Weed Contol Methods in Zimbabwe Farming Systems

Mechanical Methods
The main tools which are used to control weeds are handhoes, oxcultivators, ploughs and spike-tooth harrows. The proportion of farmers who use hoes, ploughs, cultivators and harrows were 96, 76, 23 and 14 %, respectively in six districts surveyed in Zimbabwe (Chatizwa and Nazare, http://www.atnesa.org/weeding/weeding-chatizwa-experiences-ZW.pdf). Handhoes have wooden handles which are attached to metalic blades. During weeding the metalic blade is inserted in the soil and pulled backwards. This tends to cut weeds at the root level level. It has a shaving effect on the weeds. Large weeds are pulled by hand and shaken to ensure that they do not get reestablished. Hand weeding strokes have to be done several times until the weeds are removed from the field. This method which is used on all farming systems is a slow process. Depending on the availability of labour, the farmers often taken several days to complete weeding one hectare. It is common to find some fields which escape weeding, because farmers do not have the time to remove them. Although it is difficult to timely weed using this method, it is the most common method of weed control. Most farmers own hoes. The latter can be bought from local shops or local blacksmiths. Blacksmiths can make these hoes using scrap metal. The effectiveness of handhoeing depends on soil moisture and soil type. Handhoeing is less efficient under wet conditions, because weeds may reestablish a few days after the weeding process. This method seems to increase the infestation of Cyperus rotundus, by breaking tuber dormancy. However, in some cases it enhances the dessication of the tubers, leading to a better control of Cyperus species. Handhoeing is gerally easier on sandy soils than heavy clay soils.

A tine cultivator has an adjustable v-shaped metalic frame with tines attached to it. This implement is dragged between the crop rows by two oxen. The tines will remove the weeds when the cultivator is pulled along the crop rows. Two people are involved in using this implement. One person walks infront of the oxen, determining the route to be followed by the implement. The other person will be operating the cultivator, ensuring that the implement runs between the crop rows without damaging the crops. The cultivator can be adjusted inwards or outwards to suit the width of crop rows. The disadvantage of using this implement is that it may damage the crops, if untrained people and animals are involed in this process. It also leaves uncontolled in-row weeds which need to be controlled by complementary handhoeing or pulling. The advantage is that this is a fast process and reduces labour requirement for weeding (Mabasa, S., Shamudzarira, Z., Makanganise, A., Bwakaya, F. and Sithole, T., http://www.fao.org/agris/search/display.do?f=./2003/v2901/QY2001000163.xml;QY2001000163). It is possible to achieve timely weed control.

A plough can be used as a substitute for a cultivator. It can be used with or without the mouldboard attached to the beam. It has been demonstrated under field conditions that a plough efficiently controlled weeds under field conditions in semi-arid Zimbabwe (Riches, C.R., Twomlow, S. J. and Dhliwayo, H., http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=2687). The plough tends to create ridges arround the crop row. The soil which is thrown on the crop row tends to kill the weeds by covering them. This makes it an effiecient implement in reducing labour for complementary weeding.

The spike-tooth harrow is mainly used to aid crop emergence. Sometimes after planting crops, a hard soil surface develops (crust), which prevents crop emergence. However, some weeds may emerge before the crop. The spike-tooth harrow will break the soil surface and at the same time remove the weed seedlings.

Chemical Weed Control

Chemical weed control has been maily used in the large scale commercial farming areas and invloves the use of herbicides. In the early 1940's, 2,4-D and MCPA, were used to control weeds in maize and wheat on large scale commercial farms. However, only about 5 % of the smallholder farmers use herbicides to control weeds in Zimbabwe. Currently, there are various herbicides which are registered for use in various crops in Zimbabwe. The most common herbicides used in maize, cotton and groundnuts are as follows:

Maize = Atrazine pre and post emergence, and Dual pre-emergence,

Cotton = Cotogard pre-emergence, Cotoran pre-emergence and Dual pre-emergence, and

Groundnuts = Igran pre-emergence, Dual pre-emergence and Basagran post-emergence.

Herbicides are sold at Agricura, Windmill and ZFC depots throughout Zimbabwe. Herbicides do not require a lot of labour to apply them. They also kill weeds under wet conditions, unlike mechacanical weed control methods. Lack of adequate information has prohibited farmers from using the herbicide technology. Farmers need to know basic information about the herbicides they need to use in their crops. They do not kow where they can buy these products and most importantly, how to use them. The initial capital which is needed to invest in herbicide technology tends to discourage farmers from using herbicides. Farmers need to purchase the knapsack sprayers for applying herbicides. However, the knapsack sprayer is purchased once and could be used for a number of several years. Farmers need to be taught how to do basic sprayer calibration.

