 456 392

502 474 524

436 539 535

579 516 558

465 * 470

25

B Ba Cs

* * * 24)6

* * * 26)6

* * * 26)9

466 454 453 20)8

480 527 500 43)1

561 563 536 27)7

469 * 474 38)1

SED  Crop direct drilled.

Volunteers were present in all treatments and were most numerous on plots where some straw residues remained (Fig. 1). In 1987, after direct drilling or shallow ploughing to 5 cm on chopped straw plots, more than one ear in "ve was derived from a volunteer plant compared to only about one ear in 10 where straw was burnt. Ploughing to 15 or 25 cm signi"cantly reduced volunteers compared to shallower cultivation or direct drilling.

Volunteer populations on the baled and chopped straw treatments were similar, suggesting that most volunteers originated from seed shed on to the soil surface. Plots where straw was burnt for the "rst time since 1979 behaved in a similar manner to plots continuously burnt since then (Fig. 1). Only the burnt and the chopped and spread plots were assessed for volunteers in 1988. Because the variety sown for 1988 was the same as the

Fig. 1. Ewect of straw disposal method, depth of ploughing, and direct drilling on the occurrence of volunteer cereals in the xrst year and second year after the parental crop

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variety used prior to 1987, the volunteers counted on these plots in 1988 had been carried over from the previous year. Over 14% of ears on the chopped straw direct-drilled plots were from volunteer plants. Volunteers on chopped straw plots were substantially reduced by ploughing. The burnt straw plots had low percentages of volunteers irrespective of the depth of cultivation.

3.2. >ield In most years, the highest yield was on plots where straw had been burnt (Table 8). The largest di!erences in yield were frequently between crops grown after burning straw and those after chopping and spreading straw. Crops that were direct drilled into chopped straw generally gave the lowest yield during the nine-year study. Cultivating soil to 5 cm depth to incorporate chopped straw resulted in higher yields than direct drilling through chopped straw in "ve of the nine years. The introduction of tine cultivation to 15 cm to incorporate straw after the harvest of 1982 gave slightly higher yields than tine cultivation to 5 cm depth. When ploughing replaced tine cultivation in the autumn of 1984, di!erences in yield as a result of di!erent depths of tillage became less. Yields after baling were generally intermediate between those after burning and after chopping straw, especially following direct drilling or 5 cm deep cultivation (Table 8). The average yields (1980}88) show that direct drilling into chopped straw resulted in 32)5% less

grain than direct drilling after burning straw, and, after 5 cm deep tillage, crops yielded 18% less following chopping the straw compared to burning. Irrespective of the method of straw management, a downward trend in yield was evident from 1982 until the end of the study and di!erences in yield between the treatments narrowed over the same period, except after direct drilling into chopped straw. Temporary burning for one season improved yield on all tillage plots by an average of 0)9 t ha\ compared to those obtained where straw was again returned (Table 9). The yields were also greater than were obtained after the sequence of seven years of burning straw before cultivation, except after 25 cm tillage, where yields were similar. On the direct-drilled plots, the increase in yield was 0)4 t/ha, which was less than after long-term burning. In the following autumn, the once-burnt areas reverted to chopped straw and only on direct-drilled plots was a residual bene"t apparent. However, it should be noted that yields were very low in that year where the crop had been direct drilled through chopped straw probably as a result of low ear population (Table 7).

3.3. Diseases Stem-base diseases and take-all were often least severe in plots with most surface residues (i.e. those which were direct drilled through surface straw). However, establishment in these plots was often poor and the e!ects

Table 8 In6uence of straw disposal and cultivation method on the yield of winter cereals (t/ha 85% DM). B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Cultivation depth(a), cm

