An analysis of the variation in crown size in sugar-beet (Beta vulgaris ) grown in England

The processed part of the sugar-beet plant, the beet, consists of secondary storage tissues developed from the root and hypocotyl and the crown which, botanically, is the compressed stem. Crowns contain less sugar than roots and higher concentrations of melassigenic substances (such as amino-nitrogen compounds, sodium, potassium and invert sugars) which impair the crystallisation of white sugar during processing and make beets with high proportions of crown costly to process. Two factors influence the amount of crown material in beet delivered to factories: the original size of the whole crown in intact beet (subsequently referred to as the biological crown), and the fraction removed by the topping mechanisms of beet harvesters when the crop is lifted. The residue left on the delivered beet, for which growers in the UK are not paid, is measured as crown tare at the factory. To understand the causes of variation in crown tare, an analysis was made of the trend and variation in the size of the biological crown of sugar beet varieties introduced in the UK during the last 25 yr using information from past variety trials and new trials done in 1993, 1997 and 1998. Except for a few diploids, the biological crown was generally as large and variable in recently-introduced varieties as in those grown 15-20 yr ago. Its size was strongly influenced by locational and seasonal factors which changed the plant's shoot:root ratio primarily, it is suggested, through differences in amounts of available nitrogen. There was no evidence of changes in beet anatomy in recently-introduced varieties that would result in root material being removed with the crown when samples of delivered, machine-topped beet are contractually de-crowned to estimate crown tares in the factory tarehouse. Variable amounts of crown are removed by harvesting machines. Experiments showed that a greater proportion of beet were over-topped and more root material was removed with the crown when the biological crown was small and when harvester knives were set low to deliver a minimal crown tare. In these situations, significant amounts of root material, for which growers would be paid if delivered to the factory, were left in the field. Data from differential machine-topping trials on commercial sugar beet crops at four locations in 1996 and 1997 were used to relate yield loss through over-topping to crown tare. A crown tare of at least 8% was needed to ensure that no economic yield was left in the field; below this level the losses in delivered yield and grower's income increased exponentially through over-topping. It was estimated from the factory records of individual contracts that deliveries of beet with crown tares below 8% decreased the national adjusted yield of clean beet by 58 400 t in 1997, equivalent to 0.34 t ha(-1) and 0.56% of the total delivered tonnage.