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Looking back at last year's oilseed rape harvest, Ben Freer of NIAB-TAG took the AICC audience through the yield delivering inputs of 2011.

Upping the rate of nitrogen to 280 kgs N/ha gave a useful yield increase. Increasing sulphur rates to 125 Kgs also resulted in higher yields.  Autumn disease control ranged from an increase in yield of 0.2 t/ha to 0.6 t/ha. Late sowings out yielded early sowings by 0.4 t/ha and a further 0.4 t/ha came from applying an anti-dehiscent pre harvest. Autumn insecticidal sprays barely paid nor did spring/summer fungicides.

Peter Berry of ADAS reviewed how to treat this year’s crops. In the light of the NIAB-TAG observations on increasing nitrogen rates it would be tempting to do this, but when.

Nitrogen Management

Berry said that the optimum Green Area Index (GAI) for oilseed rape is 3.5 at early flowering. It takes 50 kg of nitrogen to get 1 GAI so a total nitrogen supply of 175 kgs N/ha is required. This 175 kgs of N comes from the soil, that already in the crop plus bagged N. Determining the size of the crop gives you how much nitrogen is available from that source.

Small crops with a GAI of 0.6 will only have 30 kgs N/ha available towards the total 175.  Another 30 kgs N/ha sits in the soil, leaving 105 kgs N/ha to apply. Fertiliser nitrogen is only about 60% efficient so to get this 105 kgs N/ha one will need to apply 190 kgs N/ha. Berry suggests that this is split 100 kgs in early March followed by the other 90 kgs at green bud. If you are looking for a 4.5 t/ha crop apply a further 60 kgs N/ha at the yellow bud to early flowering stage.

Bigger crops will need less nitrogen. A crop with a GAI of 2.0 in late February will only need 75 kgs N/ha to get a GAI of 3.5 at early flowering. Berry suggests that this application should be delayed and applied as one dose at the green bud stage.

Determining the GAI of the crop before applying any fertiliser is an essential part of the equation. Establishing the GAI can be done visually using reference photos as a guide, uploading digital photos of the crop or using a smart phone app.

Crops with a GAI >1 in March can also benefit from a PGR fungicide applied at late green bud to flowering. These gibberellin inhibitors reduce crop height and lodging and can increase rooting giving a yield benefit in a dry year. The timing though is slightly different, applications pre-green bud to late green bud will benefit rooting, whilst later timings of late green bud to early flowering will reduce height. Applying these fungicides to crops with a GAI of <1 can lead to a drop in yield.

Summary

Check the GAI of oilseed rape crops over the next couple of weeks and calculate the amount of nitrogen fertiliser needed to get to a target GAI of 3.5. Assess crop size in mid-March for PGR fungicides. Apply early nitrogen to small crops but delay for crops of GAI 2 or over. If a PGR fungicde is needed split applications give maximum height regulation.

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