2 March 2001

Strobs permit Nflexibility

STROBILURIN programmes for wheat offer more flexibility in nitrogen use than triazole programmes.

They also seem to permit higher than normal optimum N use without financial penalties.

Those findings come from eight trials over the past two years by Agrovista (formerly Crop Care and Profarma), including two funded by BASF, on heavy and light land in Bedfordshire and Suffolk. Varieties were Consort, Malacca and Savannah, all second wheats.

Last year the plots received five nitrogen rates ranging from 85 to 325kg/ha (68 to 260 units/acre), full PGR treatment and T1, T2 and T3 sprays.

The first two sprays were based on the strobilurin products, Mantra (kresoxim-methyl + epoxiconazole + fenpropimorph) and Ensign (kresoxim-methyl + fenpropimorph) or the triazole, Opus (epoxiconazole), all at three-quarter rate.

"From 85 to 205kg/ha of nitrogen the yield curves of strobs and triazoles were parallel with strobs producing around 0.5t/ha more on average," says Agrovista technical director, Colin Myram. "Then the triazoles yield began to decline but the strobs did not, so at 265kg/ha the difference averaged about 0.75t/ha, and at 325kg/ha it was 1t/ha."

That reflected the strobs better disease control at higher nitrogen rates, he says. It also meant that at under 205kg/ha (164 units/acre) of N the margin over fungicide and nitrogen costs (with wheat at £70/t and N at £100/t) was about the same for both strob and triazole programmes at £575/ha (£233/acre).

But at 205 and 265kg/ha the strob margin averaged £625/ha (£253/acre) against only £590/ha (£239/acre) for triazoles.

"The optimum nitrogen rates across the trials was 205kg/ha compared with the normal 160-200kg/ha used by growers," says Mr Myram. "But we found that if you apply too much nitrogen with triazoles the margin drops away. With strobs it does not, it is more or less the same at 205kg/ha as at 325kg."

There are so many variables affecting residual soil nitrogen levels that it is hard to judge the economic optimum dressing. But with strobilurins growers can err on the generous side without financial penalty, he says. Such a strategy ensures crops receive optimum dressing.

Leaf size

The trials also measured leaf size which proved directly proportional to the N level but was not affected by fungicide type.

Mr Myram believes crops treated with Mantra and Ensign use less energy in combating disease and so devote more to producing yield. In other words strobilurins enable crops to use more of the available nitrogen.

BASFs Steve Waterhouse agrees that kresoxim-methyl allows more flexibility in nitrogen use, but stresses that N recommendations should not be changed. Applying more with kresoxim-methyl-based programmes would increase lodging potential, he warns. "One has to be cautious. If crops do get more nitrogen, their PGR programmes must be good enough.

"Those treated with kresoxim-methyl do seem more tolerant of variations in nitrogen levels and more yield responsive under a wide range of conditions." This is due to the extra activity of a key plant enzyme, nitrate reductase responsible for N assimilation, he says.

Strobs & N use

&#8226 Two years second wheat trials.

&#8226 Range of nitrogen treatments.

&#8226 Strob maintaining margins.

&#8226 N over-use and lodging warning.

STROBS AND N USE

&#8226 Two years second wheat trials.

&#8226 Range of nitrogen treatments.

&#8226 Strobs maintaining margins.

&#8226 N over-use and lodging warning.