We have conducted herbicide trials in the following communal areas: Chiweshe, Chihota, Chikomba and Tjolotjo (Makanganise, A, Mabasa, S., Jasi, L and Gatsi, L., http://www.cimmyt.org/english/docs/proceedings/africa/pdf/104_Makanganise.pdf). Before we introduced the technology, we had to determine the farmer's status of knowledge about herbicides. The farmers had vague information about this technology. On the other hand, the extension workers did not attempt to introduce it. We introduced the herbicide in such a way that it would complement the exisisting methodology of weed control. It was decided to use Atrazine in maize. The herbicide had to be applied in bands to control the in-row weeds. This approach reduced the cost of the herbicide, because the bands where the herbicide was applied needed about about half the recommended rate rates. The between-row weeds were removed by the oxcultivator, plough or hand hoeing. The herbicide reduced the labour requirement for weeding (Mabasa, S., Shamudzarira, Z. Makanganise, A., Bwakaya, F. and Sithole, T., http://www.fao.org/agris/search/display.do?f=./2003/v2901/QY2001000163.xml;QY2001000163). A comprehensive economic analysis has demonstrated the benefits of intergrating Atrazine with mechanical methods of weed control in smallholder farming areas(Gatsi, T., Kanyungwe, K., Makanganise, A. and Mabasa, S. http://www.cimmyt.org/english/docs/proceedings/africa/pdf/102_Gatsi.pdf).

One of the problems, is that the herbicides are purchased in 5 l or higher containers, yet the farmer's requirement may 1l or 2l. Furthermore, the herbicides are not found in local shops. The farmers have to travel to towns to purchase them. However, it still remains worth while to invest in herbicide technology, in order to overcome the weed problems in all the farming systems of Zimbabwe.

Effects of weeds on crops in Zimbabwe farming systems

The effects of weeds are well known, where crops are grown around the globe. If weeds are left uncontrolled they can seriously reduce crop yields. Although weed problems are common across the farming systems in Zimbabwe, their dramatic effects have been well demonstrated after the Fast Track Land Reform Programme (FTLRP) called jambanja. The fields which used to grow healthy crops, are always covered with very tall weeds during the cropping season. In very serious cases, you hardly see a crop. Before 2000, about 4000 commerical farmers owned nearly half of the agricultural land, whereas smallholder farmers owned the other half. A commercial farmer could own up to 200 or more hectares of land, whereas a smallholder farmer could own up to 5 hectares of land. After the launch of the FTLRP, some smallholder farmers and newcomers to farming were allocated land which probably range from 10 ha to more than 200 ha. I would like to emphasize here, that growing of crops continue to be elusive on large scale farms to date. A number of factors have been attributed to crop failure on large scale commercial farms, for example, unavailability of fertilizers, agrochemicals, fuel for tractors and draughts (Professor Sam Moyo has discussed these factors; internet publications). I would like to point out that the Zimbabwe government has tried to make the farming inputs available to these farmers. Inspite of a few success stories, the majority of the farmers seem to have failed to produce decent crops. I would like to discuss how weeds could possibly be causing crop production problems in Zimbabwe, with special emphasis on large scale commercial farms.


Weeds affect crops through competition for light, nutrients and water. In order to reduce the impact of weeds on crops, there is a need to understand the critical period of weed control. In the crop life cycle, there is a period when weeds should be removed in order to increase yield and there is also a period when the weeding does not rescue the crop. This means that timing of weed control is very important. The lenghth of period when the crop should be free of weeds is crucial. It should be noted that a lot of weed competition studies, where done on large commercial farms prior to attainment of Zimbabwe independence. These studies were later extended to smallholder farming areas after 1980.


It has been demonstrated that weeding at 2 weeks after crop emergence should prevent crop yield loss in both smallholder and large scale commercial farms. The influence of soil types does not seem to influence the time of weeding. However, rainfall appeared to have a strong effect on the timing of weeding crops. For example, during a high rainfall season, weeding could be delayed by 4 weeks after crop ermergence, without seriously affecting crop yields. Whereas in a dry season, weeding at 4 weeks after crop emergence could decrease crop yields. Therefore, it is generally recommended that weeds should be removed at 2 weeks after crop emergence, if crop yields are to be maximised. Weed competition studies have also demonstrated that provided weeding has been done timely, a weedfree period of 2 weeks is all that is required to maximise crop yields.


How often should weeding be done in order to achieve the required weedfree period? This is an important question. One weeding which is done timely i.e. at 2 weeks after crop emergence is enough to maximise crop yields. There was realy no advantage in weeding more than twice. What is often observed in smallholder farming areas, is that farmers have a portion of their fields timely weeded and others are weeded late i.e. post critical time of weeding. In some cases crop fields are not weeded at all, despite the fact that they were planted on time and fertilized / manured. In Shona language farmers say: "Gore rino mimwe minda yakararira". (This season some fields were never weeded at all). The situation is mainly exacerbated by mechanical methods of weed control which are deployed by smallholder farmers. It is hardly possible to mechanically weed under very wet conditions. Because farmers postpone weedings under rainy conditions, this tends to delay weedings. This problem is particularly serious on heavy soils (large commercial farms). In large scale commercial farms the proportion of unweeded fields has been scaled up.


Because of the mechanical methods of weed control which are being used, delayed weeding is often unavoidable across the farming systems. At some point after the start of the cropping season, weeding and planting operations will compete for the available labour. The farmers have to decide whether to continue with planting or start weeding the early planted crops. In most cases farmers choose to continue planting rather than start weeding operations. There is a strong need for farmers to use alternative methods of weed control, for example, herbicide technology. The later would ensure that the early planted crops are free of weeds and at the same time release labour to allow for further planting to take place.


In conclusion, there is a strong case to inform farmers about the critical time of weed removal from the crops. The farmers need to try alternative methods of weed control e.g. herbicide technology. Herbicide technology enabled former large scale commercial farmers to achieve high crop yields before the FTLRP. If the new farmers are given the necessary trainning skills in herbicide technolgy, they should be able to achieve high crop yields in Zimbabwe.