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Ian's Blog - Striped Crops

Last updated: 15 Jul 2014

 

 

Check the Fertiliser Spreader Setting

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No one likes a striped crop, especially next to a road, and they seem more common than usual this year. It’s not always easy to find the cause of stripes – it could be the fertiliser but more often it’s the setting or operation of the spreader. The pattern across the field can give a clue.

It all relies on proper working of the spreader. The maximum bout width of disc spreaders has increased by 7 metres per decade since 1950 and 50 metres or more is now claimed. So we’re expecting to launch a fertiliser particle weighing about 20 mg at around 200 kph from a disc travelling over uneven ground and have it land accurately at a spot on the ground anything up to 50+ metres away.  That is asking a lot!

Throwing

If crop colour across a whole bout width between two sets of tramlines is different to that on the next bout width, it’s left/right throw. If the darker stripe is behind the tractor, it’s throwing short. This can be due to fertiliser particles being smaller than they should be but more usually its disc speed or vane settings. If the paler stripe is behind the tractor, the fertiliser is being thrown too far. This is due again to disc speed or vane settings and is less likely to be due to the fertiliser. So spreader setting and operation are important but how are they being done?

Testing

The manufacturer’s manual is a good starting point but there are two tests that need to be done – calibration to check rate of fertiliser application and tray test to check evenness of spreading. Calibration usually can be done by farm staff but increasingly is being adjusted electronically during spreading. Tray testing is best done by professionals who can adjust settings to get best performance.

The latest annual British Survey of Fertiliser Practice found that 39% of farms that used spreaders had their machines tray tested once per year and 4% after every change of fertiliser type. On the downside, 34% had never tray tested their spreaders and 11% tested less than once per year. These percentages haven’t changed much over the past ten years.

Cost vs Yield

What’s at risk? If you do the sums, you’ll find a typical spreader, over a seven year lifetime, applies fertiliser worth more than £200,000 which generates additional crop yield worth more than £1 million.

YieldRedIncCV_600_341Spreading unevenly, especially with nitrogen, usually shows as stripes in crops but the effect on yield is not straightforward. If the rate of nitrogen applied is below the optimum for yield (as it can be for malting barley for example) the crop can appear badly striped but overall yield can be little affected. This is because on this part of the response curve, yield reduction in the paler zones receiving too little nitrogen is balanced by greater yield in the darker zones receiving too much nitrogen. It’s different at the optimum nitrogen rate for yield, near the top of the response curve, where the yield reduction in the paler zones is greater than the increase in the darker zones. Striping can look less serious but yield is reduced.

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Unevenness of spreading is measured as a coefficient of variation with 5% being a good value after testing, 10% acceptable and 25% (often found in machines when first tray tested) poor. Typically, in wheat and oilseed rape yield is 2-3% lower at a CV of 25% than at 5%. Doesn’t sound much but again, do the sums and it’s equivalent to about £25/ha. Spending around £200 per year to get a spreader professionally tray tested and set up really is money well spent that could be recouped in the first 5 ha spread.




More Research?

Mentioning malting barley reminded me that years ago at Levington we looked at effects of uneven spreading of nitrogen on bread making quality in wheat. We made up grain samples all with the same overall nitrogen content but some with very uniform grain and others where part of the grain had higher nitrogen content and part had lower. There were indications that some bread making properties were better in the uniform samples. The tests were preliminary and weren’t followed up at the time. Perhaps someone would like to take another look?

Supporting information

If 180 kg N/ha is applied to 200 ha of wheat over, say, the seven year life of a spreader, a total of 252 tonnes of N (N not bagged product) worth around £220,000 will be spread. This amount of N will generate typically around 6800 tonnes of additional grain yield (27 kg grain/kg N applied) worth around £1.2 million.