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.