THE soya bean yield is determined by the variety planted, planting date, quality of seed, plant population, fertiliser quantity applied, pest and disease control as well as efficiency in the harvesting process.
In this article we shall examine some of these yield-enhancing parameters.
High soya bean yields come with experience and appropriate training.
Many farmers have embarked on soya production motivated solely by high market prices.
When market prices of soya bean fell a few years back, many farmers abandoned the crop, citing low producer prices.
Unfortunately, the low prices were not a result of over-production but an influx of cheap imports.
Using scarce foreign currency, Zimbabwean businesses went into over-drive importing soy oil, cake and even raw beans.
That scenario was possible because foreign exchange was relatively freely available. Proponents for local soya bean production could not convince producers and even policy-makers to pause and take stock.
Today, foreign currency shortages have forced both local industry and policy makers to embrace the local content philosophy that says we should primarily rely on local production of essential crops where soils and climate are conducive.
Today, even high school students appreciate that imports represent export of jobs and de-industrialisation.
Value chain analysis shows that the benefits of local production far outweigh those of importing a commodity that can be produced locally.
The above arguments cannot sweep away the need to ensure the highest possible yields even for locally produced soya bean.
The introduction of soya bean under the Presidential and Command Agriculture Inputs schemes calls for the mobilisation of thousands of farmers many of whom will be taking up soya bean production for the first time.
Because inputs are being advanced upfront without paying, many farmers will not hesitate to sign up for the scheme.
Unfortunately, many will also not have the requisite knowledge to grow soya bean.
Every winning team requires a good coach!
For farmers, the extension department, AGRITEX, will be expected to provide the necessary training and technical advisory support.
The department requires considerable resources to equip its officers with the requisite skills and knowledge on how to grow soya beans.
The University of Zimbabwe Soyabean Promotion Programme is ready to collaborate with AGRITEX, resources permitting, to provide technical and training support to farmers and other stakeholders.
While there is reasonable expectation that extension officers will take up the task, our experience with the Soyabean Promotion Programme that ran from 1996 to 2003, showed that there is need to invest in extension officer-capacitation through training and appropriate demonstrations. Strategies for such an exercise have previously been developed and successfully tested.
What is required is to mobilise resources for the training of farmers and extension.
That way, the huge investment that Government is putting into the soya bean import substitution programme will be protected through high yields by farmers.
It is not too late for such a programme to be launched.
Radio and television programmes are also proposed, as these will go a long way to equip farmers and extension workers with appropriate knowledge and information on soya bean production.
As we said earlier, this piece was about enhancing soya bean yields.
We shall now look at some of the strategies to improve yields.
We start by assuming that the farmer has acquired the necessary inputs.
For the benefit of those who did not access previous articles in this series, we shall repeat some key information.
Good quality seed, preferably certified, applied at a rate of 100-120 kg/ha, is a requirement.
Retained seed (mbeu yomudura) is often planted by farmers as a cost-cutting measure but it can cost the farmer in terms of yields.
Seed-borne diseases accumulate over the seasons when retained seed is planted over and over. From germination, the soya bean plant is already a ‘sick’ plant.
Even with adequate fertiliser and rainfall, yields will be far less than optimal.
Nitrogen is a key nutrient for soya bean whose grain can contain up to 40 percent protein.
The cheap way to supply soya bean with nitrogen is to use rhizobium inoculant.
A sachet of 80g, costing US$5, is sufficient for 100kg/ha.
The expensive way to supply the crop with N is to apply up to 4 bags (200kg) ammonium nitrate (AN) or urea, costing up to US$140.
Farmers must make the effort to obtain rhizobium inoculants through seed suppliers or directly from the Department of Research and Specialist Services (DRSS).
Inquiries should also be made through AGRITEX offices at provincial level.
If no rhizobium inoculant is applied at planting, at 4-5 weeks, a soya bean crop will turn yellowish as a sign that the crop requires nitrogen fertiliser.
A split application of 3-4 bags (50 kg) AN will be required, split into two applications. The first application of 75-100kg of AN should be applied before flowering. The second split of 75-100 kg AN should be applied when the pods are beginning to fill.
