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.