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Wheel Traffic Effects on Tillage Draught
J. agric. Engng Res. (2000) 75, 3759382 doi:10.1006/jaer.1999.0516, available online at http://www.idealibrary.com on
Wheel Tra$c E!ects on Tillage Draught J. N. Tullberg Farm Mechanisation Centre, University of Queensland Gatton, Queensland 4343, Australia; E-mail: [email protected] (Received 29 December 1997; accepted in revised form 12 November 1999)
Tractor and/or implement wheels precede ground tools, and compact soil immediately before it is loosened in most tillage and planting operations. The work reported here was designed to quantify the impact of preceding wheel tra$c on tine draught and the energy requirements of tillage, as part of a programme to assess the bene"ts of controlled tra$c operation. Instrumented tines on a three-point linkage toolbar were used behind a 70 kW two-wheel drive tractor to measure the &tra$c e!ect' of wheels on the draught of the tillage tools following them in a clay soil. Under conditions broadly representative of Australian grain production, the draught of chisel and sweep tines operating behind tractor or implement wheels was increased by a factor of approximately two, compared with identical tines una!ected by preceding wheel tra$c. These data can be used to demonstrate that the tra$c e!ect on trailed tillage implements can increase total draught by more than 30%. This can be de"ned as the &tra$c penalty' of the operation, and indicates the extent to which a conventional tractor/implement system generates its own workload. A &tra$c e$ciency' parameter is proposed to quantify the impact of tractor and implement tra$c e!ects, and used to show that approximately 50% of a tractor's power output can be dissipated in the process of creating and disrupting its own wheel compaction. These results can be used to explain the reduction in tillage energy which occurs in controlled tra$c systems, and indicate other approaches to improving overall tractor/tillage system e$ciency. They are also the basis of speculation about the relationship between tractive e$ciency, tra$c e$ciency and soil structure damage. 2000 Silsoe Research Institute
1. Introduction The additional e!ort required to disturb soil that has been compacted by feet or wheels is common knowledge to all who till or dig the soil. Traditional agricultural systems such as those described by Chi and Zuo (1988) sometimes avoided this energy penalty by maintaining separate zones for tra$c and crop growth, but this is not easily achieved over the full cycle of operations involved in current crop production systems. The negative e!ects of tra$c on in"ltration, tilth and penetration resistance of clay soils in Australia were "rst quanti"ed by Arndt and Rose (1966), who advocated the use of improved tra$c systems to minimize the problems. The essence of the problem is well known. Optimum conditions for crop production, that is soft, friable and permeable soil, are quite unsuitable for e$cient tra$c and traction, and vice versa. Wheel tra$c increases soil 0021-8634/00/040375#08 $35.00/0
strength and the draught requirement of subsequent tillage, while tillage reduces soil strength and the e$ciency of subsequent traction. Where "eld tra$c follows a di!erent pathway for each of a series of operations, the processes of tillage and tra$c are contradictory. These contradictions are avoided in controlled tra$c farming (Taylor, 1983), where all "eld tra$c is con"ned to permanent laneways, and all crops are grown in permanent beds. This system allows soil conditions in the beds to be optimized for crop production, and the laneways optimized for traction. The advantages of controlled tra$c were demonstrated in the southeast United States by Cooper et al. (1969) and include an indirect energy economy which occurs because there is less need for deep tillage. The direct e!ect occurs because noncompacted soil requires less tillage energy than compacted soil, and traction is more e$cient when tyres are working on compacted permanent tracks.
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The penalties of wheel tra$c compaction and bene"ts of controlled tra$c have been demonstrated by a number of researchers, including Voorhees (1979), Tullberg and Murray (1988), and Chamen et al. (1990), but more recently Burt et al. (1994) concluded that tra$c had no impact on the energy required for soil preparation and planting of cotton, when using the equipment and practices regarded as standard for farmers in the southeast USA. They acknowledged, however, that their results could be a!ected by the tendency of secondary tillage equipment to operate at greater depth in non-tra$cked soil. Soil compaction e!ects generally ameliorate with time, so the most severe e!ects of tra$c, including those on tillage energy, should be observed in the performance of tillage tools disturbing soil immediately after tra$c. Tractor drive wheels are likely to be most important in this respect, although implement wheels providing depth control and frame support might also have some e!ect, particularly in extensive agriculture where trailed (rather than three-point linkage mounted) equipment is common. There a substantial literature concerned with soil compaction, traction, tillage, and their interactions with soil properties (Soane & van Ouwerkerk, 1994; Burt, 1993; McKyes, 1984; Jayawardane & Stewart, 1995; for example). The impact of the traction process on tillage e$ciency within common tractor/tillage systems nevertheless appears to have received little attention. This is surprising when wheels cover 15}40% of implement width in most individual "eld operations, and total annual tra$c intensity (total area a!ected by tra$c/crop area) is in the range from 5)0 to 0)5. (Kuipers & van de Zande, 1994). The object of this work was to assess the e!ect of tractor and implement wheels on the draught (i.e. horizontal soil force) of the tillage tools following immediately behind them, and to use these data to quantify the impact of "eld tra$c on the power requirements of tillage.
2. Methods and Materials Draught-sensing tines were designed using chisel plough shanks attached to parallel link assemblies (Fig. 1). Rearward movement of the tines was restricted by shear beam force transducers (SKT model 1500) monitored by a data logger (Datataker model DT100). The draughtsensing tines were "tted with common proprietary shares: either &chisels', curved reversible cast shares of 53 mm width normally used for primary tillage (Howard H76772 Buster Points); or &sweeps', low pro"le shares pressed from 6 mm steel, 450 mm wide, 70 mm shank width, 30 mm wing lift, normally used for weed control and seedbed preparation (McKay 15918K1).
