Science into practice: Nitrate levels in protected winter lettuce

UK lettuce growers are presently offered derogation on EC legislation concerning the maximum levels of nitrate in protected lettuce.

HDC project PC 297 investigated if there were consistent differences between cultivars for butterhead and curly types when grown in winter. Any differences in nitrate accumulation in cultivars could then be used by growers to increase understanding of accumulation in lettuce and suggest how harvesting stage and trimming could be used to reduce the risk of crops exceeding the EC limits.

Results from a January soil-planted variety trial with both types where nitrogen fertiliser was applied to achieve 100ppm N and conventional irrigation used showed no consistent difference between cultivars. Mean nitrate levels decreased between the first and second harvest dates for both types. All samples were below the EC limit of 4,500ppm. This suggests that nitrate levels in winter-planted crops could be lowered by delaying harvesting by a week so that 40-60g of the older leaves could be removed during trimming. This would affect cropping timetables. The results highlighted high head-to-head and even within-head variation in nitrate levels and this needs to be considered when setting maximum nitrate levels.

Growers are advised to continue to stick to the Code of Good Agricultural Practice, in particular sampling. For high-risk crops they should delay harvesting until heads weigh at least 220-240g to allow two or three of the oldest leaves to be removed but still achieve head weights above the 160-180g minimum. Another tip is to trim leaves with a high proportion of rib material as this contains more nitrate than the leaf material.

 


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