By Jack Desbiolles
The importance of seeding depth to maximise crop potential is recognised by most farmers.
Spending time quantifying the effect of a poorly set and operated seeding machine on crop response is time well invested. In cereals, the coleoptile length (length of the first shoot) influences how accurate seed placement has to be for optimum crop establishment.
As a general rule, the shorter the coleoptile length of a cereal variety, the more accurate the seed placement needs to be. The coleoptile protects the first leaf while pushing through to the soil surface.
When sown deeper than coleoptile length, the emergence of the first leaf is at greater risk of failure and disease.
With early sowing in warm soil, deeper seed placement significantly delays seedling emergence (equivalent to sowing later) - resulting in fewer emerging seedlings, which may be weaker and tiller less vigorously. With late sowing, however, deeper depth may hasten crop emergence that might otherwise be slowed by colder topsoils.
Seeding depth is influenced by the physical placement of seeds within the furrow and the amount of soil cover. Both the vertical seed spread and the uniformity of soil cover will influence the final variation in seeding depth. While seed boot design, and setting and matching to point type dictates the quality of seed placement, this is only half of the equation. Seed covering is another significant factor.
A more uniform seeding depth is typically achieved with press wheels, which minimise variation in soil cover, provided they leave a regular and stable furrow.
At the implement level, significant variation in seeding depth (20 to 60 millimetres) can be created with many seeding systems due to lateral soil throw between adjacent rows (furrow ridging).
Five options should be considered to manage furrow ridging:
In undulating ground, a machine"s lack of contour-following ability can also create large local variations in both tillage and seeding depth. Floating hitches, flexible frames and a range of contour-following designs for openers and seed boot systems can remedy this.
South Australian Grain Industry Trust (SAGIT) and GRDC-funded trials were held in 2002 at a clay-loam site at Minlaton and at a sandy site at Waikerie, using intermediate coleoptile length wheat.
Five seeding depths within a range of "too shallow" (10mm) to "too deep" (110mm) were set up, using a low-disturbance single shoot opener set at 0.18m row spacing and followed by press wheels.
No soil incorporated herbicide was used at sowing, which was conducted at low speed to minimise ridging. 110 kilograms DAPZn fertiliser was also deep banded in a separate operation at 110mm depth.
At Minlaton, Krichauff wheat was sown on 24 June at 78 kilograms per hectare, targeting 190 plants per square metre at 95 percent field emergence. Sowing was conducted in moist soil and significant follow-up rain (50mm) occurred at Day 3, 10 and 20 after sowing. The growing season rainfall (GSR) from April to October was 266mm (77mm below average).
At Waikerie, Clearfield Jnz wheat was sown on 25 May at 64kg/ha, targeting 140 plants/ m2 at 95 percent field emergence. At seeding there was suitable moisture below a drying 15 to 20mm topsoil, with an 11mm follow-up rainfall event 21 days after seeding. April to October GSR rainfall was just 91mm (72mm below average) - a drought.
Figure 1 (top) shows the extent to which wheat emergence was reduced by deeper seeding depth at the Minlaton site, reaching 85 percent, 73 percent and 53 percent of seeding rate, at 60mm, 85mm and 110mm depth respectively.
Deeper seeding depth also delayed maximum emergence by up to six to seven days. An emergence penalty of 12 percent also occurred at the shallowest seeding depth, explained by a proportion of seeds placed at 0 to 5mm, which did not successfully establish and/or were subject to predation. Under these experimental conditions, wheat established best within the 30 to 35mm layer.
At the Waikerie site, a similar response was achieved with slightly lower penalty levels (89 percent, 76 percent and 59 percent emergence rate at 60, 85 and 110mm depth). There was also staggered emergence at the 10 to 15mm depth, due to the drying conditions at sowing, coupled with only late follow-up rains.
In this case of marginal soil moisture at sowing, too shallow a seed placement resulted in similar effects to delayed sowing (by up to three and a half weeks).
At Minlaton, the data showed a yield drop beyond an optimum depth of 30 to 40mm. Deep seeding at 60, 80 and 100mm created yield penalties of 5 percent, 13 percent and 21 percent respectively, below the maximum yield of 2.55 tonnes per hectare. At Waikerie, due to the dry season, the crop yielded poorly (0.34 t/ha overall) with minimal treatment differences being recorded.
The Minlaton data set suggest that, even in the best case scenario (implement set for an optimum seeding depth at low speed), a 2 to 8 percent yield penalty can still be expected from uncorrected furrow ridging developing at actual sowing speed. These losses would rise to 7 to 13 percent and 15 to 19 percent if the implement was set deeper by 20mm and 40mm respectively (which is common deep-sowing practice into moisture).
The above considerations should form part of a precision farming approach to maximising paddock crop yields, by securing optimum seed environment in every furrow over the entire paddock. This means reviewing and optimising all potential factors influencing both the accuracy and the uniformity of seed placement and soil cover.
For more information:
Dr Jack Desbiolles, Agricultural Machinery R&D Centre, University of South Australia, 08 8302 3946, firstname.lastname@example.org
GRDC Research Code: MSF 1, program 4
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