Yield (c) at 85% DM, t/ha

Straw disposal 1980

1981

1982

1983

1984

1985

1986

1987

1988

Average

0


B Ba Cs

8)8 7)4 6)3

8)7 8)0 7)6

10)1 8)1 5)6

9)0 5)7 4)6

8)3 5)1 6)9

6)3 4)0 3)0

7)8 * 6)0

7)2 5)8 6)2

6)1 3)6 2)1

8)0 6)0 5)4

5

B Ba Cs

9)2 8)8 8)2

9)0 7)9 8)1

9)7 8)0 7)3

9)2 7)6 6)5

8)9 7)5 4)6

5)8 6)1 5)5

7)6 7)2 6)5

5)8 5)5 5)7

5)9 5)6 5)7

7)9 7)1 6)5

15

B Ba Cs

* * *

* * *

* * *

9)6 7)5 7)0

9)3 6)4 5)7

5)6 5)7 5)6

7)2 7)2 7)2

5)2 5)7 5)4

5)5 5)4 5)0

7)1 6)3 6)0

25

B Ba Cs

* * * 0)33

* * * 0)41

* * * 0)54

* * * 0)52

* * * 0)63

5)3 5)4 4)7 0)35

7)1 6)2 7)3 0)40

6)1 4)8 5)0 0)52

5)9 5)2 5)2 0)43

6)1 5)4 5)6

SED

  Achieved with tined implements until 1984 then by ploughing; 
 Crops direct drilled;  Includes data taken from Graham et al.

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Table 9 E4ect on yield in t/ha of interrupting straw incorporation with burning and the residual e4ect the following year Cultivation depth, cm

Yield, t/ha 1987

1988

Burnt (7th year)

Chopped and spread (7th year)

Straw burnt for 1 year

Burnt (8th year)

Chopped and spread (8th year)

Burnt in 1987

7)2 5)8 5)2 6)1

6)2 5)7 5)4 5)0 0)72

6)6 6)3 6)6 6)0

6)1 5)9 5)5 5)9

2)1 5)7 5)0 5)2 0)43

2)8 5)5 5)1 4)8

0  5 15
 25
 SED

  Crop direct drilled; 
 Tillage methods and depth changed during the experiment (see Table 1).

probably re#ect the small plant densities rather than e!ects of the straw residues per se. When considering e!ects of straw and cultivation methods, data from these plots have, therefore, been ignored; after 1984, many of these plots were not sampled. Di!erences between years in disease severity are illustrated by the means across treatments shown in Table 10. Eyespot was variable despite fungicides being applied at c. Zadoks GS30 in each year. Incidence of the disease increased between 1983 and 1985, perhaps re#ecting increased resistance to the MBC fungicide (carbendazim) that was then being applied.15 To control MBC-resistant strains, the fungicide applied at Zadoks GS30 in 1986}1988 included prochloraz, but the disease was more severe in 1987 than in any other year. Sharp eyespot occurrence was also variable but, with the exception of 1986, tended to be inversely related to eyespot, as previously observed.16 Brown foot rot was usually very slight but was unusually prevalent in 1988. As expected in consecutive crops of winter wheat (following oats (Avena sativa) in 1981), take-all tended to increase but did so relatively slowly. It was probably severe enough to decrease yield at least in 1985, 1987 and 1988.

Interactions between straw and depth of cultivation treatments were sometimes detected but were mostly explained by the e!ects of surface residues. Comparisons between treatments where the straw was burnt or chopped and incorporated (Table 11) are, therefore, based on plots cultivated to 5 or 15 cm. Eyespot was always more severe where the straw had been burnt than where it had been chopped and incorporated (Table 11). Sharp eyespot was similarly increased by burning the straw in 1984 and 1987 but was inconsistently a!ected in other years. Burning straw in 1987, on subplots where the straw had previously been chopped and incorporated, signi"cantly increased both eyespot and sharp eyespot, compared with subplots where straw incorporation continued (eyespot: 29)3 versus 19)0%; sharp eyespot 9)3 versus 3)7% straws a!ected). In 1988, when straw on both subplots was chopped and incorporated, neither disease was a!ected by burning in the previous year. In 1983, take-all was slightly, but not signi"cantly, more severe where the straw had been chopped than where it had been burnt. In the following four years it was consistently less severe in the chopped plots than in the burnt, signi"cantly so in 1984. In 1988, amounts of take-all were very similar. For all three

