Paraquat, Diuron and Atrazine for the Renewal of Chemical Weed Control in Northern Cameroon

Herbicide treatment with a Handy® sprayer. PhotoJ. Martin Paraquat, diuron and atrazine for the renewal of chemical weed control in northern Cameroon In 1995, cotton growers in Cameroon treated nearly 60 000 ha of cotton and cereal cropland with herbicides, four to five times more than in the 1980s. Three products have been found to be efficient in controlling weeds, thus facilitating crop establishment and initial growth: paraquat, a non-selective herbicide that has been widely used since 1987, along with diuron and atrazine, two generic molecules recommended (since 1992) for pre-emergence treatment of cotton and maize. uring the cropping season, cover can shelter crop pests (insects, farmers in cotton-growing diseases) or enemies of man and D regions devote most of their domestic animals (snakes). time to weed control. In rainy years, In northern Cam eroon, farmers weeds are definitely the most impor perform most maintenance tasks tant constraint on farms — delaying manually (weeding, hoeing) or agricultural work and limiting the mechanically (tilling, earthing-up). effectiveness of inputs. When weed However, they are generally unable ing is postponed, cottonseed yield to maintain weed cover at accep losses due to weed competition are table levels throughout their crop- around 20 kg/ha/day after the opti fields. Chemical weed control can mum weeding date, which is gene thus be beneficial in two major ways: rally 1 0-1 5 days after the sowing - better labour management (both on date under rural draught farming and off the farm) during intensive conditions. Weeding becomes more work periods involving tilling, laborious and takes longer beyond sowing and the first weeding; this ideal date. The regeneration - limitation of early weed competi potential of some weeds increases tion and easier subsequent mainte considerably (via cuttings or trans nance. plantation) and the earliest species In addition to chemical weed fruit. In addition to these problems of control, the Société de développe J. MARTIN early weed competition for light, CIRAD-CA, IRA-CRA Maroua, BP 33, ment du coton du Cameroun (SODE Maroua, Cameroon water and minerals, late competition COTON) recommends conducting N ew address: (mainly for water) can be a limiting mechanical interrow weeding under CNRA/ISRA, BP 53, Bambey, Senegal factor, e.g. obstructing harvesting draught farming conditions. The L. GAUDARD (climbing weeds) or contaminating company thus offers farmers weed CFDT, SO DECOTON, the harvested crops (weed plant ing tools that are adapted to their BP 302, Garoua, Cameroon debris and seeds). Finally, weed equipment and financial resources. Agriculture et développement ■ Special issue - M ay 1 997 herbicides in Cameroon annual rainfall levels of over During the 1980s, after the initial Expansion of 1 000 mm, but it is still underpopulated rapid expansion, the herbicide- and there are not many draught ani treated area remained stable at chemical weed mals. Hand sprayers are used for 40-60% of the overall area under control in cotton - ultra low volume treatments (10- crops: 1 0 000-1 3 000 ha of cotton 25 l/ha of spray mixture) - they are cropfields (i.e. 40 to 60% of the sur growing regions easy to handle and reduce water face) treated per year (10-15% of the supply problems. area sown), and 2 000-3 000 ha of In 1976, Cameroon was the first cot intensively-grown maize cropfields. ton-producing country in French- The Cameroonian Institut de la A few hundred hectares of rainfed speaking Africa to use herbicides for recherche agronomique (IRA) regu rice, groundnut and sorghum weed control in cotton and food larly tested herbicides used to cropland were weeded chemically. cropfields, i.e. maize, groundnut, control weeds in cotton cropfields, Pre-emergence herbicides were sorghum and rainfed rice (Tables 1 but they had no data or reference partially subsidized in the first few and 2). SODECOTON recom products concerning food cropfields. years, but later became too expen mended using pre-emergence herbi Herbicides widely used by SODE- sive for farmers to purchase. In 1990, cides to enhance crop intensification COTON were commercial formula the average cost of a pre-emergence in the southern part of the cotton tions, mainly binary mixtures, propo treatment against weeds in a cotton belt. This in-migration region has a sed at relatively high doses (3-4 l/ha cropfield was the equivalent of two- high agricultural potential, with of commercial product). thirds of the cost of a pesticide protection programme or of 100 kg of mineral fertilizer. Table 1. Herbicides used for pre-emergence treatments of cropfields in northern Cameroon. Important weed control Herbicide Mode of action Active ingredient Advantage Treatment dose (g/ha) stage innovations diuron root penetration 720 good efficacy - pre-emergence In 1987, SODECOTON introduced cotton on superficially of weeds paraquat, a non-selective contact rooting