Direct corn seeding. Effects of residue clearance on implant efficiency

This article describes the effects of three seeding unit designs on plant residue clearance along the central line of direct corn (Zea Mays L.) seeding and the subsequent influence on crop emergence. The equipment assemblies tested were three different combinations of the implements turbo coulter blade (TCB), double disc opener with a seed press wheel and covering/packing wheels (DDO), notched disc row cleaner with a track wheel or floating star cleaner (RC) as follows: (1) TCB+DDO, (2) RC+TCB+DDO and (3) TCB+RC+DDO. Under the field conditions tested, residue clearance before the cutting action of the turbo coulter blade (combination 2) gave rise to the best crop stand and crop uniformity. The seeding assembly that moves soil and clears away plant debris along the seeding furrow shows a direct effect on seed emergence.

Corn (Zea mays L.) shows poor leaf plasticity, scarce tillering capacity and a low prolificacy which leads to a reduced ability to compensate low plant densities. Further, if the plants do not reach the growth threshold for fixing the ear during the period from the fortnight before to the fortnight after flowering, they will be sterile (Andrade et al., 1996). This reflects the importance of achieving a uniform stand.
The presence of plant residues on the soil surface has the consequence of diminishing soil temperature which in turn impairs crop emergence (Wicks et al., 1994), requiring an increased density of seeding to achieve the same sized plant population compared to the use of conventional systems (Oplinger and Philbrook, 1992). On the other hand, the spatial distribution of residues is not uniform over the soil surface such that areas of different temperature occur. Thus, in this type of system, the seeding assembly of the planter plays an essential role since it works the soil that has not been previously tilled. Most planters have a circular blade, whose axis is horizontal and normal to the direction of travel, to cut the debris and till the line of seeding. The blade is followed by double disc furrow openers and rubber-coated packer and covering wheels (Maroni, 1994;Gargicevich, 1995). This combination has the advantage that it does not become obstructed and is of low maintenance. The drawbacks related with its use are superf icial seed distribution (Kushwaha and Foster, 1993;Soza et al., 2000), sidewall compactation and residue burying (Baker, 1994) and displacement (Balbuena et al., 2000), effects that reduce seed cover, seed/soil contact and seedling emergence.
To resolve these problems, Baumer (1999) described several seeding unit designs proposing among other things that by placing a scourer between the blade and the double disc, the penetrating capacity of the seeding drill can be enhanced, overturning of the blade cutting band increased and the residue buried by the blade displaced. However, the same author states that this arrangement can also extract stones and lead to greater machine obstruction when the residue is damp.
The need to directly sow corn with large volumes of plant debris at an early seeding date and to improve the plant population achieved has prompted the use of row cleaners in seeding assemblies. This addition also responds to the need to reduce the amount of plant debris along the line of seeding, which enhances emergence. The aim of the present study was to quantify the effect of residue clearance on corn emergence after direct seeding using a seeder equipped with a fluted circular blade commercially known as a turbo coulter blade.

Study site
The study was performed at the experimental farm Santa Ana (latitude 33 o 59 ' south, longitude 64 o 34 ' west) in the district Villa Cañás, Departamento General López, of the Santa Fe province. The soil belongs to the Santa Isabel series, a typical Hapludol of capacity of use 1-2. The soil profile shows 36 cm of dark top soil with substantial organic matter (horizon A1). This horizon gradually passes to a B2 horizon of weakly prismatic structure of a more reddish colour. Its permeability is moderately fast and it contains less clay than A1. From a depth of 95 to 100 cm, its consistency and structure diminishes to become massive and loose.

Machinery
The machine used was a 2-wheel drive (2WD) Deutz A4120 tractor with an 84.96 kW (115.6 CV) motor (norm IRAM 8005) and an 18.4-38 rear drive and front drive 11.00-16. The features of the planter used in the tests are provided in Table 1.

Experimental design
Six 60 m by 4.90 m plots (for 7 rows 0.70 m apart) were established. These plots were randomly assigned to two replicates of each of the three treatments TCB+DDO, RC+TCB+DDO and TCB+RC+DDO, corresponding to the seeding assemblies described in Figure 1.
Seeds of the corn (Zea mays L.) hybrid DK4F37 were sown on September 23, 2001. The previous grain crop was also Zea mays L. Before seeding, residue cover was determined by the transect method (Gargicevich, 1995) To control rows end and side rows effects, data for the initial and final 10 m of the plots and for the side rows of each plot were discarded. The observations made were emerged plantlets per linear metre for all the rows, corresponding to 240 observations per treatment (40 observations/row × 3 rows/replicate 2 replicates/treatment). Counts were performed 11, 14 and 18 days after seeding.
The trial designed to establish the density of seeding was dynamically performed at a tractor speed of 5 km h -1 . This was followed by counting the seeds distributed along each linear metre in uncovered furrows which resulted in 5.8 seeds per metre (82,940 seeds ha -1 ). The seeding depth was kept at 5 cm. After seeding, soil temperature throughout the crop cycle was monitored as an explanatory variable. This was achieved using a soil temperature probe capable of data storage. The implanting efficiency for each treatment was calculated as percentage seed loss (PSS) (Bragachini et al., 2000) according to the expression:

PSS = 100 [1 -(A / B × C)]
where: A = plants emerging per linear metre, B = seeds seeded per linear metre, and C = the seed germinating potential (GP/100). This last factor was provided by the firm supplying the hybrid used.
The results were subjected to analysis of variance (p≤0.05) and means (plants emerged per linear metre of furrow) were compared using Tukey's test.

Results and Discussion
In corn cultivation, surface crop residue clearance helps to reduce allelopathy problems arising when seeding over rows corresponding to the previous year and also speeds up soil warming. Early cleaner systems were fixed but these were soon replaced by moving implements that not only clear the debris but also move the soil (Baumer, 1999).
The plant residue cover achieved by the different seeding systems was found to depend upon the inclusion of the cleaner system ( Table 2). The turbo coulter bla- Figure 1. Seeding assemblies tested. Treatment TCB+DDO: turbo coulter blade, double dis opener with seed press wheel and packer wheels. Treatment RC+TCB+DDO: notched disc row cleaner with track wheel, turbo coulter blade, double disc opener with seed press wheel and packer wheels. Treatment TCB+RC+DDO: turbo coulter blade, floating star row cleaner, double disc opener with seed press wheel and packer wheels. de/furrow opener (TCB+DDO) assembly cannot be considered a substitute for seeding units equipped with a row cleaner. The effect on surface residue of including a row cleaner in front of the turbo coulter blade gave rise to best behaviour in terms of soil warming (Table 3). From a starting soil temperature of 11.7°C recorded the day before seeding at the seeding depth, daily temperature increases until the final plantlet count were: 0.36°C (TCB+DDO), 0.42°C (RC+TCB+DDO) and 0.38°C (TCB+RC+DDO). For the same date, the temperature differences between the inter-row (IR) soil and the seeding line (SL) soil (L) were 0.55°C, 1.98°C and 0.83°C (for treatments TCB+DDO, RC+TCB+DDO and TCB+RC+DDO, respectively). This finding is in agreement with observations made by Wicks et al. (1994). In our study, the difference between pre-and post-seeding soil temperature can be explained by frost recorded the night before seeding. This rise in temperature leads to changes in crop phenology, growth and index (Andrade et al., 1996). Temperature affects the length of the crop cycle from seeding until physical maturity is reached, while the photoperiod can affect the time between sprouting and flowering. Thus, the total biomass produced depends on the rate of growth from emergence to maturity.
Residue clearance prior to the action of the turbo blade (RC+TCB+DDO) led to significantly improved counts on the three dates (Table 4). Despite the final plant populations for the 3 treatments showing similar absolute counts, plant emergences for each of these dates reflect a more uniform crop stand using the RC+TCB+DDO system. This effect is of particular relevance if we consider the different treatments, since in non-uniform stands, subordinate plants yield considerable less grain, and this effect is not compensated by greater production by the dominant plants (Andrade et al., 1996) The data in Table 5 indicate that the planter with the row cleaner positioned in front of the turbo blade (RC+TCB+DDO) showed best performance in terms of crop emergence. In contrast, in a similar study involving the direct seeding of soybean, Bragachini et al. (2000) reported best behaviour using a floating star cleaner placed behind the turbo blade.
Our results and observations indicate that the turbo blade is highly efficient at cutting and is good at penetrating and moving the soil with scarce burying of plant debris. The implement opens a furrow of approximately 8 to 10 cm depth and 3.5 cm width. This is followed by closing of the furrow which is then shaped by the double disc for seed deposition guided by the side wheel. Moreover, since the soil is slightly dis-   placed, this helps the action of the covering/packing wheel with notched discs. The soil overturned on the surface of the seeding line, which is generally most visible, depends on the surface residue cover. If the soil is totally covered, no overturning of plant debris is observed along this line and there is consequently no movement of loose soil. Accordingly, as the plant cover diminishes, more loose soil appears along the line of seeding. Treatments were defined in Fig. 1.