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.
Outcomes / Key Message For Growers And Industry
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.
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.
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.
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.
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.