plants, herbicide, since pre-emergence beware of the phytotoxicity risks treatments are useless for controlling weeds in recolonised fields. Farmers atrazine root and leaf 800 good efficacy pre-emergence were quick to adopt this new highly (maize) penetration on superficially- of weeds rooting plants, efficient, rapid-acting and relatively selective for maize inexpensive product. In 1992, based and sorghum on the work of IRA, SODECOTON paraquat contact 200 to 400 rapid effect post-emergence began promoting low-dose treat easy to use of weeds, ments with two generic molecules, splitting diuron to control weeds in cotton recommended cropfields and atrazine in maize glyphosate non-selective 1 440 useful in post-emergence systemic integrated control of weeds, fields. These herbicides replaced for herbicide of perennials slow action mer binary formulations, and the (2-4 weeks) performance:cost ratio is almost glufosinate non systemic 200 or 400 effective against pre-sowing, threefold higher. Paraquat and pre contact Poaceae species pre-tillage emergence herbicides were shown action slower than and dicots to be quite complementary. Under with paraquat these suitable technical and econo mic conditions, and particularly Table 2. Binary mixtures used in pre-emergence treatments, formulated since training, logistics and credit and marketed by several companies. were provided by SODECOTON, Crop Active ingredient or combination Active ingredient dose (g/ha) renewed the interest in chemical weed control. Cotton flumeturon + prometryn 750 + 750 dipropetryn + metolachlor 720 + 480 terbutryn + metolachlor 500 + 1 000 Expansion dipropetryn 750 of herbicide-treated areas metolachlor 1 080 1992 marked a reversal in the herbi Maize atrazine + alachlor 1 000 + 1 000 cide treatment trend concerning cot atrazine + metolachlor 750 + 750 ton and maize cropland. In 4 years, Agriculture et développement ■ Special issue - M ay 1 997 herbicides in Cameroon The use of paraquat, diuron and atrazine Paraquat can be used alone or in extemporaneous mixtures with pre emergence herbicides. Since 1987, there has been a steady increase in the use of paraquat alone to control weeds in cotton cropfields (around + 40%/year, up to nearly 16 000 ha in 1995). However, there has been a reduction in its use in maize fields (less than 500 ha in 1 995). This reduction could be explained by the spectacular increase in treatments Manual inter-row weeding after weeding with an ass-drawn plough. with atrazine alone, i.e. almost Photo J. Martin 5 000 ha in 1995, or + 250%/year since 1992. There was a more mode rate increase in treatments with diuron alone in cotton cropfields (+ 40%/year). Since 1992, combined Herbicide blend: extemporaneous mixture of contact and pre-emergence herbicides treatments (paraquat + pre □ emergence herbicides) have increa Pre-emergence herbicide: since 1992, : | diuron at 720 g/ha on cotton, atrazine sed substantially in cotton and maize 4 0 - at 800 g/ha on maize cropfields, i.e. nearly 22 000 ha and 10 000 ha in 1995, or + 1 35% and Contact herbicide: paraquat 200-00 g/ha 220%/year, respectively. 3 0 - Geographically, herbicide use has Cotton expanded considerably from the sou thern region (Touboro and, to a les 2 0 - ser extent, Caroua regions) to the centre of the cotton-growing region (Guider region, with around 4 300 ha treated, including more Maize than 900 ha with food crops), and up to the northern part of the cotton- growing region (more than 2 600 ha UhhIU—I treated in 1995, including almost wnn M 1 1988 I 1990 I 1992 I 1994 I 500 ha with food crops). 1989 1991 1993 1995 1989 1991 1993 1995 Year The cotton-growing area has Figure 1. Cotton and maize cropland treated with herbicides in northern Cameroon generally increased due to the deva (source: SODECOTON). luation of the CFA franc, thus affecting all cotton-producing coun tries of the franc zone. This change forced Cameroonian farmers to use there was a 4-fold increase in the increase in the surface area treated herbicides to a greater extent in their area under chemical weed control, relative to that sown: 12-28% for cotton and food cropfields (DUGUE while the area treated with pre-emer cotton and 40-90% for intensively 6 DOUNIAS, 1995). gence herbicides increased 6-fold cropped maize. Atrazine treatments over the same period (Figure 1 ). were also started in fields of "tradi tionally-grown" maize (without ferti In 1995, chemical treatments were lizer): 3 700 ha in 2 years, or 13% of Paraquat carried out to control weeds on the area. There was also a modest, around 60 000 ha of cropland, i.e. but definitely significant increase in treatments 42 000 ha of cotton and 16 000 ha of the use of atrazine to control weeds maize, along with 2 000 ha of sorg in sorghum cropfields and diuron in Paraquat treatments were introduced hum and groundnut. Between 1991 groundnut fields (1% of the overall in 1987, at a time when gramoxone and 1 995, there was a marked area).

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