Managing soyabeans: Of weeds and harvesting


IN the last article we looked at the challenges of managing soyabean under moisture limited conditions.
We highlighted the importance of supplementary irrigation and of moisture-capturing or conserving land preparation such as ripping.
We also saw that indeterminate soyabean varieties may survive mid-season droughts better as they resume growth when rains return.
Determinate varieties have a fixed growth cycle: once they reach flowering, if drought comes and flowers drop off, they are unable to resume growth or produce new flowers with the return of the rains.
Farmers may take note and plan on how to manage moisture stress.
This week we focus on managing weeds especially as we go towards harvesting soyabean crops in a few weeks time.
A major pre-occupation for soyabean farmers at this time is clearing their crops of weeds.
Apart from lowering yields through competition with crops, weeds interfere with harvesting operations.
Most of the weeds are now mature and occur as isolated stands in the soyabean fields.
Mature weeds do not respond well to selective herbicides; so weeds must be removed by hand.
This should be done before the weed plants set seed.
Farmers must particularly watch out for ‘wandering jew’ or gezi, which grows luxuriantly once the soyabean canopy opens up as the leaves fall.
Other broad leaf weeds also come up especially with late rains.
The weeds take advantage of the break-up of the leaf canopy as the crop ripens and sheds leaves.
More light will be coming through supporting vigorous, but unwanted weed growth.
In preparation for combine harvesting, green weeds must be removed by hand or by ‘burning off’ with the herbicides such as paraquat.
This can be done a week or so prior to harvesting.
Green weeds tend to choke the vents in the harvester.
They get cut up and mix with and discolour the soyabean grain.
Grain mixed with green weeds may need to be spread out to dry and to be further winnowed prior to marketing or processing.
All this will entail extra labour and costs, thereby reducing profits.
So farmers must control weeds early in the cropping cycle by using pre-emergent herbicides.
These prevent weeds from germinating.
If weeds do come up, post-emergent herbicides must be applied at an early stage (three -five leaf stage) to ensure effective weed control.
Problem weeds during harvesting are those that are green and juicy (e.g. gezi). These choke the combine harvester and slow down the rate of harvesting as frequent stops are made to clear the vents.
It is obvious that the combine harvester scatters the chaff as it goes through the field.
This effectively distributes weed seeds over the field compounding the weed challenge.
Weeds must be removed before they mature and set seed.
The thorny upright starbur or chidhongi makes hand harvesting difficult due to its thorns.
Those harvesting have to spend time disentangling the thorny starbur from the soyabean to allow for easy handling. Portions of the field infested with starbur and other offensive weeds will be avoided by hand harvesters resulting in yield losses to the farmer.
Those weeds must be removed prior to harvesting.
Weeds with a twining habit such as morning glory enmesh pods and stems and make hand harvesting a real nightmare.
Trying to disentangle the soyabean from the weed twines often results in many pods breaking off from the stems.
The pods may even shatter under hot conditions increasing harvesting losses.
The solution is to control weeds early in the cropping cycle.
Soyabean grain which is full of weed seeds may be rejected by buyers.
When a crop full of weeds (what one might call ‘sora bean’) is harvested, whether by hand or machine, weed seeds are also mixed with the seed.
If the combine harvester is not properly cleaned after harvest, weed seeds can be spread to other fields or farms where the machine is hired to harvest.
This way, weeds may be spread, thereby increasing production costs.
Seed soyabean must therefore be carefully cleaned to avoid spreading weed seeds.
Farmers who plant retained seed are advised to thoroughly clean their seed stocks to minimise the spread of weeds through planting materials.
We have emphasised the importance of controlling early weeds and removing them from our maturing soyabean crops.
We have shown that weeds interfere with crop harvesting resulting in increased costs and grain losses.
In the next segment we shall focus on harvesting and storage as we follow the soyabean cropping cycle.


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