Most soya bean crops in Zimbabwe and other countries like Brazil and the US are inoculated with rhizobium to supply the crop with ‘cheap’ nitrogen fertiliser.
Soya bean yields increase dramatically with increase in yields up about 200-250kg/ha of compound fertilisers.
Some farmers will apply booster foliar nutrients.
These are fertilisers dissolved in water and applied by boom or knapsack spray on to leaves before the soya bean flowers.
The plants grow more vigorously and develop many more flowers leading to higher than normal yields.
Correct procedures and timing are critical when applying boosters.
We must emphasise that soya bean is very sensitive to weed competition.
Early weeding is recommended but use of herbicides is recommended for large plantings.
To ensure high yields, weeds must be controlled when they are still young so that they are killed by the recommended dosage of herbicide.
Late application of herbicides on old weeds is ineffective and results in severe yield losses.
Normally, if pre-emergent herbicides are applied appropriately and in time, post-emergent weeds will be very few and yields will not be affected significantly.
For susceptible soya bean varieties (most old varieties), fungicides against rust disease must be applied as preventive sprays before flowering, even if there are no signs of rust disease.
Deep ploughing or discing when soil is moist (20 – 30 mm rainfall event).
Yields are often lost due to poor land preparation.Too many soil clumps (magadhi) will reduce germination; a low population equals low yields!
Except for those using specialised no-till planters, we recommend good deep ploughing and removal of all volunteer green plants using

‘Round-up’ or paraquat.
Land preparation must be thorough to give a good tilth.
Pre-mergent herbicides must be correctly and promptly applied to avoid harvesting ‘sora’ beans in place of soya beans,
Soya bean yields have been increased through increasing plant populations per ha. Planting populations must be controlled through appropriate spacing between rows (30 – 50 cm) and within rows (5 – 7 cm).
Spacing wider that 45 cm must only be used if mechanical cultivators are to be used for weed control. In the US and Brazil, yields of up to seven metric tonnes per ha have been achieved with the same varieties through narrowing the row spacing, in some cases down to 15-17 cm. This results in higher plant populations.
In all such cases, a high level of management is required to ensure adequate nutrients, moisture and little or no weed competition.
Break-even yields are often quoted at 2 ton/ha. In practice, with good management, yields can range from 3 to 6 ton/ha. Early planting e.g. in early November (under irrigation/early rains) enhances potential for high yields as it provides a long growing season.
If population is low, early-planted soya bean compensates by developing many tillers (branches).
Late planted soya bean crops give low yields because the plants stop growing and go into the reproductive phase once day-length begins to shorten in late February to March. Soya bean requires 12-14-hour day-length to grow to full maturity and give best yields.
Growth of soya bean is also reduced as the plants are sensitive to falling temperatures.
The latter situation takes place in high altitude areas such as the eastern highlands of Zimbabwe. Soya bean crops in these areas must be planted early so that the crops enjoy the best of warm days and a long photo-period
Availability of irrigation water to take the crop across mid-season dry spells also ensures good yields. Soya bean is very sensitive to drought at flowering.
A dry spell during that critical period results in flowers dropping down, reducing the number of pods formed. Another critical period is at the pod-fill stage.
Again dry spells will reduce pod-filling, resulting in low yields.
Poor weed control and low plant populations significantly reduce soya bean yields! Hand-hoeing is not suitable where large areas have been planted.
Farmers must use herbicides, but apply them early and correctly to obtain high yields. Continuous farmer-education and technical support are critical for successful soya bean production.
Let us close this piece by reminding farmers that soya bean yields are lowest in sandy soils and improve as the clay content increases.
Soya bean yields are lowest in drier areas and increase with the amount of rainfall, Irrigation will help increase yields. Normally, rainfall that gives a good maize crop will be adequate for a soya bean crop.
We wish all those taking up soya bean production the best possible yields!
Professor Sheunesu Mpepereki is a leading soyabean expert and soil scientist working at the University of Zimbabwe. He can be contacted at: mpepereki@gmail.com