Recent research into nitrogen fertilisation of winter barley in the UK has indicated that a greater proportion of the total N fertiliser should be applied earlier in the growing season than was previously recommended. Research over the last few seasons at Oak Park set out to determine if similar recommendations should be made for Irish conditions.

The work at Oak Park compared the grain yield of crops that received their first application of fertiliser N in late February/early March (the early treatment) with crops that received the first N in mid-March (the mid treatment), shortly before GS30. A third treatment where the first N was not applied until much later than is recommended (early April – the late treatment) was also included to see how crops would react to delaying the first N.

For the purposes of the experiment, all N was applied as CAN (ie P and K were applied independently of the N according to soil test results). A total of 180kg N/ha was applied to all treatments. As well as altering the timing of the first application of N, the proportion of the total amount to be applied in the first application was varied, with either 30%, 50% or 70% of the total being applied in the first application.

The trial also considered two and three split programmes. For two split programmes, the remaining N for each treatment was applied approximately 15 to 20 days after the first application; for the three split programmes, 20% of the total was retained and applied between just before GS37, the flag leaf emerging stage. The results of three of the experiments are presented in Figure 1.

There were very obvious visual differences between the different treatments during the season; the treatments that got the early nitrogen were visually greener and denser throughout April and much of May than the crops that got their first N in mid-March. It would have been difficult to imagine during this period that there would not be a yield difference between the two timings at harvest. The crops that got their first N in early April looked very poor during this period.

However, once the crops had headed out, visual differences between the timing treatments were much smaller and it was difficult to pick out the different treatments. When it came to harvest, there were generally only small and statistically insignificant differences in grain yield between the late February/early March treatment and the mid-March start dates for N application.

Indeed, even where the first N was delayed until early April, when the crops were approaching GS31, grain yield was often unaffected compared with where the first N was applied earlier, although delaying until this late is not recommended due to the risk of stimulating late maturing tillers at harvest.

As regards the proportion of N applied in the first application, there was no advantage, and in some cases a slight reduction in yield, from increasing the percentage of the total N applied in the first application above 30%. The work also indicated that there was usually no advantage, in terms of yield, to using three splits compared to two splits.

However, where unfavourable conditions occur after application, particularly of the main split, retaining some of the nitrogen for a third split may be advantageous. In terms of quality, there was little difference in hectolitre weights between the late February/early March dates and the mid-March dates irrespective of how much was applied in the first application.

So what are the practical implications of this work? The work indicates that there is some flexibility regarding when the first N is applied to winter barley and that although early N may make crops look better early in the season, crops that get their first N later catch up during the season. Therefore, where ground conditions are poor in late February/early March, N can be delayed until the crop is approaching GS30 without loss of yield.

While this is suitable for situations where P and K have been applied beforehand, where the first N is being applied as a compound with other nutrients, particularly P, it should be applied as soon as conditions allow in late February. Obviously the calendar date at which winter barley reaches GS30 will vary depending on site and sowing date but at Oak Park GS30 typically occurs for late September/early October-sown barley sometime around or soon after St Patrick’s Day.

It will occur earlier further south and for earlier drilling dates and so in these areas, for early sown barley, GS30 could well occur in the first half of March or in very early regions in late February. In the current season, there are some reports of much more advanced than normal winter barley crops (approaching GS30 already) and some are questioning if there is a need to apply nitrogen earlier than normal to these crops.

In short, there would seem to be no advantage to applying N to winter barley before late February in any season as the crop will not be growing very much and therefore it will have a very low N requirement. In addition, very early applications of N to winter barley are known to increase the risk of lodging later on and will also promote mildew, which is widespread on many crops.

In terms of what proportion of the total to apply in the first split, the work at Oak Park indicated that there is no benefit to applying any more than a third of the total N in the first split. This makes sense as uptake of N by the crop will be relatively slow during March, particularly early March, as temperatures are normally low and the crop is still growing slowly.

The main split should be applied to winter barley at or before GS31 and, combined with what was applied in the first split, at least 75% of the total should have been applied by this stage.

While the work at Oak Park didn’t show any consistent benefit of using a third split, many growers will opt to use a third split, mainly to avoid having to make a very large second application, some of which could be lost if unfavourable conditions occur after application. Third splits should be considered where in excess of a total of 150kg N/ha is being applied. If a third split is being applied, it should be applied between GS32 and GS37.

Read more

Special focus: fertilisers