Table 10 Diseases a4ecting stem bases (% straws) and roots in 1983+88. (Data, apart from take-all ratings, were transformed to logits for analysis and the values shown are back transformed means for each year) Crop year Eyespot, % straws Sharp eyespot, % straws Brown foot rot, % straws Take-all, % plants severe Take-all rating

1983

1984

1985

1986

1987

1988

4)6 11)8 2)3 6)1 105

8)0 5)1 0)7 3)3 107

17)2 11)4 0)4 12)4 157

4)0 3)8 0)4 2)8 105

23)9 4)5 0)0 19)7 170

5)2 13)6 22)2 19)8 173

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Table 11 Eyespot, sharp eyespot and take-all in plots where straw was burnt, or chopped and spread each year. (Values, apart from take-all ratings, are logit transform values with, in parentheses, the corresponding percentage values obtained by back transformation, and are the means for plots cultivated to 5 or 15 cm.) Crop year

1983

1984

1985

1986

1987

1988

Mean !1)04 (11)1) !1)27 (7)3) *

Eyespot, % straws Straw burnt Straw chopped SED

!1)13 (9)4) !1)59 (4)0) 0)145

!1)08 (10)4) !0)72 (19)2) !1)20 (8)3) !0)78 (17)2) 0)120 0)111

!1)57 (4)1) !1)65 (3)6) 0)177

!0)54 (25)4) !0)83 (16)1) 0)150

!1)20 (8)3) !1)59 (4)0) 0)201

Sharp eyespot, % straws Straw burnt Straw chopped SED

!0)84 (15)6) !0)48 (27)7) !0)86 (15)1) !0)86 (15)3) !1)68 (3)4) !0)70 (19)8) 0)142 0)173 0)247

!1)85 (2)4) !1)73 (3)0) 0)238

!1)36 (6)2) !1)76 (2)9) 0)203

!0)85 (15)5) !1)04 (11)1) !0)92 (13)6) !1)27 (7)3) 0)136 *

Take-all rating Straw burnt Straw chopped SED

110 126 10)8

117 109 14)0

195 156 21)0

168 169 13)6

136 111 9)0

186 165 13)9

diseases, average amounts in plots where the straw had been baled and removed were intermediate between those in the burnt and chopped plots but were not consistently so in each year. E!ects on eyespot and sharp eyespot of di!erent cultivation depths where the straw was burnt were mostly small and inconsistent (Table 12). However, there were signi"cant e!ects on eyespot in 1987 and 1988 and sharp eyespot in 1988, both diseases being least severe in direct-

152 139 }

drilled plots. Take-all was invariably most severe in plots that were cultivated either to 5 or 15 cm and usually least severe in those ploughed to 25 cm or direct-drilled. Cephalosporium stripe was more prevalent on the chopped straw and baled plots than following burning with more infected shoots found on direct-drilled plots compared to cultivated areas, except in 1984 (Table 13). However, cultivation depth did not greatly in#uence the incidence of the disease, which occurs randomly in the

Table 12 Eyespot, sharp eyespot and take-all in plots cultivated to di4erent depths after burning the straw in each year. (Values, apart from take-all ratings, are logit transform values with, in parentheses, the corresponding percentage values obtained by back transformation) Crop year

Cultivation depth (cm)

