Spread the word on fertiliser
He told the recent tillage conference that if the incorrect rate is applied, grass or crop yield is lost or fertiliser is wasted, and crop or grass quality can be reduced. In cereals, crop lodging causes greater losses. Variations within the spreading width mean some of the crop gets too little, some too much.
Uneven spreading is obvious if green striping is seen in the crop or grass, but losses are also incurred at lower levels of unevenness where no striping is visible.
The cost of inaccurate spreading is difficult to estimate, but could be up to €80/ha.
The twin-disc spreader design dominates the arable market due to its simple robust design and ability to achieve even-spreading over wide bout-widths, in test-hall conditions.
The design of fertiliser drop point, discs and vanes is critical for good spreading and, where required, the adjustment of these components to suit the bout width, and fertiliser type is also vital.
Machines that produce well-shaped spread patterns are generally easier to set, and less influenced by small differences in fertiliser quality and field conditions.
Larger sized, well-rounded particles are easiest to spread. Where blends are used, all components should be of a similar granule characteristic. The setting of the spreader for evenness and its calibration for the correct application rate, are vital for efficient fertiliser use and are aided by manufacturers’ setting information, which is increasingly based on fertiliser quality.
While fertiliser spreaders will continue to develop, manufacturers must strive to produce spreading mechanisms that perform well in the field, and have the least reduction in performance compared to the test hall.
There are four main steps to ensure even and accurate fertiliser spreading:
* Selecting the correct machine
* Calibrating the application rate
* Choosing fertiliser that will spread well
* Setting the machine for even spreading.
Using contractors to apply fertiliser does not eliminate the need for careful monitoring. Machine type, settings and fertiliser quality are just as important as when spreading yourself.
* Machine selection: The most important attribute of a fertiliser spreader is its ability to spread evenly and accurately at the desired bout width. Hopper capacity, chassis design, shutter control, etc, are all buying considerations, but they are minor compared with spreading ability.
In an article in the Todays Farm magazine for Teagasc clients, Mr Forristal says it is incorrect to presume that ‘evenness’ in spreading is only important with tillage crops. Even spreading is just as important on grassland, although narrower bout widths can make it easier to achieve.
All of today’s spreaders are broadcasters which throw fertiliser from a central position across the bout width, and rely on an overlap with the adjacent run to give an even distribution.
The design of the spread-mechanism such as discs and vanes (or spout), and fertiliser delivery to the spreading elements is extremely complex, and determines the spread pattern.
There are vast differences in the spreading ability of different makes of spreader despite using the same basic design.
A performance test is the only reliable way of knowing how good the spread pattern is. Spreaders are tested using between 50 and 200 collecting boxes (25cm or 50cm wide) placed across the full spread width of the machine, in a purpose-built test hall.
After spreading over the boxes, the contents are individually weighed and the results analysed. The variation in the spread pattern is gauged by the co-efficient of variation (CV) value.
While a value of less than 15% is acceptable in the field, the lower the CV, the better.
In the windless conditions of test halls, CVs of less than 5% can be achieved.
From a series of independent tests, a trend in fertiliser spreader design is evident. Spreaders with outward spreading discs are capable of achieving good CVs, but generally require very careful setting to suit the bout width and the fertiliser type.
The more recent inward-spreading disc spreaders have a very good basic spread pattern that requires less setting for different fertilisers, etc, to achieve good results.
Manufacturers also carry out their own spread tests, but their results need to be interpreted very carefully.
Reciprocating spout machine have a more limited spread width.
Pneumatic spreaders are expensive, there are maintenance and corrosion issues; but they are better in windy conditions or with poor quality fertiliser, and shut-off is sharper.
* In the field: Most spreaders require some adjustment or setting, depending on fertiliser type and bout width.
The adjustments needed, depending on your spreader model, may include such factors as disc speed, vane type and position, disc type, fertiliser drop point, hopper height and/or angle.
The starting point is the manufacturer’s instruction manual. Many manufacturers now have support on the internet for spreader setting.
Fertiliser quality can have a big effect on spreading evenness. Knowing the physical characteristics of the fertiliser, in particular, the size of the granules, is essential.
Simple sieve boxes can help determine fertiliser characteristics, while some spreader manufacturers also have a service that supplies details from analysis of a supplied sample.
Generally, fertiliser should have 80% of its particles in the 2mm to 4mm size range with blend components (N, P, K) each having similar size distribution.
Some manufacturers supply simple test kits with a small number of trays (three to seven) to guide adjustment in the field.
These need to be used very carefully to get a reliable result.
* Calibration: All fertiliser spreaders require some level of calibration to ensure that the correct rate is being applied.
Relying on last year’s settings and trying to tweak it a little as each field is spread is not good enough.
The calibration system varies with the fertiliser spreader type, but typically involves stopping, or removing, or covering a disc to allow the fertiliser to be collected over a timed period (flow rate).
The exact forward speed of the tractor is then calculated. With forward speed and fertiliser flow rate known, accurate application rates can be achieved.
One manufacturer has introduced a simple flow-bag system for assessing flow rate through their fertiliser spreader.
While this is not a direct calibration of the spreader, it is a simple and logical guide which would quickly allow the flow rate of different fertilisers to be checked.
