Weed management crucial

FARMERS across the country are optimistic of a bumper harvest during the 2022/2023 season since most parts of the country are receiving rains.

However, a good harvest depends on sound crop management.

The incessant rains most parts of the country have been receiving have made it difficult for farmers to control weeds and this poses a threat on potential yield. Farmers risk losing up to 80 percent of their potential yield if they fail to effectively deal with the weed menace in the first 10 weeks after germination.

At their core, weeds are unwanted plants that crave for sunlight, water and nutrients as their crop counterparts. However, in the race for Zimbabwe to be food-sufficient, weeds are a big obstacle. Left uncontrolled, weeds can take over a field and ruin a farmer’s chances of a bountiful harvest and can derail Zimbabwe’s efforts to attain Vision 2030.

Knowing the damage weeds pose to potential yields, the Government has rolled out weed management training programmes across the country.

Renowned agronomist Mr Munashe Moyo warned that farmers are likely to lose a lot if they fail to deal with the weed menace that is associated with incessant rains.

“As rains continue to pound throughout the country, farmers need to employ good weed management practices.

“With these rains, weeds sprout and outdo crops in the fight for nutrients and sunlight, resulting in reduced yield and crop quality,” Mr Moyo said.

A differentiation between weeds and crops is of paramount importance and should be the first step taken in the management of weeds. Weeds mimic crops as a survival mechanism. Therefore, farmers should know the different types of weeds available to scout for them effectively.

Type of weeds

Weeds can be distinguished according to habitat, origin and degree of harm, among other characteristics, with the most common being the plant morphology and life span of weeds. These can help in identifying weeds during scouting.

Plant morphology is the physical appearance of a weed. There are broad leaf weeds. These have wider leaf structures like bonongwe (pigweed [amaranthus thunbergia]). Narrow-leaf weeds have slimmer leaf structures and fall into two subgroups — sedges and grasses.

Sedges resemble grasses that have solid or three-sided stems in thick clusters. They often grow in shallow waters or moist soils and pfende (nutsedge [cyperus rotundus]) is one example.

Then there are grasses that grow to have round, hollow stems and hard, closed joints (nodes) with alternating leaf blades on each side like the Shamva grass (rottboellia cochinchinensis) or tsangadzi/shanje (couch grass [Cynodon dactylon]).

The lifespan of weeds is the classification of weeds by using the time or duration in which they exist — annual, biennial and perennial. Annual weeds complete their life cycle in a year or less, for example, bonongwe/mowa (pigweed).

Biennial weeds complete their life cycle in two years. The first year is for vegetative growth and the second one is for flowering and seeds.

An example of a biennial weed is rukato (milk thistle [silybum marianum]). Perennial weeds complete their life cycle in more than two years. They are propagated by the weeds themselves, rhizomes, bulbs and stolons.

Scouting

Scouting helps farmers identify and control weeds early, when they are in their most vulnerable state since some species become more challenging to manage as they become larger. Scouting should be repeated at several times during the production cycle of crops. Timely weed scouting is meant to inform the farmer of weed species, location and density.

This helps in designing an effective integrated weed management (IWM) programme. IWM, also referred to as integrated weed control, is an approach that integrates both chemical and non-chemical practices to control weeds. IWM is meant to suppress weed population below the economic injury level of the crop yield.

Mechanical control

This involves the employment of physical methods to kill and suppress weeds.

The methods include pulling, digging, discing and ploughing. The success of mechanical methods depend on the weed. Hand pulling escaped weeds is critical to prevent seeds from entering the soil seed bank, particularly for herbicide- resistant weeds.

Chemical control

This entails the use of herbicides to kill and inhibit weed growth. Herbicides are often used before planting, but can also be used after planting in conjunction with safeners or herbicide-tolerant crops. Farmers should consider several factors, including crops grown, season, weather and weed pressure when deciding which type of herbicide is best and when to spray.

Environmental concerns

The use of herbicides has become indispensable in weed management. Although herbicides are designed for weeds, they can be toxic to humans and wildlife if not properly used. Herbicides vary greatly in their environmental impact, and more specifically, their toxicity and persistence in the environment.

Herbicides may also bring about a deterioration in the soil and water quality if they persist in the environment.

However, these problems can be minimised by using suitable herbicides and following the appropriate handling rules. The persistence depends on the soil factors, climatic conditions and herbicide properties.

The Government is playing its part by stepping up efforts to improve plant health by training farmers on weed management. This is meant to ensure weeds are suppressed and guarantees improved yields and, ultimately, food sufficiency.

The Second Republic, led by President Mnangagwa, has prioritised food security and is targeting a US$8,2 billion agricultural economy by 2025, contributing 20 percent of the Gross Domestic Product underpinned by the National Development Strategy 1 as Zimbabwe moves toward an upper middle-class economy by 2030.

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