1983

1984

1985

1986

1987

Eyespot, % straws

0  5 15 25 SED

!0)87 (15)0) !1)09 (10)2) !1)18 (8)5) * 0)251

!0)97 (12)5) !1)14 (9)2) !1)01 (11)6) * 0)170

!0)68 (20)3) !0)78 (17)3) !0)65 (21)3) !0)79 (17)2) 0)148

!1)79 (2)7) !1)45 (5)2) !1)70 (3)2) !1)43 (5)4) 0)139

!0)94 (13)1) !0)62 (22)3) !0)46 (28)6) !0)40 (31)1) 0)280

!1)80 !1)19 !1)21 !1)19 0)260

Sharp eyespot, % straws

0  5 15 25 SED

!0)70 (19)6) !0)89 (14)3) !0)79 (16)9) * 0)200

!0)55 (25)0) !0)43 (29)8) !0)53 (25)7) * 0)341

!0)90 (14)1) !1)06 (10)8) !0)67 (20)7) !0)67 (20)8) 0)272

!2)28 (1)0) !1)94 (2)0) !1)76 (2)9) !1)87 (2)3) 0)332

!1)92 (2)1) !1)37 (6)1) !1)35 (6)3) !1)75 (2)9) 0)324

!1)16 (9)0) !0)96 (12)7) !0)73 (18)7) !0)76 (18)0) 0)160

Take-all, rating

0  5 15 25 SED

83 100 120

132 144 127

138 176 196 156 26)8

99 115 120 106 19)3

108 192 198 170 35)0

125 173 164 155 19)7

 Crop direct drilled.

* 18)3

* 17)3

1988 (2)6) (8)4) (8)2) (8)5)

306

D . G . C HR I S T IA N E¹ A¸ .

Table 13 Percentages of shoots exhibiting symptoms of cephalosporium after di4erent methods of straw disposal and tillage. B, Ba, and Cs refer to burnt straw, baled straw and chopped straw, respectively Depth of tillage, cm

Cephalosporium symptoms, % shoots 1984

0  5 15 25 SED

1985

1986

1987

1988

B

Ba

Cs

B

Ba

Cs

B

Ba

Cs

B

Ba

Cs

B

Ba

10)1 13)9
 * *

31)8 18)4
 * * 6)39

41)9 44)3
 * *

4)7 7)7 10)9 5)2

50)8 12)9 10)9 19)9 6)18

65)8 20)5 13)4 20)2

10)3 6)9 4)0 2)4

31)7 8)4 10)3 10)7 4)22

29)2 14)0 11)3 14)3

0)9 0)6 0)2 0)6

* * * * 0)67

1)4 2)9 0)8 1)6

1)2 0)2 0)9 0)5

* * * * 1)32

Cs 11)8 0)9 0)9 0)9

  Crops direct drilled; 
 tined, thereafter ploughed.

crop, making it impractical to estimate the e!ect of Cephalosporium on yield when other diseases are present.

4. Discussion Over the period of study, the average yield was always greater where straw was burnt irrespective of tillage. Whatever the factors responsible for the di!erences in crop establishment, growth and yield where residues were present, they did not a!ect each crop to the same degree each year. The e!ect of residues on cultivation, and crop performance during this study are discussed below in descending order of probable importance. Using a tined cultivator, it is di$cult to achieve shallow incorporation of considerable quantities of straw and yet also achieve a satisfactory seedbed. The mixing action of the tines does not bury straw well and, as a result, straw can be poorly incorporated in the cultivated layer with the majority on or near the soil surface. The straw needs to be chopped; short straw is incorporated more readily with a tined implement than long straw.17 When shallow ploughing (5 cm deep) replaced tine cultivation in 1985, a greater proportion of the straw was incorporated, but the depth of soil covering the straw was insu$cient to ensure that the straw remained buried after secondary tillage to prepare a seedbed. As a result, the seedbed after 5 cm deep ploughing was similar in appearance to that when 5 cm deep tine cultivation was used. Incorporating chopped straw to 15 cm resulted in slightly better plant establishment compared to shallower depths (Table 3). Shallow ploughing has been shown to be a satisfactory method of straw incorporation in one study in Scotland.18 Deeper cultivation, particularly by ploughing, almost eliminated yield di!erences within years in this study but a consequence of the heavy yields

