Managing soyabean crops: Contending with moisture stress


THE current 2013/14 season has seen an increase in area planted to soyabean as farmers moved away from cotton and in some instances tobacco.
This is a welcome development given the strategic importance of soyabean in our economy.
As pointed out earlier many of the farmers taking up soyabean have little or no knowledge about managing the crop.
The greatest challenge at this point in the cropping cycle is weeds.
Weeds are a menace in both young and mature soyabean crops.
Due to incessant rains in many parts of the country, weed pressure has been high and farmers have been pre-occupied with clearing weeds from their fields.
We applaud the chemical manufacturers for providing herbicides in smaller pack sizes, mostly one litre doses enough for a hectare.
The opening of many outlets in the farming areas is also a welcome development. Prices have been on the high side with some bordering on profiteering.
How else do we describe the situation where a one litre container costs US$4 retail in Harare and up to US$15 out in the farming areas.
These high prices of herbicides seriously erode earnings from what is otherwise a lucrative crop.
Much of the soyabean crop is now at the pod-filling stage.
This is a critical stage requiring adequate rainfall.
The recent dry spells especially in the northern parts of the country have not helped soyabean crops.
In some fields whole patches have literally dried up.
Soyabean can tolerate short dry spells before flowering stage.
After the onset of flowering, drought results in loss of flowers which leads to reduced yields.
During the pod-filling stage, drought will result in empty or partially filled pods. The lower pods which form first may contain mature seeds, but upper pods may contain a few green immature seeds or no seeds at all.
If the dry spell persists for more than a week or two, the soyabean leaves will turn brown and dry up, but will not fall off.
In other cases the leaves fall off, but the stems remain green; a clear sign of drought-induced leaf losses.
If rains return, indeterminate varieties may resume growth and flowering to give some yield if the plants have not died.
Determinate varieties are those that stop growing once flowering has commenced and will not resume growth if the rains return.
The current dry spell in the northern parts of Zimbabwe spells trouble for those late-planted soyabean crops now at flowering and pod-fill stage.
If the rains return, indeterminate varieties will start to grow and produce new flowers.
Indeterminate varieties will not and yields may suffer.
It is advisable then to grow both determinate and indeterminate varieties as a risk management strategy where irrigation is unavailable.
Usually at the crop maturity stage, soyabean leaves turn yellowish and then brown and gradually fall to the ground.
The plants will have reached what is termed ‘physiological maturity’.
At physiological maturity, additional nutrients (fertilisers) or irrigation or rain water will not help to increase the yield level.
Irrigating a crop that has reached physiological maturity is also a waste of resources as no additional yield gain is possible.
However, late in the season soyabean crops may lose many lower leaves either because of dry weather conditions or diseases like red leaf blotch.
The stem and pods will still be green.
As long as the crop remains with several green leaves, it means grain filling is still taking place; light irrigation or rainfall will help to increase seed size and weight. Many of the soyabean crops have reached this stage.
In relatively shallow and light soils, moisture stress is now common.
For those with capacity, this is the time to irrigate, but for the rest we must pray for more rains, seriously.
The best yields will be realised where the soyabean crop does not experience moisture stress from the flowering stage to maturity.
This is the period when supplementary irrigation is useful to carry the crop through to maturity.
Irrigation is a risk-reduction strategy; otherwise soyabean is a rain-fed crop.
Supplementary irrigation can be done with four – six hour cycles.
The trick is not to wait until the plants are wilted.
Light irrigation must be applied as soon as the dry spell rears its ugly head.
By listening to the weather forecasts, farmers can get an idea as to what the rainfall pattern is likely to be in the next few days.
Then they can plan accordingly.
An important point to note about drought stress in soyabean crops is its relation to land preparation.
Deep ploughed or ripped lands retain moisture better.
So the ripper must be used as a soil moisture management strategy.
Rip lines will allow the soil profile to capture and retain moisture for longer than a similar un-ripped field of similar slope.
Run-off is higher in un-ripped lands as infiltration is poor.
A ripper is an essential tool for those cropping on relatively shallow or sloping lands where run-off is high.

l To be continued


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