Main Objectives
Several considerations point to the need to re-evaluate the physiological and agronomic interactions between plant numbers and N rates for modern high-yielding crops. The more relevant of these are:
- Continental growers who consistently achieve yields greater than 100 t/ha generally use higher plant populations and more N than those currently recommended for UK sugar beet.
- Apart from a few experiments done in the 1970s, most of the research on which current UK recommendations are based treated plant populations and N rates as individual rather than interacting factors. The effects of varying plant number, for instance, have usually been measured using a single, recommended rate of N, and those of varying N rate a single, recommended plant population density.
- Increased grower concern as to whether the greater frequency of pale, late-season leaf canopies and declining amino-N concentrations in delivered beet indicate shortages of N especially now that recommendations for optimal plant population densities has increased.
- Our current understanding of how crops acquire and utilise N indicates that whereas total dry matter production is driven by the uptake of N per hectare - which primarily determined by the total amounts of N available from the soil and is independent of plant number – the way the dry matter is partitioned to the storage root and sugar is strongly influenced by the amounts of N per plant which does depend on plant number. This implies functional interactions between plant number and crop N on the production and distribution of crop dry matter that could be optimised to increase sugar yield.
- The advent of ‘whole-crop’ delivery in which substantially more crown material that derives from the shoot is processed. The growth of the shoot is more responsive to N and more strongly influenced by plant number than the storage root. This, too, would be expected to affect the plant number/N relationships and yield.
- Two recent, small-scale ‘look-see’ BBRO experiments produced significant yield gains from increased plant number and extra N (References 1 & 2). They do not, however, provide a sufficiently extensive database upon which to base recommendations
However, these indicators only circumstantial - they are persuasive but not conclusive. A more extensive series of experiments is required to establish reliable plant number/N rate yield-response surfaces for high-yielding crops grown under modern conditions on the major UK soils that will more positively indicate the optimal plant numbers and N rates for different situations.
Latest Report
Outcomes / Key Message For Growers And Industry
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The 2016 data shows very little interaction between N rates and plant population and provides little evidence that higher plant population require higher N rates.
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At both sites, the 50,000 population density was clearly sub optimal and this was expected given the current recommended population density of 100,000/ha. At Bracebridge, there were few significant differences between the 90,000 and higher (110,000, 130,000 and 150,000) population densities.
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At the Hibaldstow site, the 90,000 population density resulted in lower yields that at the higher population densities. Thus was a higher yield of the two sites and reinforces the need to establish the recommended population densities, especially to realise the potential of higher yielding sites.
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There was a consistent reduction in root content (%) above 90% N/ha at both sites albeit this effect was more variable at Hibaldstow. This effect was not significantly influenced by plant population.
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The two sites in 2016 had higher than expected soil N levels at drilling and the recommended rate of N was 100kg N/ha. This reflected in the relatively shallow response curve that reinforced the current recommendations.