Fig. 1. Draught-sensing tine assembly: a, parallel links; b, transducer; c, tine shank; d, chisel or sweep; e, toolbar
The draught sensing tines were mounted on a 4 m wide three-point linkage toolbar (e) "tted with adjustable depth control wheels (f ) at its extremities (Fig. 2). The toolbar was mounted on the three-point linkage of a tractor modi"ed for work in permanent wide beds, with front and rear wheel track centres set to 3 m, and stabilizers to prevent lateral movement of the toolbar relative to the tractor. Four draught-sensing tines were "tted to the toolbar at 1 m centres so the two outer &wheeltrack' tines (a) operated directly behind the tractor wheel centrelines, and tilled soil immediately after wheeling. The two inner &control' tines (b and c) worked in soil not previously wheeled by the tractor (Fig. 2). An &implement wheel' (d), "tted with a 10;16 ribbedtread, low-pro"le, implement tyre, was used in many tests to simulate the e!ect of the depth control/frame support wheels of trailed implements (Fig. 2). This was free to move in a vertical plane on the front of the tractor, and tra$c soil on the centreline and ahead of control tine (b). The implement wheel was operated at 200 kPa tyre pressure and supported a weight of 9)0 kN. The tractor was a two-wheel drive unit of 70 kW pto power (John Deere 4040) with a total weight of 65 kN. The rear axle was "tted with two 16)9;38 R1 tread bias ply tyres, operated at a pressure of 120 kPa and supporting a static weight of 44 kN (with implement lowered) throughout the work reported here. All tests were carried out at the University of Queensland Gatton College farm, on self-mulching alluvial black earth of the Lawes or Blenhiem pro"le classes (kg
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Fig. 2. Schematic plan view of tractor and toolbar: a, tractor wheeltrack tines; b, implement wheeltrack tine; c, control tine; d, implement wheel; e, toolbar; f, depth wheels
5)15, clay&55%, silt&25%, sand&25%) (Northcote, 1992). Soil conditions approximating those of Australian dryland grain production were obtained by selecting plot areas from "elds at the appropriate stage of land preparation. No special preparation was involved other than the choice of areas which appeared to be uniform. Before each set of tests, four 40 mm core samples were taken from widely spaced locations for gravimetric water content determination of the 0}100 mm (topsoil) and 100}500 mm (subsoil) layers. Mean soil data are summarized in Table 1.
Tine draught for any one soil condition and depth was measured while traversing a pair of adjacent parallel plots of 160 m length in opposite directions, at a speed of approximately 7)2 km/h. The logger scanned transducer outputs at 0)1 s intervals, and recorded the mean draught force measurement at 10 s intervals for each tine. Each test was carried out on a new area of soil, to avoid tra$c and tillage e!ects from previous tests. The possibility of systematic error due to tines following pre-existing wheel tracks was minimized by orienting plots at approximately 303 to the longer dimension of "elds. Systematic errors due to a non-horizontal toolbar were avoided by checking that the mean draught of the two outside wheeltrack tines (operating in nominally identical conditions) was similar. Toolbar depth wheel settings were maintained at a constant value, after initial adjustment of tillage depth using the distance between a transverse 2 m straight edge, and the point of the control share. The &depth' values quoted here thus represent a nominal value, uniform across the machine in relation to the undisturbed, nonwheeled surface. Because wheeltrack tines operated in the wheel rut, their depth of operation relative to the soil surface at the wheeltrack centreline was always smaller than the nominal value. A similar depth measurement technique was attempted for wheel ruts, but results were highly variable. Chisel points were used in test A, where dry surface soil mulch overlay a moist, compact layer. The implement wheel was not available during this test, representing primary tillage e!ects to a depth of 220 mm. Depths used in subsequent tests with sweeps in secondary tillage and planting conditions rarely exceeded 125 mm. Tests B and C represented conventional (i.e. bare fallow) secondary tillage and planting, respectively. Test D was carried out in sorghum stubble with some weed growth, representing planting or weed control in a reduced tillage (heavy residue) situation. Test E, with dry surface soil, represented a conventional secondary tillage operation delayed until soil moisture was suboptimal.
Table 1 Soil condition and moisture content during tests Test
A B C D E
Tillage type
Primary tillage (chisels) Secondary tillage (sweeps) Planting (sweeps) Reduced till. (sweeps) Secondary tillage
Soil condition
Wheat harvest followed by residue mulching As A, then 2;chisel tillage; minimal residue. Area B, after more seedbed preparation Heavy residue, weeds, harvester wheel marks Area B, after 2 weeks further drying
Soil moisture, % (100 mm
'100 mm
15)7 22)7 25)7 24)7 14)5
26)0 26)2 29)0 24)8 27)7
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A variety of other observations were made with the instrumented tillage unit behind other tractors and with varying wheel arrangements. This included draft measurements with and without a front tyre ahead of a tractor rear wheel, and attempts to evaluate the e!ect of di!erent in#ation pressure and weight on the same tyre(14)9;38 bias ply) using asymmetrical wheel positions and an o!set ballast weight.
The subsequent brief evaluation of two-wheel-drive tractor front tyre e!ects provided no evidence that the lighter, high-pressure leading tyre had any signi"cant e!ect on the draught increase caused by the rear wheels. Assessment of the e!ect of varying weight and pressure on the same tyre gave very variable results, and further work is required on each of these topics.
4. Discussion 3. Results Relatively smooth draught versus depth characteristics were obtained for wheeltrack and control tines in all cases except test C, where some inconsistency occurred at 75 mm depth. This might well be associated with the rather unsatisfactory technique used to establish nominal tillage depth, and the same factor might also be responsible for the di$culty experienced in "nding a single draught to depth model which would provide a consistent representation of the data. The data are presented in Fig. 3 as mean treatment values, with error bars representing the 95% con"dence interval. In all cases, the draught of tractor and implement wheeltrack tines was less than that of control tines at nominal depths (50 mm. This e!ect occurred because wheeltrack tines did not engage the soil until tillage depth (relative to the non-wheeled surface) exceeded wheeltrack depression (i.e. rut depth). When wheeltrack tines did engage the soil, tractor and implement wheeltrack tine draught always increased more rapidly with depth than control tine draught. In all cases of operation in normal soil moisture conditions (tests A}D), the draught of wheeltrack tines was signi"cantly greater than that of control tines in the usual operating depth range. For chisel tines in primary tillage at depths '150 mm, the mean draught of wheeltrack tines was greater than that of control tines by a factor of approximately 2)2. For sweeps in secondary tillage at depths '75 mm, tractor and implement wheeltrack tine draught was greater than control tine draught by factors of approximately 2)0 and 1)8, respectively. When surface soil was drier, the e!ect was di!erent. In delayed tillage with sweeps (test E), and in primary tillage with chisels (test A), wheeltrack tine draught appeared similar, or less than that of control tines at depths (100 mm. Draught values were also small for tillage of dry surface soil, which appeared to rearrange the existing aggregates, with little apparent e!ect on aggregate size distribution. In both cases, however, at depths '100 mm, wheeltrack tine draught appeared to increase relative to control tine draught as the tine penetrated deeper and encountered moist soil.