during the period 1980}1984 was that a large amount of straw was present, making it more di$cult to e$ciently incorporate residues. However, the amount of straw incorporated was unlikely, in itself, to have greatly in#uenced yield, as incorporation of up to four times the normal straw yield did not reduce grain yield in an adjacent experiment at this site.19 Extensive "eld trials on the e!ects of tillage on straw disposal carried out on a range of soils elsewhere in England during most of the period covered by this experiment showed that ploughing-down straw and stubble did not greatly a!ect crop yield on clay soils.3 It is unlikely that reduced yield is associated with nutrient de"ciency since phosphorus, potassium and other minerals are returned with both incorporating and burning straw. It was di$cult to direct drill seeds where straw had been chopped and spread. Straw a!ected the e$ciency of sowing by either blocking the drill coulters or by preventing the coulter from properly penetrating the soil, with the result that seed/soil contact was poor. Straw, particularly when fresh and dry, can be di$cult to compress and absorbs some of the downward pressure of the disc coulter. It was found that this e!ect could be overcome, to some extent, by adding weight to the drill and drilling using a slow forward speed. However, the work rate was too low to be acceptable commercially. Other designs of drill have been tested elsewhere on a similar clay soil, but none o!ered bene"ts that justi"ed changing from the disc design adopted.20 Unpublished data from an adjacent "eld experiment where straw was removed before direct drilling and then replaced resulted in more plants being established, but the e!ect on yield was inconsistent in the two years that it was tested. Between autumn 1982 and 1985 (crop years 1983 to 1986), slug attack had a signi"cant e!ect on crop establishment, particularly on treatments with straw residues where more slugs were found than on burnt straw

I N TE R AC TI O N S O F S TR A W DI S P OS A L M E TH O D S AN D D I RE CT D R I L LI N G O N W I N TE R W HE A T

treatments. Slug damage probably accounted for most of the di!erence in emergence between treatments in the crops established between 1982 and 1985 (Table 3). In the autumn of 1986 and 1987 (crop years 1987 and 1988), when slug damage was slight, treatment di!erences in the number of plants disappeared or were reversed, with more plants present on treatments with straw residues than on burnt straw plots. This was mainly attributed to a greater number of volunteers. Glen et al.21 suggested that the slug population increased in the presence of straw because it provides an additional food source to the sown crop, and this may contribute to slug survival. Further, straw may hide slugs from predators. It is not known why the slug population declined in crop year 1986 and remained low in 1987 and 1988. Cultivation had a greater e!ect in reducing slug damage than expected from its e!ects on slug numbers alone. In directdrilled and shallow-cultivated areas, the poor seed/soil contact resulting from the presence of surface straw residues undoubtedly resulted in seeds being much more readily found by slugs than in deeper cultivated areas where seeds could be sown at the correct target depth in close contact with the soil. Close seed/soil contact resulting from "ne tilth, greater drilling depth and compaction are all known to reduce the ability of slugs to "nd and kill wheat seeds.22+24 Drilling at greater depth has been shown to increase wheat yield as well as reducing slug damage.25 Because much damage occurred to a crop before it had fully emerged, frequent, careful observation of slug activity is necessary so that controls can be applied in good time. Toxic leachates from decomposing straw have been suggested as another factor that may in#uence establishment and growth.26 However, in the "eld, it would be di$cult to demonstrate the e!ect of toxins over other in#uences. Therefore, if there is an e!ect, it is unlikely that its importance can be quanti"ed. Immobilization of nitrogen (N) by the decomposition of straw has been linked to yield reductions.27, 28 However, in the "rst part of this study, nitrogen applied in the autumn, as well as in the spring, did not greatly in#uence yield.8 In a separate experiment at this site, straw reduced N uptake during early growth but the e!ect was transient and yield was not a!ected.17 The plough has traditionally been used as an implement of good crop hygiene, burying straw, weed seeds, pests and pathogens to reduce their in#uence on the following crop. Burning provides an alternative but it can be less e$cient than ploughing in the control of weeds and volunteers.29 In this experiment, volunteers were more abundant in the absence of burning especially when a crop was direct drilled or sown after shallow incorporation of straw (Fig. 1). This may be due to more volunteer grains on the surface or residing in the seeded layer.

