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Sowing winter Oil Seed Rape in the first two weeks of August has helped many crops survive cabbage stem flea beetle grazing in recent years.

Scientific evidence and practical experience over the past two seasons, in particular, points to the value of getting crops securely beyond their most vulnerable cotyledon stage before the peak of flea beetle migration.

Even under the lower CSFB pressures and more favourable moisture conditions of last autumn, the latest National Cabbage Stem Flea Beetle Management Study conducted by Bayer, ADAS and NIAB with almost 200 growers across the country shows earlier sowing led to noticeably better crop survival to harvest than drilling in the traditional late Augst/early September slot; an almost identical trend to the very much more challenging 2019/20 season (Figure).

Figure: Overall winter OSR crop survival by sowing date (2020/21 and 2019/20)

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Source: Bayer National CSFB Management Studies (2021 on 2020)

“Providing there is sufficient moisture, our own substantial annual grower surveys have consistently shown early drilling enables crops to tolerate levels of flea beetle grazing that might otherwise devastate them,” points out NIAB break crop specialist, Colin Peters.

“In 2019, for instance, we saw over two thirds of crops sown before mid-August incurring mild, little or no autumn damage from cabbage stem flea beetle. This declined to around 40% with late August sowings and only just over 10% for those in the first week of September.

“Rapid, minimum disturbance OSR drilling immediately after combining can take advantage of surprisingly high levels of soil moisture beneath most cereal canopies,” he observes.

“Earlier-sown crops also benefit from substantially more thermal time to establish themselves ahead of winter than those sown later. Indeed, it can mean an additional 4-5 leaves/plant ahead of winter as well as significantly increased rooting. This means they are better placed to tolerate winter water-logging and pigeon grazing as well as any CSFB larval challenges.

“Higher early spring Green Area Indices (GAIs) may also significantly reduce the amount of nitrogen fertiliser required for the most productive canopies,” adds Mr Peters. “What’s more, earlier-drilled crops are generally at less risk from phoma and subsequent stem cankering, so have higher infection thresholds for spraying.

“Early sowing should all but eliminate the need for autumn insecticide spraying, helping to support the most active populations of key beneficials valuable in natural CSFB control.”

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