WE have discussed many aspects of soya bean production in previous articles in this series.
The last article dealt with pests and diseases in soya bean crops.
This time we are looking at harvesting.
When do we harvest and how?
Key issues to consider in harvesting soya bean include variety planted, the days from germination to maturity, pod clearance, days to shattering, lodging (falling over) and the plant population.
How does the farmer know that his/her soya bean crop is ready to harvest?
Normally soya bean leaves are dark green.
As the crop matures, the leaves begin to turn golden yellowish and then brown.
Once the leaves begin to lose their green colour, the crop has reached its ‘physiological maturity’, a term used by scientists to mean that adding more water and nutrients will not increase the yield of the crop.
Golden yellow patches appear across the field of maturing soya bean.
The sea of green gives way to golden yellow and eventually khaki brown.
One by one the leaves will drop to leave each plant looking ‘naked’ with all the pods hanging in small bunches on the stems.
Each pod will have seeds ranging from two-to-four in number. The seeds range in colour, from whitish yellow to a cream colour.
There are cases where some lower leaves may drop off, leaving a few dark green ones at the top.
Such a crop is not yet mature.
The few green leaves left will still be supplying the pods with more food to reach maturity.
We must mention though that leaves can also drop due to diseases such as soya bean rust, frog-eye leaf spot or red leaf blotch.
In such cases, the stems, pods and even the leaf stalks may remain green.
The crop is not mature.
While the leaves turn golden yellow and then brown, the pods will also turn khaki brown as they slowly dry out losing most of their moisture.
A rattling sound is produced in the pods when the plant is shaken.
This sound grows sharper as the pods dry out.
This provides the signal to start harvesting the soya bean crop.
The fallen dry soya bean leaves form a soft carpet on the ground (manyowa) which, if ploughed in, improves the soil organic matter content, water-holding capacity and residual fertility for other crops grown in rotation.
In drought-induced immature ripening, most leaves dry out, but remain attached to the stem.
The seeds look wrinkled and often greenish to show that they were still growing when the plant was deprived of water.
Such seeds have very little oil.
Small shrivelled seeds may also be found in crops where termites have attacked the crop’s roots, destroying most of them such that the plant dries up before the seeds have matured.
In termite-infested areas, large patches in soya fields may appear to have matured prematurely due to termite root damage.
Serious yield losses are experienced.
Once the crop has matured and dropped its leaves, farmers must check to see if it is ready to harvest.
The sharp rattle in the pods is a good indicator of dryness. When pods readily crack open when squeezed between fingers that also is a good sign of drying.
Soya bean has a window period from the time the pods dry up to the time they spontaneously shatter throwing all the seeds to the ground.
This period from maturity to shattering may range from as short as two weeks for older varieties up to four-or-five weeks for the newer commercial varieties.
Farmers are strongly encouraged to harvest their soya bean crop as soon as it is dry enough to thrash.
Further drying can be done in the open after the crop has been taken in.
If the mature crop is hit by rain showers before harvesting, farmers must avoid cutting and heaping but leave it in the field where it will rapidly dry out once rains stop.
In the same vein, farmers must avoid cutting and heaping soya bean in the field for more than a day.
It will shatter or may be caught up in unexpected rain showers common in some areas during harvest time.
If rained while heaped, seeds may begin to germinate or even rot.
Soya bean must be harvested during this ‘window’ period to avoid shattering losses.
Two main methods are used for harvesting soya bean – hand and machine harvesting.
Harvesting of soya bean can be done by a combine harvester. Farmers are encouraged to book combine harvesters in advance to ensure timely harvesting.
Combining charges vary and may range from US$80 to US$100 per hectare.
Rates may be quoted with or without diesel fuel.
In most cases, the farmer is required to supply the fuel for combine harvesting.
Combine harvesters are used in large scale soya bean production for areas exceeding five hectares.
The harvester has a head which cuts the stems near the ground, thrashes, winnows and places the grain in a special bin.
Pod clearance refers to the distance from soil level to the lowest point where pods are attached to the stem.
The combine blades must cut below this point to take in all the beans.
If pods are too close to the ground (small pod clearance), some will be too low to be harvested leading to yield losses.
The harvester, if properly set, recovers anything from 70 to 90 percent of the crop.
The percentage recovery is higher if the the machine is properly serviced and tuned and, the crop has been well-maintained to minimise lodging and remove green weed infestation.
All soya bean plants that have fallen over (lodged) cannot be picked up by the harvester.
It is normal practice to engage labour to ‘clean up’ or glean the soya field of all soya missed out by the harvester otherwise losses will be high.
Soya bean fields must be weed-free.
Weeds, especially those that are green clog the air passages in the harvester reducing the machine’s efficiency.
If green weeds are present, they must be uprooted by hand if labour is available.
Alternatively, the weeds can be ‘burned’ with a herbicide such as paraquat or round-up prior to combine harvesting.
Alternatively, special threshers powered by small diesel or petrol motors have been developed, tested and availed on the local market through a partnership between the Department of Soil Science and Agricultural Engineering, University of Zimbabwe, and certain local equipment manufacturers.
These units are available for hire and can be moved from farm to farm.
Their numbers are still relatively small.
On average, each thresher can do one-to-three tonnes per day. Most of the threshers require labour to cut/uproot the soya bean plants prior to feeding them into the machine.
Some of the threshers are hand-operated, while others are motorised.
The development of motorised threshers is an area requiring more investment in research and development as the majority of our farmers are in the small-to-medium scale category where large combine harvesters are inappropriate.
These efforts should be part of the Government’s mechanisation policy thrust.
The traditional method is hand-harvesting of soya bean.
From first light in the morning up to about mid-morning, the soya plants can be handled without pod shattering.
The plants are cut with a hoe or sickle below the lowest pods on the stem and placed on tarpaulins or other suitable surface (e.g. thick plastic), spread out so that they dry out.
The plants are harvested in the early morning when they are sufficiently moist from overnight dew, to prevent shattering.
Around mid-day when the pods have dried out, the soya bean plants are thrashed with sticks to shatter the pods to release the seeds.
The chaff is winnowed off and the grain spread out to dry prior to bagging and marketing or storage.
Samples can be taken to the nearest Grain Marketing Board (GMB) depot for moisture checking.
This must not exceed 12,5 percent.
Over-drying also reduces weight and loses the farmer money.
Hand harvesting output per person averages 100 kg per day but experienced farm labourers can each thrash and winnow 150 to 250 kg of clean soya bean per day.
Labour for hand-harvesting needs to be mobilised in good time to avoid shattering losses.
Plans in Zimbabwe to expand soya bean production under the Presidential Inputs and Command Agriculture programmes must include vigorous efforts to address head-on the issue of mechanised harvesting technologies.
Labour is no longer readily available or affordable given anticipated larger areas to be planted.
Alternatively, the threshing can be effected by a span of oxen driven to walk repeatedly over the heaped soya bean so as to shatter the pods to release the grain.
Labour can then be mobilised to winnow the soya grain.
Another way of threshing soya bean is to drive a tractor over the heaped dry soya bean plants.
The heap is turned over regularly to expose un-shattered pods and to allow for drying.
Winnowing can then proceed in the normal way.
Some people fear that the soya seeds will be split during the thrashing, under the cattle hooves or tractor wheels.
In practice, only a small percentage (5-10 percent) of seeds are split.
Local GMB quality guidelines provide for a maximum of 20 percent split beans in Grade B, where the bulk of commercially grown soya bean is classified.