These results demonstrate that when tines follow a tractor or implement wheel, their draught is approximately twice that of identical tines operating in non-wheeled soil in the moisture and operating depth conditions typical of northern Australia. This e!ect of tra$c is similar in magnitude to that reported for tillage of the same soils by Tullberg and Luhrs (1994), and for planter groundtools (narrow tines) in the loess ("ne silt) soils of northwest China by Du (1996). Radford (1999) has also demonstrated a doubling of zero-till planter draught following trampling by grazing cattle under wet conditions. &Tra$c e!ect' is de"ned here as the ratio of the draught of a groundtool operating in soil compacted immediately beforehand by preceding tra$c, to that of an identical and adjacent groundtool operating in non-tra$cked soil at same depth relative to the original, undisturbed soil surface. Tra$c e!ect t thus requires de"nition of the V tra$c treatment causing the e!ect (subscript x), the groundtool on which the e!ect is measured, and the soil conditions. Using this approach, current results indicate the tra$c e!ect of a 6)5 t two-wheel drive tractor t "tted R with 16)9;38 tyres at 120 kPa in#ation pressure was approximately 2)2 for primary tillage with chisels and 2)0 for secondary tillage or planting with sweeps. The tra$c e!ect of an implement wheel t "tted with 10;16 tyres at G 200 kPa in#ation pressure when supporting 0)9 t in secondary tillage and planting was approximately 1)8, in all cases operating under typical conditions for tillage and planting of the heavy clay soils of northern Australia. &Tra$c penalty' ¹ can be de"ned as the increase in V draught of a complete implement due to preceding tra$c, expressed as a decimal of the draught of that implement in non-tra$cked soil. The contribution of any one wheel or set of wheels (subscript x) to the tra$c penalty is a simple function of tra$c e!ect t and tra$cked width V w , where this is expressed as a proportion of total V implement width, and the subscript x indicates the wheel(s) causing the e!ect: ¹ "w (t !1) (1) V V V The total tra$c penalty for a tractor/implement system ¹ is normally the sum of tra$c penalties due to tractor Q
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Fig. 3. Tractor and implement wheel trazc ewects on tine draught/depth characteristics: (a) Test A, primary tillage with chisels; (b) Test B, secondary tillage with sweeps; (c) Test C, planting with sweeps; (d) Test D, zero-till planting with sweeps; (e) Test E, delayed tillage with sweeps: , tractor wheeled; , implement wheeled; , non-wheeled
¹ and implement ¹ tra$c e!ects, because implement R G wheels are generally positioned to avoid tractor wheel ruts
The relationship between unit draught in kN/m implement width for non-tra$cked soil D and that found in convenS tional operation D , for tillage depths '50 mm, is then A
¹ "(¹ #¹ ) Q R G
D "D (1#¹ ) A S Q
(2)
(3)
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During this work the mean value of draught measured for individual tines with sweeps was 1)07 kN/ tine for non-tra$cked soil, under normal secondary tillage conditions of moisture content and depth. At the common tine spacing of 0)305 m, this indicates a unit draught of approximately 3)5 kN/m implement width for these tines, if interactions between tines are ignored. This value is remarkably small for tillage equipment of this type. For trailed sweep tillage equipment used in extensive agriculture, tractor and implement tra$cked width ratios, w of 0)2, and w of 0)15, respectively, would be R G typical. The present results indicate tra$c e!ects t of 2)0 R and t of 1)8, respectively, for similar groundtools and soil G types. Applying Eqns (1) and (2), the tra$c penalty due to tractor wheels, ¹ is 0)2, and due to implement wheels R ¹ is 0)12, giving a system tra$c penalty ¹ of 0)32. G Q Applying Eqn (3), the unit draught for conventional operation D is 4)63 kN/m, which is within the range A (4)5}5)5 kN) found for sweep tillage equipment during an extensive survey of tractor/tillage implement performance in these soil types (Tullberg et al., 1984). This simple analysis shows that a tra$c penalty &32% can easily occur as a consequence of the tra$c e!ect under conditions representative of Australian grain production, and this e!ect appears broadly consistent with the reduction in tillage energy requirements noted by Voorhees (1979) and Chamen et al. (1990) for nontra$cked soil. The tra$c penalty will vary with the width ratios tra$cked by tractor and implement, but observation suggests that the total of these ratios is rarely less than 0)25 for pneumatic tyred equipment, and sometimes exceeds 0)5. The impact of tra$c e!ects on tillage energy requirement has been recognized here as a consequence of controlled tra$c research. Clearly, tra$c penalties can be avoided by not tilling the permanent wheel lanes in controlled tra$c systems. They can also be modi"ed within conventional (non-controlled tra$c) systems by reducing the proportion of soil tra$cked by tractor and implement. In controlled tra$c farming, tractors are usually "tted with tyre equipment of minimal width (w (0)15), loadR bearing implement wheels are con"ned to permanent tracks, and tra$c penalties do not occur when tra$cked soil is neither tilled nor planted. The energy economy of controlled tra$c can thus be seen as the overall result of eliminating the tra$c penalty (&32%), reducing tilled width due to permanent lanes (&15%), and increasing tractive e$ciency on the "rmer surface provided by permanent lanes (&5%), to produce a substantial (&50%) cumulative e!ect. This e!ect is inconsistent with the results of Burt et al. (1994). Such large di!erences in outcome are di$cult to
account for, except in terms of the draught increase occurring with the increase in tillage depth noted by these researchers for implements working non-tra$cked soil. Their results might thus be a simple consequence of the &draught control' tendency (to adjust depth in order to maintain draught) observed by the author in a number of popular tillage and planting implements. In conventional (random) tra$c systems, the tra$cked width of tractors is generally constrained by the need to maintain moderate contact pressures and avoid excessive ruts. Some reduction in tra$cked width might occur as a result of the greater contact length achieved with radial ply tyres, but a larger e!ect would be expected from the use of crawler tracks or rubber belts. The contact length of these is approximately four times that of a tractor tyre (Esch et al., 1990), allowing a substantial reduction in tra$cked width at the same mean contact pressure. Crawler tracks might thus reduce the tractor component of tra$c penalty by a factor of two, compared with an equal-wheel four-wheel drive. Tractor and implement components of tra$c penalty are usually additive because implement designers place the wheels of tillage and planting implements where depth control will not be compromised by tractor wheel ruts. Load-bearing and depth control functions might be separated however, removing the need for load-bearing wheels on non-tra$cked soil. This development should eliminate or substantially reduce the implement component of tra$c penalty. A rubber-belt tractor and implement with non-contact depth control might thus be an optimal arrangement for minimizing tra$c penalties in conventional random traf"c systems, using currently available equipment. Data quoted here suggests that this would reduce the tractor and implement components of the tra$c penalty by 10 and 12%, respectively. The resulting draught reduction* 22% * should have corresponding impacts on tractor power, fuel requirements and cost, provided the tra$c e!ect of rubber belts or crawler tracks is no larger than that of pneumatic tyres. No tra$c e!ect tests have yet been carried out with track or rubber-belt tractors, but assessments of their performance and soil impact (e.g. Esch et al., 1990; Gassman et al., 1989), suggest that a greater tra$c e!ect is unlikely. This is also consistent with the observation that tillage draught was noticeably smaller with the steeltrack crawler tractors, which also tra$cked a smaller proportion of implement width, and were widely used on the heavy soils of northern Australia until recently. There is no formal data on this topic, but observations of reduced draught with crawler tractors have frequently been voiced by farmers, and convincing data on comparative fuel use is sometimes available (e.g. Alexander, 1999).
