I NOTICED in Mafuta Resettlement area, Raffingora, that some farmers had left weeds to outgrow their soya crop.
Late application of ‘Fusilade’ results in poor weed control.
So timing of when to apply herbicides is critical.
Once you plant without pre-emergent herbicides, you know you will need post-emergent herbicides.
Farmers should have these ready when the crop emerges because this allows for timely application when weeds are still young.
Post emergent herbicides must also be applied while the canopy is still open so the herbicide is deposited on the leaves of weeds growing between the rows.
Talking about the crop canopy, one must consider how it shades out weeds and keeps soil moist as there is no direct sunlight heating the soil surface.
Inter-row spacing of 30 – 45 centimetres (cm) allows for full canopy cover within four – five weeks after planting.
So we are saying narrow row spacing is another way of controlling weeds.
Those who control weeds by cultivating need wider rows to allow equipment to move in without damaging the crop.
In such cases where rows are 50 – 60 cm wide within row spacing is reduced to give a higher plant population overall.
The general advice on herbicides is to read the label, seek information from those who know including local extension (AGRITEX) officers and other experienced farmers.
An important reminder to farmers is that herbicides are dangerous toxic chemicals.
Great care must be exercised when using them.
Also they must be kept away from children and under lock and key. Many suicides are blamed on victims drinking pesticides.
Spray equipment must be calibrated correctly to avoid over- or under-dose of pesticide.
Let me comment on other aspects of the current season’s soyabean crop at this time.
The soyabean crop that the Mafuta farmer was spraying with herbicides looked yellow, with clear symptoms of nitrogen deficiency. Normally if Rhizobium inoculant had been applied at planting, the crop would have looked dark green and healthy.
Farmers who planted soyabean without Rhizobium inoculant and whose crops look yellow are advised to broadcast by Vicon or even by hand, 1,5 -2 x 50kg bags of ammonium nitrate (AN) per hectare as soon as yellowing is observed at start of flowering.
The AN top dressing for soyabean should be applied when all dew has dried out to avoid leaf burn by the highly soluble fertiliser.
A second application of 1,5 – 2 x 50 kg bags of AN is required at pod-fill stage when the soyabean seeds start to form in the pods.
The total amount of AN per hectare will come to 3 – 4 x 50 kg bags.
The 3 – 4 bags of AN are equivalent to just one sachet of Rhizobium inoculant that costs US$5 and covers one hectare.
As most of you will know the single sachet of Rhizobium inoculant is stirred with one litre of water and mixed with soyabean seeds before planting.
Inoculant contains bacteria called Rhizobium.
These bacteria (also called rhizobia) form nodules on the soyabean roots inside which they manufacture ammonium fertiliser for the plant.
Let me add that in sandy soils, even with proper inoculation, some soyabean crops may look yellow because of infection by the tiny round worms called nematodes.
This is common in tobacco lands planted to soyabean.
Soyabean is not advisable to rotate with tobacco.
Talking about pests, leaf eaters, leaf rollers and semi-looper caterpillars are actively chomping and chewing the soyabean leaves during this vegetative phase of the soya crop cycle.
Only when the percentage leaf area destroyed approaches 30 percent should the farmer spray insecticides such as Carbaryl, monocrotophos, dimethoate and other insecticides.
Cotton insecticides will control soyabean leaf eaters.
Spraying when only a small leaf area has been eaten is not economic.
Soyabean rust which is a fungal disease that infects leaves starting with lower ones requires a preventive spray of fungicide at start of flowering, i.e. 50 days after planting.
Those planting older varieties must look out and spray against leaf rust. Newer varieties are more resistant.
It appears though that soyabean leaf rust may be on the decline as a major disease.
Time will tell.
Finally let me revisit the issue of foliar fertilisers.
The crop can be fed through applying liquid fertiliser to the leaves. Several companies are stocking these materials and farmers are encouraged to apply them.
This is particularly important for many farmers who planted soyabean without basal fertiliser.
Foliar fertilisers boost soyabean yields significantly.
Is it true that soya is a wonder bean?
We shall explore the evidence in the next article which will look at the benefits of soyabean and its potential contribution to the fulfillment of our food security in the context of the Zim-ASSET blueprint.