Green bean weed control

In trials that I conducted when farming years ago on quite a large scale, I found a number of herbicides to be effective as a pre-emergence application. However, many were either not registered for beans in South Africa or were registered only for dry beans.

Because bean varieties vary in their resistance to herbicides, chemical companies will invariably make a safety commitment only on popular bean varieties planted on a large scale, and these are invariably dry bean varieties. No herbicide manufacturer would want to face claims of damage to a new green bean variety.

CONDUCT YOUR OWN SMALL-SCALE TESTS

Fortunately, it’s not difficult to carry out your own small-scale trials of a particular herbicide.

Dual worked very well for me as a pre-emergence product. There are a number of related products to choose from; ask your crop protection representative for details.

This chemical group is very effective at controlling grassseeded species and a number of (not all!) broad-leaf weeds. Yellow nutsedge ( Cyperus esculentus) can also be controlled if done properly. If you are using one of these herbicides, irrigate it into the soil so that it can reach the nutsedge bulbs before they sprout.

Any herbicide that can control all the weeds effectively is also likely to suppress the bean plants. Unfortunately, this means that with safer herbicides, some species of broad-leaf weeds will germinate with the beans.

The herbicide Basagran, a product registered for broadleaf weeds, can help solve this problem. However, I found that in hot, dry weather, it was effective only against smaller weeds. As the weather cannot be controlled, I considered how I would be able to apply the product to the younger weeds without damaging the beans, which are vulnerable to the product while still young.

SMALL WEEDS CAN BE KILLED WITH A LOWER CONCENTRATION OF PRODUCT

I rigged up a knapsack sprayer with two Teejet 8004 tapered jet nozzles, each directed above the middle of the row. These nozzles are used on a spray boom where an overlap is required. To avoid overdosing where the spray overlaps, less product is delivered at the two ends.

The idea was to reduce the volume of spray mixture on the beans to prevent any damage. The sprayer was therefore held at a height that allowed the edges of the spray to reach only the bottom leaves of the plants.

In trials, I found that a lower concentration of product was enough to kill small weeds. I was thus able to get excellent and reliable control with just 1ℓ/ha, instead of the recommended rate of between 2ℓ/ha and 3ℓ/ha. The correct stage to apply the chemical is at the commencement of the second trifoliate leaf.

HOW TO CALCULATE PRODUCT CONCENTRATION

Some farmers may think that using a knapsack is a step backwards in this day and age, but it is an efficient and speedy method. A 20ℓ knapsack covers about 0,1ha.

Begin by filling the tank with water and spraying the plants at a comfortable pace until it is empty. You can then calculate exactly how much product to add.

You can use a tractor if the rows are particularly even and the soil is level.

• Bill Kerr is a vegetable specialist and a breeder of a range of vegetables. Email him at farmersweekly@caxton.co.za. Subject line: Vegetable production.

fw archive ABOVE: When spraying with a knapsack sprayer, hold the nozzle so that the edges of the spray reach only the bottom leaves of the bean plants.