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Consideration of possible errors in the measurements and assumptions entailed in this work suggest that these estimates of tra$c penalty are likely to be conservative, because the draught of tines working in permanently non-tra$cked soil would be less than those measured in experiment plots prepared using conventional (random tra$c) tillage. Tra$c penalty calculations set out here also exclude the e!ect of factors such as the draught required to overcome implement wheel rolling resistance. These considerations all indicate that the draught penalty occurring in normal "eld practice is unlikely to be less than that demonstrated here. In current thinking, tractive e$ciency, the ratio of drawbar power to axle power, is the accepted criterion of tractor performance. If the tra$c penalties demonstrated here are generally applicable across a range of soils, implements and tra$c systems, a new approach to the evaluation of soil/vehicle performance will be needed. A new parameter*&tra$c e$ciency'*might be postulated as the complement of tra$c penalty (1!¹), and describe that proportion of implement draught necessarily used for tillage and planting, rather than overcoming the tra$c e!ects of the tractor/implement system. The product of tractive e$ciency and tra$c e$ciency would then represent an overall tillage system energy e$ciency, de"ning that proportion of axle power necessarily dissipated in the soil, and providing an excellent criterion of the mechanical productivity of farm vehicle operations. Its complementary function would then represent energy dissipated in unnecessary soil deformation, perhaps the major anthropogenic input to soil degradation. Taking 75 and 32% respectively as typical values for tractive e$ciency and tra$c penalty, this overall tillage system e$ciency would be approximately 50%. It is interesting to speculate that tractive e$ciency and tra$c e$ciency might be related in practice, because the complement of tractive e$ciency is that proportion of tractor power dissipated largely in soil deformation. The complement of tra$c e$ciency (tra$c penalty) represents that proportion of implement power used to undo some aspects of this deformation within the tilled layer. There is an attractive symmetry in the notion of equality between energy input to the compactive and loosening processes of soil deformation. Clearly, this could be true only of soil within the tilled layer, but inspection of the base of this layer (by shoveling away loose tilth) indicated that sweep tillage depth was rarely adequate to remove visible tra$c e!ects on the sub-tillage layer. The greater cloddiness of tilled wheeltracks was also clearly evident throughout this work. These observations suggest that the e!ects of wheelinduced soil deformation had profound e!ects on a physical scale greater in depth and smaller in terms of its e!ect on soil structure than those of sweep tillage, at least
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under the conditions of these experiments. This is, perhaps, hardly surprising when the energy per unit soil surface area dissipated by tractor tra$c is substantially greater than that of tillage. Tra$c e!ects on draught appeared to follow similar characteristics with both sweep and chisel tines, but no tests were carried out with other tillage groundtools, such as discs or mouldboards.
5. Conclusions The impact of tractor and implement wheel tra$c was related to the performance of tined tillage equipment operating under the moisture and depth conditions typical of extensive grain production systems in the heavy clay soils of northern Australia. (1) The draught of sweep and chisel tines operating in soil tra$cked by a 6)5 t, 2 wheel-drive tractor was increased by more than 100% compared with identical tines operating in adjacent, non-tra$cked soil. The draught of sweep tines in soil tra$cked by an implement tyre supporting approximately 0)9 t was similarly increased by approximately 80%. (2) In common tractor/implement con"gurations, these tra$c e!ects can result in an increase implement draught, or tra$c penalty, of more than 30%. The absence of this tra$c penalty is an important component of the improved energy e$ciency found in controlled tra$c farming. (3) Minimizing the proportion of implement width &traf"cked' by tractor and implement should reduce the magnitude of the tra$c penalty. An implement with non-contact depth control, powered by a crawler track or belt-type tractor, appears likely to be the optimal arrangement in this respect. (4) A tra$c e$ciency parameter, deducting the tra$c penalty from unity, might be useful, and have interesting relationships with tractive e$ciency and energy inputs to soil structural degradation. (5) These phenomena did not occur during tillage of dry soil. Di!erent tra$c e!ects might be found with different traction systems, soil types, and implement groundtools.
Acknowledgements This work was supported by the Australian Centre for International Agricultural Research, under project nos. 9209 and 96143. Valuable contributions were made by Brett Jahnke, Minh Kinh, Julian Winch and Levi Victor-Gordon.
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References Alexander D (1999). Personal communication about fuel use at Jimbour, Queensland, 1999 Arndt W; Rose C W (1966). Tra$c compaction of soil and tillage requirements. Journal of Agricultural Engineering Research, 11(3), 170}187 Burt E C (1993). Soil}tyre/track interaction: current and future research needs. Journal of Terramechanics, 30(5), 317}323 Burt E C; Reeves D W; Raper R L (1994). Energy utilization as a!ected by tra$c in a conventional and conservation tillage system. Transactions of the American Society of Agricultural Engineers, 37(3), 759}762 Chamen WTC; Vermeulen D G; Campbell D J; Sommer C (1990). EEC cooperative project on reduction of soil compaction. ASAE Paper No. 90}1073 Chi Renli; Zuo Shuzhen (1988). Development and evolution of the zonal tillage concept in China; a historical review. Proceedings of the 11th ISTRO Conference Edinburgh. Vol. 2, pp 601}606 Cooper A W; Trouse A C; Dumas W T (1969). Controlled tra$c in rowcrop production. In: Proceedings of Seventh International Congress of Agricultural Engineering, Baden-Baden, Germany, pp 1}6 Du Bing (1996). Energy requirements of planter groundtools. Unpublished Report, China Agricultural University. Esch J H; Bashford L L; Von Bargen K; Ekstrom R E (1990). Tractive performance comparisons between a rubber belt track and a four-wheel drive tractor. Transactions of the ASAE, 33(4), 1109}1115
