ON AVERAGE, fertiliser accounts for 38% of cereal variable costs and on mixed grass leys the figure is over double that. Shaving even a small amount off those costs is therefore a worthwhile exercise.

An even spread pattern is the key to making the most of fertiliser inputs. Variation across the working width of disc spreaders is often the cause of striping in crops. But stripes only appear when variation rises above 20%. Below this there is no visible effect but the financial implications can be significant.

We provide a guide to help avoid the losses caused by inaccurate fertiliser application – it should take about three hours to complete.

1. general condition

First of all check over the general condition of the machine. Any debris in the hopper or wrapped around the agitators should be removed. What is the condition of the discs and vanes?

2. level and height

It is very important to make sure that the spreader rides level and at the correct height. This Amazone ZA-M should run 80cm above the growing plants – this may alter as the crop matures. Adjust the top-link so that it is level fore/aft – some manufacturers may recommend a slight angle.

Then stand back from the tractor and by adjusting the link-arms try to ensure that the hopper is parallel with the cab roof.

3. vanes

Next check the vanes. When applying fertiliser to young crops the tips should be level and angled upwards for top-dressing late in the season. Assess the vanes for wear. Heavy pitting leads to an uneven spread pattern.

4. application rate

Set the application rate. The manufacturer’s instruction manual will provide a range of hopper aperture settings to achieve the desired rate. For example, where 198kg/ha is to be spread at 10km/hr the aperture should be set to open to 35 on the ZA-M”s adjustment scale.

5. rate check

Check the rate – do a bucket flow test. Remove a disc and hang a bucket underneath the hopper aperture. Next check the instruction book or work out the area that would be covered travelling at the desired speed in 15secs or 20secs. Remember to halve this as you are only collecting from one hopper outlet.

Here, running at 10km/hr for 15secs covering half of a 24m bout, we would cover 0.05ha. Multiply the desired rate by this area to find how much material should run out of the hopper – eg 198kg/ha x 0.05ha = 9.9kg.

With the spreader running at the required pto speed open the slider and allow fertiliser to run out for the required time. Weigh the amount collected – remembering to deduct the bucket’s weight – and compare with the desired amount.

Adjust the hopper aperture and retest until the correct figure is reached. Repeat this process for the other disc feed.

6. working width

Set the working width. The settings alter depending on the material to be applied. The manufacturer”s instruction manual will show the required settings for various different types of product. Position the vanes at the correct angle marked by a scale on the disc. Long and short vanes will have different settings.

7. tray test

Check the spread pattern. There are a number of ways to conduct this but, put simply, lay a row of trays out to catch fertiliser as the spreader passes over them. Collecting the material then gives an indication of a machine”s spread pattern. With the Amazone ZA-M set to spread across 24m we laid out trays across a 24m width, leaving a gap at each end for the tractor to pass through.

The tractor then runs up one side and back in the opposite direction at the other end. Although this means that we can only assess one side of each pass at a time, the trays represent the area between each tramline.

The reason for doing it this way is that most twin-disc spreaders actually spread material across double their actual working width, ie a 24m spreader covers 48m in one pass. But because the spread of material diminishes across the working width, the overlap from the next bout is used to ensure an even spread pattern. So by running up and back the full spread width is replicated.

The 50cm x 50cm boxes are then collected and their contents poured into tubes. Held in a rack and collected in the correct order, these represent the spread pattern to show any unevenness.

Where too much material collects towards the outside of the spread width, adjust the longer vane on the disc. If fertiliser is concentrated closer to the tractor’s wheelings, it is probably the shorter vane that requires tweaking. This is the case for the Amazone ZA-M machine used here – it is worth checking individual manufacturer’s settings.

Don’t forget to check border spreading devices to avoid spinning fertiliser into hedges and ditches.

fwmachinery@rbi.co.uk