27 February 1999

MORE NITROGEN THE MERRIER?

If you are using strobilurin fungicides this year, consider applying more fertiliser N to captilise on the full yield potential of your crop. Helen Riby investigates.

TWO full doses of strobilurins will form the basis of Alistair Leakes fungicide strategy on the integrated plots in this years Focus on Farming Practice (FOFP) trials at Stoughton Estate near Leicester. But given that their ability to increase green leaf retention and crop yield, is it time to re-write the nitrogen curve?

"All the evidence from the manufacturers and indeed from Profarmas own field trials indicates that the strobilurins performance is different to that of traditional fungicides.

"We are not sure why, but it seems likely that the plants natural defence mechanism is activated in some way so that after two full doses, the yield benefit achieved is significantly greater than would be expected from the products fungicidal activity alone. Something else is going on in the metabolism of the plant," explains Mr Leake – project manager for the CWS Agriculture, Hydro Agri and Profarma sponsored integrated trials programme.

So should growers apply more nitrogen than the current optimum?

"Try to think of the total yield potential of your wheat crop as being the full contents of a wooden barrel," he says.

"Each of the slats represents one of the inputs you need to grow your crop – sunlight, water, variety, soil, crop protection, nitrogen and so on. However, the slats are of varying lengths and the barrel is leaking.

Clearly it is the shortest slat which is limiting yield and if you can lengthen that slat you will gain a step increase in the yield achieved.

The strobilurins have effectively lengthened the pathogen control slat so that we now need to consider what might be the next limiting factor."

"One of the things that we know from using the Hydro N Tester in the FOFP research, is that there is a direct correlation between greenness and the amount of nitrogen applied.

Fine tuning

The N-Tester is used to fine tune nitrogen applications in the season by measuring the greenness of the leaf. Typically our organic wheat which receives no nitrogen fertiliser has a reading of 500 compared to the integrated wheat at 650 and the conventional, which receives the most nitrogen, at 700. We have seen that the strobilurins prolong leaf greenness and have therefore possibly also lengthened another of those barrel slats, the ability of the leaf to intercept and use sunlight."

Put another way if the optimum nitrogen rate is calculated to grow a 10t crop but the crop is now capable of yielding 12t because of the strobilurin effect, it might be that nitrogen is now limiting yield, and that by recalculating and increasing the N rate a 13t crop could be grown.

The trial was set up to look at three nitrogen treatments, 150 kg/ha, 200 kg/ha and 250 kg/ha. Comparisons were made between the best conventional triazole chemistry (Opus plus Erysto for mildew control at growth stage (GS) 30/31 followed by a repeat dose of Opus at GS37) and two alternative strobilurin treatments.

In the first of these strobilurin was applied at half rate at GS30/31, and again at GS37 and for the second, two full strobilurin doses were used, again applied at the same two growth stages. The trial was fully replicated.

The plots were combined separately and the grain weighed off. Later the Hydro deep N soil tester was used to find out how much nitrogen was left behind in the soil.

"We can look at the results in a number of ways, the response to nitrogen, the response to fungicides and the interaction between the two," explains Mr Leake. "To begin lets consider the response to nitrogen. At 150kg/ha the average yield was 7.43t/ha, at 250kg/ha it was 7.08t/ha due to lodging and at 200kg/ha, the target optimum for this site, the yield was 8.13t/ha. So, no surprises there."

It was a similar story for the response to fungicide with the full rate strobilurin clearly outperforming the other treatments.

Now what about the interaction? The worst case scenario turns out to be the 150kg/ha of nitrogen combined with the triazole treatment. This combination produced a respectable yield of 10.4t/ha and with wheat at £80/t, a gross margin over input cost of £614/ha.

The mid-range performance came from 250kg/ha of nitrogen with the two half rate doses of strobilurin. This yielded 10.8t/ha and a gross margin of £635/ha.

By far the best result, however, came from the 200kg/ha of nitrogen and the two full doses of strobilurin which together produced a substantial 12.3t/ha and a gross margin of £719/ha.

But integrated farming is not exclusively concerned with yield. Environmental factors are also a prime consideration, hence the deep N testing after the plots were harvested.

"The findings from the deep N sampling reinforce the yield results," says Mr Leake "At the 150kg/ha nitrogen rate there was 73kg left in the soil after harvest while at the 200 kg/ha rate, a bigger crop was grown and the residual nitrogen was just a little higher at 76kg/ha. At 250kg/ha, however, we had definitely overcooked it. Not only did crop lodging cause yields to fall but we also had 90 kg/ha left in the soil. This regime was therefore more likely to increase subsequent losses by leaching."

"Some people think that the prophylactic nature of the strobilurins could present a problem to integrated farming but that would only be an issue if there is ever a season when the wheat crop doesnt need a fungicide and from my experience we always need a disease treatment," he adds. "The strobilurins also have a very good environmental profile so there can be no objection on that front either."

Integrated

Of course, if you think that integrated farming is only about reducing inputs then switching to a fungicide regime which requires more to be used may seem bizarre. But integrated farming is about better targeting and using inputs more effectively not necessarily reducing them.

"From an integrated perspective, theres no harm at all in applying extra N or using more fungicide as long as the crop can make good use of it and its economically worthwhile. By increasing output we may be more efficient – particularly if we relate this to the quantity of inputs used per tonne of crop produced, rather than just inputs used per hectare. First and foremost integrated farming must be profitable farming," he insists.

So does all this mean that there is no need to re-draw that nitrogen curve?

"Well not necessarily. On reflection, I think we made the nitrogen bands rather too wide. An extra 2t or so of wheat doesnt need a whacking 50kg extra nitrogen, but it might need an extra 15 or 30kg so, to answer the question, we need to narrow the range down perhaps looking at 180, 210 and 230kg/ha. Weve certainly proved that the optimum rate isnt going to shoot up but we need to look again to see if a more subtle increase can push our 12.3t crop up to 13t," concludes Mr Leake.