Gassman P W; Erbach D C; Melvin S W (1989). Analysis of track and wheel soil compaction. Transactions of the ASAE, 32(1), 23}29 Jayawardane N S; Stewart B A (1995). Subsoil Management Techniques (Advances in Soil Science Series). Lewis Publishers, Boca Raton, FL Kuipers H; van de Zande J C (1994). Quanti"cation of tra$c systems. In: Soil compaction in Crop Production (Soane B D; van Ouwerkerk C eds), pp 417}445. Elsevier, Amsterdam McKyes E (1984). Soil Cutting and Tillage. Elsevier, Amsterdam Northcote K H (1992). A Factual Key for the Recognition of Australian Soils. Rellim Technical Publications, Co!s Harbour, NSW Radford B (1999). Personal communication*Cattle grazing e!ects on planter draught and depth. Soane B D; van Ouwerkerk C (1994). Soil Compaction in Crop Production. Elsevier, Amsterdam Taylor J H (1983). Bene"ts of permanent tra$c lanes in a controlled tra$c crop production system. Soil and Tillage Research, 3, 385}395 Tullberg J N; Rickman J F; Doyle G J (1984). Reliability and the operation of large tractors. The Agricultural Engineer, 39(1), 10}13 Tullberg J N; Murray S (1988). Controlled tra$c in subtropical grain production. Proceedings of the 11th ISTRO Conference, Edinburgh, Vol. 1, pp 323}327 Tullberg J N; Luhrs A (1994). Energy requirements for wheeltrack tillage. Second International Conference on Soil Dynamics, Silsoe. Abstracts, pp 95}96 Voorhees W B (1979). Energy aspects of controlled wheel tra$c. Proceedings Eighth ISTRO Conference, Germany
Wheel Tra$c E!ects on Tillage Draught J. N. Tullberg Farm Mechanisation Centre, University of Queensland Gatton, Queensland 4343, Australia; E-mail: [email protected] (Received 29 December 1997; accepted in revised form 12 November 1999)
Tractor and/or implement wheels precede ground tools, and compact soil immediately before it is loosened in most tillage and planting operations. The work reported here was designed to quantify the impact of preceding wheel tra$c on tine draught and the energy requirements of tillage, as part of a programme to assess the bene"ts of controlled tra$c operation. Instrumented tines on a three-point linkage toolbar were used behind a 70 kW two-wheel drive tractor to measure the &tra$c e!ect' of wheels on the draught of the tillage tools following them in a clay soil. Under conditions broadly representative of Australian grain production, the draught of chisel and sweep tines operating behind tractor or implement wheels was increased by a factor of approximately two, compared with identical tines una!ected by preceding wheel tra$c. These data can be used to demonstrate that the tra$c e!ect on trailed tillage implements can increase total draught by more than 30%. This can be de"ned as the &tra$c penalty' of the operation, and indicates the extent to which a conventional tractor/implement system generates its own workload. A &tra$c e$ciency' parameter is proposed to quantify the impact of tractor and implement tra$c e!ects, and used to show that approximately 50% of a tractor's power output can be dissipated in the process of creating and disrupting its own wheel compaction. These results can be used to explain the reduction in tillage energy which occurs in controlled tra$c systems, and indicate other approaches to improving overall tractor/tillage system e$ciency. They are also the basis of speculation about the relationship between tractive e$ciency, tra$c e$ciency and soil structure damage. 2000 Silsoe Research Institute
1. Introduction The additional e!ort required to disturb soil that has been compacted by feet or wheels is common knowledge to all who till or dig the soil. Traditional agricultural systems such as those described by Chi and Zuo (1988) sometimes avoided this energy penalty by maintaining separate zones for tra$c and crop growth, but this is not easily achieved over the full cycle of operations involved in current crop production systems. The negative e!ects of tra$c on in"ltration, tilth and penetration resistance of clay soils in Australia were "rst quanti"ed by Arndt and Rose (1966), who advocated the use of improved tra$c systems to minimize the problems. The essence of the problem is well known. Optimum conditions for crop production, that is soft, friable and permeable soil, are quite unsuitable for e$cient tra$c and traction, and vice versa. Wheel tra$c increases soil 0021-8634/00/040375#08 $35.00/0
strength and the draught requirement of subsequent tillage, while tillage reduces soil strength and the e$ciency of subsequent traction. Where "eld tra$c follows a di!erent pathway for each of a series of operations, the processes of tillage and tra$c are contradictory. These contradictions are avoided in controlled tra$c farming (Taylor, 1983), where all "eld tra$c is con"ned to permanent laneways, and all crops are grown in permanent beds. This system allows soil conditions in the beds to be optimized for crop production, and the laneways optimized for traction. The advantages of controlled tra$c were demonstrated in the southeast United States by Cooper et al. (1969) and include an indirect energy economy which occurs because there is less need for deep tillage. The direct e!ect occurs because noncompacted soil requires less tillage energy than compacted soil, and traction is more e$cient when tyres are working on compacted permanent tracks.
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The penalties of wheel tra$c compaction and bene"ts of controlled tra$c have been demonstrated by a number of researchers, including Voorhees (1979), Tullberg and Murray (1988), and Chamen et al. (1990), but more recently Burt et al. (1994) concluded that tra$c had no impact on the energy required for soil preparation and planting of cotton, when using the equipment and practices regarded as standard for farmers in the southeast USA. They acknowledged, however, that their results could be a!ected by the tendency of secondary tillage equipment to operate at greater depth in non-tra$cked soil. Soil compaction e!ects generally ameliorate with time, so the most severe e!ects of tra$c, including those on tillage energy, should be observed in the performance of tillage tools disturbing soil immediately after tra$c. Tractor drive wheels are likely to be most important in this respect, although implement wheels providing depth control and frame support might also have some e!ect, particularly in extensive agriculture where trailed (rather than three-point linkage mounted) equipment is common. There a substantial literature concerned with soil compaction, traction, tillage, and their interactions with soil properties (Soane & van Ouwerkerk, 1994; Burt, 1993; McKyes, 1984; Jayawardane & Stewart, 1995; for example). The impact of the traction process on tillage e$ciency within common tractor/tillage systems nevertheless appears to have received little attention. This is surprising when wheels cover 15}40% of implement width in most individual "eld operations, and total annual tra$c intensity (total area a!ected by tra$c/crop area) is in the range from 5)0 to 0)5. (Kuipers & van de Zande, 1994). The object of this work was to assess the e!ect of tractor and implement wheels on the draught (i.e. horizontal soil force) of the tillage tools following immediately behind them, and to use these data to quantify the impact of "eld tra$c on the power requirements of tillage.
2. Methods and Materials Draught-sensing tines were designed using chisel plough shanks attached to parallel link assemblies (Fig. 1). Rearward movement of the tines was restricted by shear beam force transducers (SKT model 1500) monitored by a data logger (Datataker model DT100). The draughtsensing tines were "tted with common proprietary shares: either &chisels', curved reversible cast shares of 53 mm width normally used for primary tillage (Howard H76772 Buster Points); or &sweeps', low pro"le shares pressed from 6 mm steel, 450 mm wide, 70 mm shank width, 30 mm wing lift, normally used for weed control and seedbed preparation (McKay 15918K1).
