Fodder Beet - P & K offtake

Survey of commercial fodder beet crops

In order to establish the typical values for potassium and phosphorus content of fodder beet, a survey was carried out to collect representative root samples from 30 commercial fodder beet crops across the UK during the harvest of 2000. The tops were not harvested and included in the survey because it is now very rare for these to be taken from the field. A very wide range of locations was involved from The Lizard in Cornwall, West Pembrokeshire in Wales through the Midlands to North Yorkshire. Soil type ranged from light sand to heavy loam with generally satisfactory pH and phosphate levels but soil K levels ranging from low Index 1 to high Index 3. The surveyed crops included a range of modern varieties and received varying manurial treatment. Three quarters of the 30 crops received FYM or slurry usually in conjunction with mineral fertilisers. Salt was only used on approximately 25% of the crops.

Methods

Around 50kg of roots per crop were collected, 80% from clamps, the others were hand- harvested in the field. Clamp sampling provides the most accurate reflection of actual crop removal from the field for this forage crop which is not topped to a specified factory standard but to the individual farmer's own ability or requirements. As the aim of the survey was to establish what nutrients are removed from the field in practice, the variation in top tare and crown content of samples was accepted as representing the normal range of commercial practice. The total root samples were processed to a "brei" sample (homogenous fresh sample of roots produced by chopping roots and sub-sampling as undertaken for commercial sampling of sugar beet crops) following normal procedures in a mobile tarehouse by IACR Brooms Barn who also prepared a dry matter sample. Dry matter samples were analysed by NRM Ltd using normal acid extraction to assess total phosphorus and total potassium.

Measurements

The % dry matter of fresh roots was measured together with total P & total K.
Yields were not measured but were subjectively assessed by co-operating farmers on a "trailer loads/experience" basis. Most crops were estimated to be yielding in the range 60-80 t/ha range (25-32 t/acre) with a few poorer crops around 45-50 t/ha (18-20t/acre) and one or two excellent crops over 80t/ha.

Results

The sample size was not large enough to identify relationships between "site factors" and "measured factors" e.g. variety or soil type versus dry matter : soil fertility versus root P or K content : etc. The "site factors" (soil type, manuring treatment, variety, etc.) were only recorded in order to exclude any crops which could not be regarded as normal. Whilst there was a range in dry matter and P & K content values, this was expected and none were felt to be so extreme as to warrant exclusion - see tables 3a and 3b.

Dry matter ranged from 12.3 to 19.4% with an average of 15.2%. These results are at the low end of the expected DM range for the varieties involved and this is likely to be a direct consequence of the extreme wet conditions prevailing over the harvest period.
Phosphorus (P) and potassium (K) were measured in the dry matter but are expressed here as P₂O₅ and K₂O in the fresh roots (kg nutrient per tonne of roots) as this is the normal basis for expressing nutrient removal.

Table 2 Phosphate & potash content of roots (kg/t fresh roots)

	P2O5	K2O
Lowest	0.4	2.6
Highest	0.8	5.9
Mean	0.6	4.0

Table 3a Distribution of root P values
(number of crops)

Table 3a Distribution of root K values
(number of crops)

The root K content was not influenced by manure use or sodium application, nor by the rate of potash applied in this survey.

The survey results are very similar to those from the earlier forage crop study (see table 1) and are also endorsed by removal data from other countries which quote specific fodder beet figures.