307

Volunteers can be worse in crops with a low plant population because there is less competition from the sown crop.30 Despite fungicides, eyespot was prevalent in some years. However, its symptoms and those of sharp eyespot were mostly slight; thus, it is unlikely that either had much e!ect on yield. Even if they were damaging, they do not explain the e!ects of incorporating straw on grain yield because both diseases tended to be more severe where straw was burnt than that where it was not. The explanation for this is uncertain but similar e!ects have been observed in other experiments.31 Take-all was almost certainly damaging in some years but, again, there was no evidence that it explained the e!ects of the straw treatments on grain yield. E!ects of cultivation treatments in plots where the straw had been burnt (and where results were not, therefore, in#uenced by the presence of crop debris) were unexpected. Thus, take-all was always much less severe in the direct-drilled plots than in the cultivated ones, as previously reported.32 In contrast, the disease also tended to be less severe after ploughing to 25 cm depth than after shallower ploughing. These apparently contradictory results may be explained by the relatively faster destruction of inoculum in the microbiologically rich upper layers of the soil in direct-drilled plots than in the cultivated and, conversely, its burial by deep ploughing.33 Cephalosporium was site-speci"c and not found on other parts of the "eld where the experiment was located. It is unusual to record high levels of the disease in cereals in the UK,34 but its ability to infect may have been aided by root damage caused by slugs.35 Its incidence is closely linked to the amount of straw present and, according to Bruehl,36 monoculture and leaving straw on the surface, or not ploughing deeply encourages its survival. In this study, the decline in the incidence of the disease could have resulted from the decline in the population of slugs recorded on the experiment or, alternatively, from the application of lime in 1986 to correct acidity.37

5. Conclusions These experiments have shown that the best yields of winter cereals were achieved when straw was burnt, irrespective of tillage. However, this option is no longer open to farmers in the UK. The results obtained provide strong evidence that, in heavy clay soils, sowing cereals after shallow cultivations or by direct drilling in the presence of straw residues is unreliable and may restrict both early crop growth and yield. The causes of poor crop establishment were not clearly identi"ed and may have varied with the season. It is probable that there was more than one cause including poor seed burial and slug

308

D . G . C HR I S T IA N E¹ A¸ .

damage and they were interactive. Ploughing at least to 15 cm depth, is recommended in order to improve reliability in crop establishment, minimize the presence of volunteer cereals and to help maintain crop yield and product purity. Diseases probably a!ected yield but they are unlikely to have been entirely responsible for the large e!ects of straw and tillage that were detected. The average yields, obtained following di!erent straw management systems, indicate that it was the presence of straw, rather than the mass of straw present, which most in#uenced yield.

8

9

10

11

12

Acknowledgements 13

We thank Mr M. E. S. Dart for the use of the site at North"eld Farm, former colleagues at the Letcombe Laboratory, especially Mrs D P Miller and Ms Caroline Nash, for valuable technical assistance, and Dr R Q Cannell, now at Virginia Polytechnic Institute and State University, Blacksburg, USA, for helpful advice. We acknowledge the contribution to the "eld agronomy made by the late Mr. M. R. E. Bulpitt, and technical assistance from Mr N. F. Milsom and Mr C. N. Wiltshire in estimating slug numbers and damage. IACR receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom.

14

15

16

17

18

References 1

2

3

4

5

6

7

Ellis F B; Lynch J M &&Why burn straw?''. ARC Research Review, 1977, 3, 29}33 Oliphant J M The e!ect of straw and stubble on the yield of winter wheat after cultivations or direct drilling. Experimental Husbandry, 1982, 38, 60}68 Lord E I &&Yields'' in Changing Straw Disposal Practices. HGCA Research Review (11). London: Home-Grown Cereals Authority, 1988 Cannell R Q; Ellis F B; Christian D G; Graham J P; Douglas J T The growth and yield of winter cereals after direct drilling, shallow cultivation and ploughing on noncalcareous soils, 1974}80. Journal of Agricultural Science, Cambridge, 1980, 94, 345}359 Ellis F B; Christian D G; Cannell R Q Direct drilling, shallow tine cultivation and ploughing on a silt loam soil, 1974}80. Soil Tillage Research, 1982, 2, 115}130 Christian D G; Bacon E T G A long-term comparison of ploughing, tine cultivation and direct drilling on the growth and yield of winter cereals and oilseed rape on clayey and silty soils. Soil Tillage Research, 1990, 18, 311}331 Patterson D E; Chamen W C T; Richardson C D Long-term experiments with tillage systems to improve the economy of cultivations for cereals. Journal of Agricultural Engineering Research, 1980, 25, 1}35

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