Fig. 1. Draught-sensing tine assembly: a, parallel links; b, transducer; c, tine shank; d, chisel or sweep; e, toolbar
The draught sensing tines were mounted on a 4 m wide three-point linkage toolbar (e) "tted with adjustable depth control wheels (f ) at its extremities (Fig. 2). The toolbar was mounted on the three-point linkage of a tractor modi"ed for work in permanent wide beds, with front and rear wheel track centres set to 3 m, and stabilizers to prevent lateral movement of the toolbar relative to the tractor. Four draught-sensing tines were "tted to the toolbar at 1 m centres so the two outer &wheeltrack' tines (a) operated directly behind the tractor wheel centrelines, and tilled soil immediately after wheeling. The two inner &control' tines (b and c) worked in soil not previously wheeled by the tractor (Fig. 2). An &implement wheel' (d), "tted with a 10;16 ribbedtread, low-pro"le, implement tyre, was used in many tests to simulate the e!ect of the depth control/frame support wheels of trailed implements (Fig. 2). This was free to move in a vertical plane on the front of the tractor, and tra$c soil on the centreline and ahead of control tine (b). The implement wheel was operated at 200 kPa tyre pressure and supported a weight of 9)0 kN. The tractor was a two-wheel drive unit of 70 kW pto power (John Deere 4040) with a total weight of 65 kN. The rear axle was "tted with two 16)9;38 R1 tread bias ply tyres, operated at a pressure of 120 kPa and supporting a static weight of 44 kN (with implement lowered) throughout the work reported here. All tests were carried out at the University of Queensland Gatton College farm, on self-mulching alluvial black earth of the Lawes or Blenhiem pro"le classes (kg
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Fig. 2. Schematic plan view of tractor and toolbar: a, tractor wheeltrack tines; b, implement wheeltrack tine; c, control tine; d, implement wheel; e, toolbar; f, depth wheels
5)15, clay&55%, silt&25%, sand&25%) (Northcote, 1992). Soil conditions approximating those of Australian dryland grain production were obtained by selecting plot areas from "elds at the appropriate stage of land preparation. No special preparation was involved other than the choice of areas which appeared to be uniform. Before each set of tests, four 40 mm core samples were taken from widely spaced locations for gravimetric water content determination of the 0}100 mm (topsoil) and 100}500 mm (subsoil) layers. Mean soil data are summarized in Table 1.
Tine draught for any one soil condition and depth was measured while traversing a pair of adjacent parallel plots of 160 m length in opposite directions, at a speed of approximately 7)2 km/h. The logger scanned transducer outputs at 0)1 s intervals, and recorded the mean draught force measurement at 10 s intervals for each tine. Each test was carried out on a new area of soil, to avoid tra$c and tillage e!ects from previous tests. The possibility of systematic error due to tines following pre-existing wheel tracks was minimized by orienting plots at approximately 303 to the longer dimension of "elds. Systematic errors due to a non-horizontal toolbar were avoided by checking that the mean draught of the two outside wheeltrack tines (operating in nominally identical conditions) was similar. Toolbar depth wheel settings were maintained at a constant value, after initial adjustment of tillage depth using the distance between a transverse 2 m straight edge, and the point of the control share. The &depth' values quoted here thus represent a nominal value, uniform across the machine in relation to the undisturbed, nonwheeled surface. Because wheeltrack tines operated in the wheel rut, their depth of operation relative to the soil surface at the wheeltrack centreline was always smaller than the nominal value. A similar depth measurement technique was attempted for wheel ruts, but results were highly variable. Chisel points were used in test A, where dry surface soil mulch overlay a moist, compact layer. The implement wheel was not available during this test, representing primary tillage e!ects to a depth of 220 mm. Depths used in subsequent tests with sweeps in secondary tillage and planting conditions rarely exceeded 125 mm. Tests B and C represented conventional (i.e. bare fallow) secondary tillage and planting, respectively. Test D was carried out in sorghum stubble with some weed growth, representing planting or weed control in a reduced tillage (heavy residue) situation. Test E, with dry surface soil, represented a conventional secondary tillage operation delayed until soil moisture was suboptimal.
Table 1 Soil condition and moisture content during tests Test
A B C D E
Tillage type
Primary tillage (chisels) Secondary tillage (sweeps) Planting (sweeps) Reduced till. (sweeps) Secondary tillage
Soil condition
Wheat harvest followed by residue mulching As A, then 2;chisel tillage; minimal residue. Area B, after more seedbed preparation Heavy residue, weeds, harvester wheel marks Area B, after 2 weeks further drying
Soil moisture, % (100 mm
'100 mm
15)7 22)7 25)7 24)7 14)5
26)0 26)2 29)0 24)8 27)7
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A variety of other observations were made with the instrumented tillage unit behind other tractors and with varying wheel arrangements. This included draft measurements with and without a front tyre ahead of a tractor rear wheel, and attempts to evaluate the e!ect of di!erent in#ation pressure and weight on the same tyre(14)9;38 bias ply) using asymmetrical wheel positions and an o!set ballast weight.
The subsequent brief evaluation of two-wheel-drive tractor front tyre e!ects provided no evidence that the lighter, high-pressure leading tyre had any signi"cant e!ect on the draught increase caused by the rear wheels. Assessment of the e!ect of varying weight and pressure on the same tyre gave very variable results, and further work is required on each of these topics.
4. Discussion 3. Results Relatively smooth draught versus depth characteristics were obtained for wheeltrack and control tines in all cases except test C, where some inconsistency occurred at 75 mm depth. This might well be associated with the rather unsatisfactory technique used to establish nominal tillage depth, and the same factor might also be responsible for the di$culty experienced in "nding a single draught to depth model which would provide a consistent representation of the data. The data are presented in Fig. 3 as mean treatment values, with error bars representing the 95% con"dence interval. In all cases, the draught of tractor and implement wheeltrack tines was less than that of control tines at nominal depths (50 mm. This e!ect occurred because wheeltrack tines did not engage the soil until tillage depth (relative to the non-wheeled surface) exceeded wheeltrack depression (i.e. rut depth). When wheeltrack tines did engage the soil, tractor and implement wheeltrack tine draught always increased more rapidly with depth than control tine draught. In all cases of operation in normal soil moisture conditions (tests A}D), the draught of wheeltrack tines was signi"cantly greater than that of control tines in the usual operating depth range. For chisel tines in primary tillage at depths '150 mm, the mean draught of wheeltrack tines was greater than that of control tines by a factor of approximately 2)2. For sweeps in secondary tillage at depths '75 mm, tractor and implement wheeltrack tine draught was greater than control tine draught by factors of approximately 2)0 and 1)8, respectively. When surface soil was drier, the e!ect was di!erent. In delayed tillage with sweeps (test E), and in primary tillage with chisels (test A), wheeltrack tine draught appeared similar, or less than that of control tines at depths (100 mm. Draught values were also small for tillage of dry surface soil, which appeared to rearrange the existing aggregates, with little apparent e!ect on aggregate size distribution. In both cases, however, at depths '100 mm, wheeltrack tine draught appeared to increase relative to control tine draught as the tine penetrated deeper and encountered moist soil.
