Quickly calculating grain shrinkage

When grain is accepted by merchants at a moisture content level which is higher than that contracted, the weight loss due to drying must be calculated. This enables the actual value of the grain to be worked out. This charge and also the charge applied for then drying the grain can be avoided through having suitable facilities for on-farm grain drying.

The loss in weight during drying is known as water shrinkage and occurs when grain is dried, whether in the field or mechanically. The weight of dried grain may be calculated, by using shrinkage factors or the HGCA grain moisture calculator. These tables may cause confusion as for a given percentage reduction in moisture content there is a larger percentage reduction in weight. The shrinkage tables can be applied to all combinable crops, they are not influenced by crop type.

Just as a quick demonstration of how this works in practise. If you fill a D 18 Seedargrain drier with wheat at 20% mc it will hold 18 tonnes (all Seedarcapacities are based on wheat at 20% mc and with a specific weight of 74 kg/hal). To calculate the final weight of the wheat after drying and cooling to a final moisture of 15% follow this process:

• Look up the factor in the HGCA Final Weight of 1 Tonne after drying (kg) the intersection between the initial and final moisture contents – 941.2 kg
• Convert the figure to tonnes (divide by 1000) – 0.9412 tonnes
• Multiply the wet weight by the factor – 18 x 0.9412
• Answer – you are left with 16.938 tonnes at 15%

This demonstrates that for a 5% reduction in moisture content you actually see a 5.9% reduction in weight.

Total shrinkage may then be calculated by adding the water shrinkage to handling losses during the drying process. Research in the USA by the United States Department of Agriculture shows that on-farm handling losses can be anywhere between 0.22% and 1.71% and commercial installation handling losses range between 0.64% and 1.33%. How these figures translate to UK farming practises is not certain.

Being able to calculate total shrinkage is one of the first steps in constructing an economic case for on-farm grain drying. It is part of the process that accurately quantifies the return on investment (ROI) of mobile grain driers. I will continue this process in the coming weeks.