PGRO Crop Update (22.08.14)

Field bean desiccation

As well as increasing production costs there may also be loss of crop from the passage of the sprayer. Desiccation will not advance seed maturity and has a slow effect on green stems. However if the crop is infested with green weedy material or has a few late set pods which are still green, application of a desiccant will aid combining. It is important to apply the desiccant at the correct stage of maturity. Application before this stage may result in reduced yield or loss of seed quality.

The most widely used material is diquat. A non ionic surfactant can be added. Apply when 90% of the pods are dry and black and most of the seed is dry. At this stage most of the leaves have senesced and fallen but the stems are still green. The contact action is fast and harvesting can be carried out 4-7 days later. It can be used on crops for animal feed, human consumption or seed.

Glyphosate is not a true desiccant but can be used as a pre-harvest treatment to control perennial weeds. It must not be used on crops destined for seed.

Drying and storage for combining peas and field beans

The quality standard for peas and beans is usually 14% moisture content (MC) with 2% impurities, or a combination of the two that should not exceed 16%. Drying can be more difficult with peas and beans than with cereals due to seed size, and whilst damaged produce is acceptable for compounding, mouldy produce is not. Peas and beans should not be over-dried and at higher temperatures texture may be affected and peas may split.

The large seed size of beans makes drying difficult as beans have a low resistance to air flow. It takes time to move moisture from the inside to the outside hence slow, gentle drying with ambient air is best.

The table below details the maximum recommended drying temperatures:

 
Maximum drying temperatures - Peas
Product
%MC
Max drying temperature
Seed
>24%
37ºC
Seed
<24%
43ºC
Human consumption
>24%
43ºC
Human consumption
<24%
49ºC

 

Maximum drying temperatures - Beans
Product
%MC
Max drying temperature
Seed
>24%
34 - 38ºC
Seed
<24%
38 - 43ºC
Human consumption
 
43 - 49ºC

 

Any type of dryer may be used for peas and beans, but those operating at low temperatures are safer.

Floor-ventilated bins are easy and relatively safe to operate. When the initial moisture content is high, the transfer of the produce from bin to bin and the use of warmed air together with adequate ventilation may be necessary to avoid mould developing in the upper layers.

Radially-ventilated bins allow faster drying than floor-ventilated bins, but care must be taken not to overheat peas and beans.

On-floor drying using ambient or warmed air can be used, and provided there is sufficient volume of air
and adequate ventilation, peas of relatively high moisture content can be dried using this method. Care must be taken not to load beans too deep if moisture content is high and if lateral ducts are spaced wider than 1 m.

Continuous flow driers designed to work on a short period/high temperature basis need more careful operation than other systems for peas. Use of continuous flow driers should be avoided for beans where quality is important since they may cause cracking.

For safe storage, the maximum moisture content of peas depends upon the method and the length of time they are to be stored. Peas may be safely stored for up to 4 weeks at 17% MC, but if they are to be stored until the following spring, the moisture content should not be above 15%. If the peas are in bulk with forced ventilation or frequently moved, the moisture content can be 1% higher. Storage in dark areas is recommended for beans destined for the human consumption market to delay the development of tannins which cause beans to discolour. Beans must be dried down to 14% MC for long-term storage in bulk - this is important since beans are often stored for some time before they are sold.