These results demonstrate that when tines follow a tractor or implement wheel, their draught is approximately twice that of identical tines operating in non-wheeled soil in the moisture and operating depth conditions typical of northern Australia. This e!ect of tra$c is similar in magnitude to that reported for tillage of the same soils by Tullberg and Luhrs (1994), and for planter groundtools (narrow tines) in the loess ("ne silt) soils of northwest China by Du (1996). Radford (1999) has also demonstrated a doubling of zero-till planter draught following trampling by grazing cattle under wet conditions. &Tra$c e!ect' is de"ned here as the ratio of the draught of a groundtool operating in soil compacted immediately beforehand by preceding tra$c, to that of an identical and adjacent groundtool operating in non-tra$cked soil at same depth relative to the original, undisturbed soil surface. Tra$c e!ect t thus requires de"nition of the V tra$c treatment causing the e!ect (subscript x), the groundtool on which the e!ect is measured, and the soil conditions. Using this approach, current results indicate the tra$c e!ect of a 6)5 t two-wheel drive tractor t "tted R with 16)9;38 tyres at 120 kPa in#ation pressure was approximately 2)2 for primary tillage with chisels and 2)0 for secondary tillage or planting with sweeps. The tra$c e!ect of an implement wheel t "tted with 10;16 tyres at G 200 kPa in#ation pressure when supporting 0)9 t in secondary tillage and planting was approximately 1)8, in all cases operating under typical conditions for tillage and planting of the heavy clay soils of northern Australia. &Tra$c penalty' ¹ can be de"ned as the increase in V draught of a complete implement due to preceding tra$c, expressed as a decimal of the draught of that implement in non-tra$cked soil. The contribution of any one wheel or set of wheels (subscript x) to the tra$c penalty is a simple function of tra$c e!ect t and tra$cked width V w , where this is expressed as a proportion of total V implement width, and the subscript x indicates the wheel(s) causing the e!ect: ¹ "w (t !1) (1) V V V The total tra$c penalty for a tractor/implement system ¹ is normally the sum of tra$c penalties due to tractor Q
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Fig. 3. Tractor and implement wheel trazc ewects on tine draught/depth characteristics: (a) Test A, primary tillage with chisels; (b) Test B, secondary tillage with sweeps; (c) Test C, planting with sweeps; (d) Test D, zero-till planting with sweeps; (e) Test E, delayed tillage with sweeps: , tractor wheeled; , implement wheeled; , non-wheeled
¹ and implement ¹ tra$c e!ects, because implement R G wheels are generally positioned to avoid tractor wheel ruts
The relationship between unit draught in kN/m implement width for non-tra$cked soil D and that found in convenS tional operation D , for tillage depths '50 mm, is then A
¹ "(¹ #¹ ) Q R G
D "D (1#¹ ) A S Q
(2)
(3)
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During this work the mean value of draught measured for individual tines with sweeps was 1)07 kN/ tine for non-tra$cked soil, under normal secondary tillage conditions of moisture content and depth. At the common tine spacing of 0)305 m, this indicates a unit draught of approximately 3)5 kN/m implement width for these tines, if interactions between tines are ignored. This value is remarkably small for tillage equipment of this type. For trailed sweep tillage equipment used in extensive agriculture, tractor and implement tra$cked width ratios, w of 0)2, and w of 0)15, respectively, would be R G typical. The present results indicate tra$c e!ects t of 2)0 R and t of 1)8, respectively, for similar groundtools and soil G types. Applying Eqns (1) and (2), the tra$c penalty due to tractor wheels, ¹ is 0)2, and due to implement wheels R ¹ is 0)12, giving a system tra$c penalty ¹ of 0)32. G Q Applying Eqn (3), the unit draught for conventional operation D is 4)63 kN/m, which is within the range A (4)5}5)5 kN) found for sweep tillage equipment during an extensive survey of tractor/tillage implement performance in these soil types (Tullberg et al., 1984). This simple analysis shows that a tra$c penalty &32% can easily occur as a consequence of the tra$c e!ect under conditions representative of Australian grain production, and this e!ect appears broadly consistent with the reduction in tillage energy requirements noted by Voorhees (1979) and Chamen et al. (1990) for nontra$cked soil. The tra$c penalty will vary with the width ratios tra$cked by tractor and implement, but observation suggests that the total of these ratios is rarely less than 0)25 for pneumatic tyred equipment, and sometimes exceeds 0)5. The impact of tra$c e!ects on tillage energy requirement has been recognized here as a consequence of controlled tra$c research. Clearly, tra$c penalties can be avoided by not tilling the permanent wheel lanes in controlled tra$c systems. They can also be modi"ed within conventional (non-controlled tra$c) systems by reducing the proportion of soil tra$cked by tractor and implement. In controlled tra$c farming, tractors are usually "tted with tyre equipment of minimal width (w (0)15), loadR bearing implement wheels are con"ned to permanent tracks, and tra$c penalties do not occur when tra$cked soil is neither tilled nor planted. The energy economy of controlled tra$c can thus be seen as the overall result of eliminating the tra$c penalty (&32%), reducing tilled width due to permanent lanes (&15%), and increasing tractive e$ciency on the "rmer surface provided by permanent lanes (&5%), to produce a substantial (&50%) cumulative e!ect. This e!ect is inconsistent with the results of Burt et al. (1994). Such large di!erences in outcome are di$cult to
account for, except in terms of the draught increase occurring with the increase in tillage depth noted by these researchers for implements working non-tra$cked soil. Their results might thus be a simple consequence of the &draught control' tendency (to adjust depth in order to maintain draught) observed by the author in a number of popular tillage and planting implements. In conventional (random) tra$c systems, the tra$cked width of tractors is generally constrained by the need to maintain moderate contact pressures and avoid excessive ruts. Some reduction in tra$cked width might occur as a result of the greater contact length achieved with radial ply tyres, but a larger e!ect would be expected from the use of crawler tracks or rubber belts. The contact length of these is approximately four times that of a tractor tyre (Esch et al., 1990), allowing a substantial reduction in tra$cked width at the same mean contact pressure. Crawler tracks might thus reduce the tractor component of tra$c penalty by a factor of two, compared with an equal-wheel four-wheel drive. Tractor and implement components of tra$c penalty are usually additive because implement designers place the wheels of tillage and planting implements where depth control will not be compromised by tractor wheel ruts. Load-bearing and depth control functions might be separated however, removing the need for load-bearing wheels on non-tra$cked soil. This development should eliminate or substantially reduce the implement component of tra$c penalty. A rubber-belt tractor and implement with non-contact depth control might thus be an optimal arrangement for minimizing tra$c penalties in conventional random traf"c systems, using currently available equipment. Data quoted here suggests that this would reduce the tractor and implement components of the tra$c penalty by 10 and 12%, respectively. The resulting draught reduction* 22% * should have corresponding impacts on tractor power, fuel requirements and cost, provided the tra$c e!ect of rubber belts or crawler tracks is no larger than that of pneumatic tyres. No tra$c e!ect tests have yet been carried out with track or rubber-belt tractors, but assessments of their performance and soil impact (e.g. Esch et al., 1990; Gassman et al., 1989), suggest that a greater tra$c e!ect is unlikely. This is also consistent with the observation that tillage draught was noticeably smaller with the steeltrack crawler tractors, which also tra$cked a smaller proportion of implement width, and were widely used on the heavy soils of northern Australia until recently. There is no formal data on this topic, but observations of reduced draught with crawler tractors have frequently been voiced by farmers, and convincing data on comparative fuel use is sometimes available (e.g. Alexander, 1999).
