Spraying oilseed rape at mid-flowering with a conventional tractor-mounted sprayer may not be as damaging to final yield as many growers think, research by Bayer CropScience has revealed.

Eight German trials (see table) during 2006 and 2007 across a range of varieties similar to those grown in the UK found that yield losses across the bout width were generally very low (about 1%) up until growth stage 65, the firm’s Chris Cooksley says.

“If you normally spray early to avoid damaging the crop, these results show you probably wouldn’t do as much damage as you think. At these stages (GS 57-65) the crop’s still flexible, so recovers well, despite being pushed over quite a lot by the tractor. But later on (GS 69), the stems are larger and stronger, which means they can break, causing secondary growth problems and possible harvesting difficulties.”

Mr Cooksley acknowledges that many growers may be eager to spray early, particularly after the bad sclerotinia problems over the past two years.

“All sclerotinia products are protectants, so you have to get on before the disease gets established. But going too early risks products running out of steam and allowing late infection into the crop – as occurred in regions such as Kent last year and Scotland in 2006,” he says.

A Bayer/ Velcourt trial last year in Kent found that applying Proline (prothioconazole) at 0.5 litres/ha at green/yellow bud proved less effective than the later early/mid-flower timing, he said.

“The wet weather arrived middle/end of flowering and the results showed that the early green/yellow bud spray had run out of steam, as the crop yielded 4.61t/ha compared to 4.9 t/ha for the later treatment (untreated yield was 4.3 t/ha).

“That’s an extra 6% yield from the better timing, which easily overcomes the 1% yield penalty the German study shows is caused by travelling through the crop later on.”

Yield losses caused by tractor spraying

Growth stage

Crop height (cm)

% yield loss compared with unsprayed

   

18m spray width

24m spray width

57

105

0.5%

0.3%

65

140

1.0%

0.6%

69

155

1.5%

1.0%

German trials used a tractor on standard tyres with 1.6m track width and 53cm ground clearance.