GrowNotes

Spray application manual

Module 22: Integration of the sprayer with other farm equipment

22.5 Boom width, sprayed width and nozzle spacing

Published 24 January 2025 | Last updated 20 January 2025

In situations where the spray operator would like to have the opportunity to match the position of the nozzles on the boom with the crop row or inter-row, they must consider the overall width of the boom, as well as the nozzle spacing, using the same units of measurement as the crop row width on the seeding equipment.

Boom width could be a multiple of the width of the seeding equipment, remembering that the physical width of the boom may not be exactly the same as the sprayed width. Sprayed width is normally determined by the number of nozzles multiplied by the nozzle spacing (where end nozzles on the boom have the same fan angle as other nozzles on the boom).

Tip

Do not include additional wheel-track nozzles when considering boom width, these should only be considered when determining the applied rate of product to enter into the rate controller.

Nozzle spacing and configuration on the boom

Booms may be plumbed with either a ‘broadacre’ set-up, or a ‘row crop’ set-up. Row- crop plumbing (also known as centre-plumbed booms), will have a nozzle outlet in the centre of the sprayer and an equal number of nozzles either side of this (the total number of nozzles is not an even number).

Hence, a metric boom that is physically 36m wide with nozzles at a 0.5m spacing that is centre-plumbed should have 73 nozzles fitted, meaning that the sprayed width would be 36.5m. Whereas a 36m boom plumbed with a broadacre set-up, with a 0.5m nozzle spacing, may be supplied with 72 nozzles producing a sprayed width

of 36m. This difference in sprayed width may be very important for obtaining an overlap of spray patterns at the end of the boom between each pass of the sprayer.

It is important for the operator to confirm how the boom is to be plumbed and the sprayed width before delivery to ensure it will fit in with their existing or future planting configurations.

Tip

For some applications, such as pre-emergent and residual herbicides, the operator may not want additional overlap at the ends of the boom. Where an overlap is not wanted, the operator may choose to replace the last nozzles with a standard fan pattern at the either end of the boom with offset nozzles (half spray patterns). When doing this the operator should adjust the width of the outer section in the rate controller to reflect the actual sprayed width of that section.

Centre-plumbed booms can allow for nozzles to be plumbed to match up with the inter-row spacing when an even number of crop rows has been planted, provided the units if measurement for the nozzle spacing matches the units of measurement for the row spacing. Where an uneven number of rows have been planted, the nozzle outlets may line up with the planted row.

Nozzle spacing aligned with the crop row width

Nozzle spacing to match the row version 1
Figure 1 When nozzle spacing aligns with crop- row width, directed and banded sprays into the inter-row, or onto the crop itself, become possible. Source: Graham Betts (1.5 MB JPG)
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Matching nozzle spacing to crop row width

Growers and spray operators in the northern GRDC region who have traditionally used wider row spacings (e.g. 40 inches or more, or 1.016 metres or more) have matched nozzle spacing to crop-row width, or fractions of the row widths (e.g. one-half or one-quarter), for many decades, even before accurate GPS systems became widely available.

However, the increased level of precision that GPS and autosteer capabilities now have available means that these techniques can now be utilised in farming systems with much narrower row spacings. There will be some practical limits to how narrow the nozzle spacing can be.

Find out more

For more information on inter-row shielded spraying and the application of bands, go to Module 17: Pulse- width modulation

Benefits of matching nozzle spacing to crop row width

To maximise the flexibility of the system, having nozzle spacings at intervals of at least half of the crop-row width opens up several opportunities, including:

  • improved stubble penetration

  • reduced boom height, leading to reduced drift potential

  • improved overlap of spray patterns for fungicide applications

  • the ability to apply banded liquid fertiliser to the inter row

  • the ability to apply directed sprays onto the crop row, and

  • inter-row shielded spraying.

Practical limitations of using narrower nozzle spacings

One of the practical limitations of using a narrower spacing is that the flow rates required to deliver the desired application rate (litres per hectare) generally requires that small-orifice nozzles may be required.

Smaller orifice sizes, such as an 01 orifice, can potentially create problems with nozzle blockages (unless mixing is always correct, and the filtration is very good). Additionally, the range of nozzle types and spray qualities available in an 01 orifice size is greatly reduced when compared to larger orifice sizes.

Example using 01 orifice sizes (the smallest practical orifice size available)

On a standard, single line boom fitted with 0.25m nozzle spacings, using an 01 orifice produces an equivalent flow rate to using an 02 orifice at a 0.5m spacing on the boom. At an operating pressure of 3.5 bar (mid-range for a low-pressure air-induction nozzle) this would deliver 57L/ha at 18km/h.

In practical terms, either a 0.25m nozzle spacing for metric units, or a 10- inch spacing for imperial units, is the narrowest nozzle spacing that should be considered for the majority of applications in grain-production systems.

The impact of mixing imperial and metric units of measurement (row spacing and nozzle spacing)

Where the nozzle spacing is an exact match to the crop-row spacing or is established at half or one-quarter intervals of this spacing, it allows the operator to use banded and directed spraying, as well as inter-row applications or shielded spraying.

If the measurement units are mixed, e.g. imperial row spacings and metric nozzle spacings (or vice versa), a mismatch will occur. The wider the boom width is, the more out of alignment the nozzle outlets and crop row or inter row will become.

For example, for a boom that has been centre-plumbed (row crop set-up) using an imperial spacing of 20 inches to spray a crop that has been planted using a metric 1-metre row spacing and an even number of planted rows, the centre nozzle of the boom should align with the inter-row space of the 1-metre rows.

Using a 20-inch nozzle spacing, which is actually 0.508m, the next nozzle out from the centre of the boom will be 0.8cm off centre, two nozzles out from the centres will be 1.6cm off centre, 20 nozzles out will be 16cm off centre, and 36 nozzles out from the centre will be 28.8cm off centre from the inter-row.

For ‘over-the-top’ or ‘blanket sprays’ mismatching measurement units is not likely to have a great impact on the result, provided the boom height is set to obtain a double overlap, and the spray patterns from each pass of the sprayer overlap sufficiently.

However, mismatched row and nozzle spacing prevents directed spraying onto the crop row or attempts to apply banded applications to the inter-row space.