Peter Gambara

SINCE the start of the year, most areas have continued to receive persistent rains, raising concerns by some farmers that the current rainfall activity will cause damage to the crops. Firstly, farmers can take advantage of the rains by split-applying top dressing in maize.

When I worked for the Agricultural, Technical and Extension Services (Agritex) some years back, we used to emphasise that “dzokororo ine simba” (repeating teachings to farmers benefits them immensely). I would therefore like to re-emphasise that top dressing can be split into up to four applications, that is, at two, four, six and eight weeks.

If a farmer split-applies top dressing, the crop is likely to benefit immensely as it is likely to absorb the small amounts that are being applied, rather than if the whole amount had been applied at once.

The CZB Agro-Yield scheme recommends 350 to 400 kilogrammes of top dressing per hectare, therefore a farmer can split the 400kg/ha into four applications as suggested above. The chances are high that if the whole amount had been applied at once, there were chances that it could have been washed away by the heavy rains, sometimes leaving the crop yellowish.

Maize can reach knee height at two to four weeks depending on how much top dressing it receives. The less nitrogen it receives, the slower it grows, whereas, if it receives 100kg/ha top dressing at two weeks and another 100kg/ha at four weeks after emergence, it will be more aggressive than a crop that only received basal fertiliser at planting.

As pointed out earlier, maize needs over half its nitrogen requirements between knee height and tasselling stages, therefore, a top dressing between those two stages is also crucial. If a farmer fails to top dress a crop between knee height and tasselling stage, the farmer would have deprived the crop of over half of its nitrogen requirements.

Every year, there tends to be a shortage of top-dressing fertiliser at this time of the year, as almost every farmer is looking for it. A good farmer stock piles his/her nitrogen fertiliser well in advance. It does not help running around all over town, when the crop is already looking miserably yellow.

The second problem that farmers have had to deal with, is the fact that sometimes, they may not get a chance to apply herbicides to their crops before heavy rain falls or heavy rains fall immediately after applying a herbicide. I would like to re-emphasise what I said earlier about weed control. Weed control is best done at planting. Pre-emergent herbicides kill weed seed even before it germinates by stopping the formation of new root cells of the weeds. Trying to control a weed that has already germinated is always going to present its problems.

While pre-emergence herbicides that are applied at planting would normally require 1, 25 to 2,5mm of moisture for the herbicide to be activated, too much rain will dilute or wash away an applied post emergent herbicide. If the herbicide is washed away from the weeds by heavy rains, obviously, the herbicide becomes less effective.

Most herbicides, therefore, require a rain delay period for them to be effective. These rain delay periods are normally indicated on the herbicide label. The rain delay period varies with the different herbicides that farmers use. While for some herbicides, it is essential that they be applied for at least five to six hours before being rained off, for others like Atrazine, no rain delay period is shown on the label and for Classic herbicide, a farmer should not apply if rains are expected within an hour.

Farmers should be guided by two general considerations. Firstly, they should always seek to establish if it is going to rain or not. Surely, if the weather forecasts a heavy downpour early or mid-morning, why risk applying the herbicide? Secondly, in most areas, if it has to rain, the rains would normally come in the afternoon, therefore its best time to spray is first thing in the morning. The chances are that the rains will give you five to seven hours before they come.

I did warn farmers about the dangers of ignoring the threat of fall armyworm. Most farmers were surprised at the amount of damage that the armyworm had caused to their crops while they were away on holiday. One benefit of the current heavy rains is that it aids to wash the caterpillars off the leaves and drown those in the whorl as the water accumulates in the funnel of the maize.

Naturally, when farmers see evidence of fall armyworm attack, most panic, some have tried to phone me while in hardware shops, to ask which chemical to buy.

Let me repeat it here (dzokororo ine simba). Chemicals that have been tested and approved by the Plant Protection Institute in Zimbabwe that are available for use on the fall armyworm include Blast Super, Super Dash, Nemesis, Emamectin Benzoate, Lambda (Karate), Ecoterex 0.5GR, Belt 480SC, Belt Expert, Ampligo, Coragen 20SC, Steward 150EC, Missile 150SC, Blanket 150SC, Indoxacarb 150SC, Vantex 60CS and Carbaryl 85 percent WP.

Farmers should be reminded that the fall armyworm caterpillars they are trying to kill hide in the whorl of the maize plant and hence their chemical application should target that whorl. Simply spraying over a crop that is almost knee high with a boom sprayer might be a waste of time and chemical.