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Consideration of possible errors in the measurements and assumptions entailed in this work suggest that these estimates of tra$c penalty are likely to be conservative, because the draught of tines working in permanently non-tra$cked soil would be less than those measured in experiment plots prepared using conventional (random tra$c) tillage. Tra$c penalty calculations set out here also exclude the e!ect of factors such as the draught required to overcome implement wheel rolling resistance. These considerations all indicate that the draught penalty occurring in normal "eld practice is unlikely to be less than that demonstrated here. In current thinking, tractive e$ciency, the ratio of drawbar power to axle power, is the accepted criterion of tractor performance. If the tra$c penalties demonstrated here are generally applicable across a range of soils, implements and tra$c systems, a new approach to the evaluation of soil/vehicle performance will be needed. A new parameter*&tra$c e$ciency'*might be postulated as the complement of tra$c penalty (1!¹), and describe that proportion of implement draught necessarily used for tillage and planting, rather than overcoming the tra$c e!ects of the tractor/implement system. The product of tractive e$ciency and tra$c e$ciency would then represent an overall tillage system energy e$ciency, de"ning that proportion of axle power necessarily dissipated in the soil, and providing an excellent criterion of the mechanical productivity of farm vehicle operations. Its complementary function would then represent energy dissipated in unnecessary soil deformation, perhaps the major anthropogenic input to soil degradation. Taking 75 and 32% respectively as typical values for tractive e$ciency and tra$c penalty, this overall tillage system e$ciency would be approximately 50%. It is interesting to speculate that tractive e$ciency and tra$c e$ciency might be related in practice, because the complement of tractive e$ciency is that proportion of tractor power dissipated largely in soil deformation. The complement of tra$c e$ciency (tra$c penalty) represents that proportion of implement power used to undo some aspects of this deformation within the tilled layer. There is an attractive symmetry in the notion of equality between energy input to the compactive and loosening processes of soil deformation. Clearly, this could be true only of soil within the tilled layer, but inspection of the base of this layer (by shoveling away loose tilth) indicated that sweep tillage depth was rarely adequate to remove visible tra$c e!ects on the sub-tillage layer. The greater cloddiness of tilled wheeltracks was also clearly evident throughout this work. These observations suggest that the e!ects of wheelinduced soil deformation had profound e!ects on a physical scale greater in depth and smaller in terms of its e!ect on soil structure than those of sweep tillage, at least
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under the conditions of these experiments. This is, perhaps, hardly surprising when the energy per unit soil surface area dissipated by tractor tra$c is substantially greater than that of tillage. Tra$c e!ects on draught appeared to follow similar characteristics with both sweep and chisel tines, but no tests were carried out with other tillage groundtools, such as discs or mouldboards.
5. Conclusions The impact of tractor and implement wheel tra$c was related to the performance of tined tillage equipment operating under the moisture and depth conditions typical of extensive grain production systems in the heavy clay soils of northern Australia. (1) The draught of sweep and chisel tines operating in soil tra$cked by a 6)5 t, 2 wheel-drive tractor was increased by more than 100% compared with identical tines operating in adjacent, non-tra$cked soil. The draught of sweep tines in soil tra$cked by an implement tyre supporting approximately 0)9 t was similarly increased by approximately 80%. (2) In common tractor/implement con"gurations, these tra$c e!ects can result in an increase implement draught, or tra$c penalty, of more than 30%. The absence of this tra$c penalty is an important component of the improved energy e$ciency found in controlled tra$c farming. (3) Minimizing the proportion of implement width &traf"cked' by tractor and implement should reduce the magnitude of the tra$c penalty. An implement with non-contact depth control, powered by a crawler track or belt-type tractor, appears likely to be the optimal arrangement in this respect. (4) A tra$c e$ciency parameter, deducting the tra$c penalty from unity, might be useful, and have interesting relationships with tractive e$ciency and energy inputs to soil structural degradation. (5) These phenomena did not occur during tillage of dry soil. Di!erent tra$c e!ects might be found with different traction systems, soil types, and implement groundtools.
Acknowledgements This work was supported by the Australian Centre for International Agricultural Research, under project nos. 9209 and 96143. Valuable contributions were made by Brett Jahnke, Minh Kinh, Julian Winch and Levi Victor-Gordon.
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Gassman P W; Erbach D C; Melvin S W (1989). Analysis of track and wheel soil compaction. Transactions of the ASAE, 32(1), 23}29 Jayawardane N S; Stewart B A (1995). Subsoil Management Techniques (Advances in Soil Science Series). Lewis Publishers, Boca Raton, FL Kuipers H; van de Zande J C (1994). Quanti"cation of tra$c systems. In: Soil compaction in Crop Production (Soane B D; van Ouwerkerk C eds), pp 417}445. Elsevier, Amsterdam McKyes E (1984). Soil Cutting and Tillage. Elsevier, Amsterdam Northcote K H (1992). A Factual Key for the Recognition of Australian Soils. Rellim Technical Publications, Co!s Harbour, NSW Radford B (1999). Personal communication*Cattle grazing e!ects on planter draught and depth. Soane B D; van Ouwerkerk C (1994). Soil Compaction in Crop Production. Elsevier, Amsterdam Taylor J H (1983). Bene"ts of permanent tra$c lanes in a controlled tra$c crop production system. Soil and Tillage Research, 3, 385}395 Tullberg J N; Rickman J F; Doyle G J (1984). Reliability and the operation of large tractors. The Agricultural Engineer, 39(1), 10}13 Tullberg J N; Murray S (1988). Controlled tra$c in subtropical grain production. Proceedings of the 11th ISTRO Conference, Edinburgh, Vol. 1, pp 323}327 Tullberg J N; Luhrs A (1994). Energy requirements for wheeltrack tillage. Second International Conference on Soil Dynamics, Silsoe. Abstracts, pp 95}96 Voorhees W B (1979). Energy aspects of controlled wheel tra$c. Proceedings Eighth ISTRO Conference, Germany