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). was the only formulation (200 g/l)

Agriculture et développement ■ Special issue - M ay 1 997 herbicides in Cameroon

- treatments with paraquat alone, Users are less aware of these risks without being mixed with a pre­ and have no protective gear and clo­ emergence herbicide; thing (glasses, masks, boots, gloves, - pre-tillage treatments, to obtain etc.), which are difficult to obtain weed-free fields, with a more sustai­ and to wear under tropical ned effect than with post-tillage treat­ conditions. ments. Pre-tillage treatments are sometimes advanced by several days in order to reduce the herbicide Risks of oral intoxication dose. Reference toxicological levels offer only a starting point for assessing ¡¡¡F; . . 'i ? . ,|g .| ■ It is important to note that paraquat risks associated with the handling Ô-, enabled the development of a new and spraying of commercial formula­ crop establishment technique, i.e. tions (decanting and proportioning), direct-sowing without tilling on since the toxicity of a molecule weed mulch. In such conditions, depends on the concentration. paraquat is often used in extempora­ Paraquat, at 20% concentration neous mixtures with a pre-emergence (commercial formulation), has been herbicide, thus reinforcing its action classified as highly hazardous to and ensuring a more sustainable dogs (BCPC, 1987) and man, i.e. clearing effect. This new technique swallowing a few drops can be fatal was applied on 1 5 000 ha of cotton to small children (SEVERIN & cropfields in 1995, and has been ful­ TISSUT, 1991 ; PAN & CTA, 1993). ly adopted in the southern part of the However, there is very little risk of cotton-growing region. Excellent efficacy of herbicides in a cotton accidental ingestion, e.g. by confu­ field 20 days post-treatment. In the sion with a drink, because of the centre, the two untreated rows show highly repulsive effects of formula­ a predominantly Poaceae plant Limiting treatment risks tions that comply with FAO/WHO population. safety standards. Paraquat, although not very hazar­ Photo J. Martin dous in normal usage conditions, is highly toxic when swallowed Suitable dilutions accidentally (WHO, 1994). Never­ Paraquat spray mixtures diluted to theless, in hot climates, there is more 2% (400 g/ha) are moderately hazar­ meeting the Food and Agriculture risk of intoxication through dous, while those diluted to 1 % Organization/World Health Orga- dermal absorption or inhalation. nization (FAO/WHO) safety standards. This monopoly ended recently.

Paraquat is a non-selective contact herbicide that is extensively used by farmers in northern Cameroon to facilitate the establishment of rainfed crops, and to reinforce the action of pre-emergence herbicides in reinfes­ ted fields. It is highly useful when the rainy season is early, or sowing is extended or late.

Treatment conditions rapidly beca­ me diversified, based on experience acquired by farmers and SODE- COTON supervisory staff: Insufficient weed control efficacy 30 days post-treatment in a field overrun with - treatments with double (or even C. benghalensis. Manual weeding combined with herbicide treatment was too late. triple) doses relative to the initial In the absence of herbicide, the cotton plants have practically disappeared 200 g/ha dose, to deal with high under C. benghalensis cover. weed conditions; Photo J. Martin

Agriculture et développement ■ Special issue - M ay 1997 herbicides in Cameroon

"high" dose (400 g/ha) can be recommended when the weed cover is dense or there are tufts of weeds partially buried by tilling. The "low" dose (200 g/ha) is recommended to avoid treatments at concentrations above 1 %. To obtain the same efficacy, optimal conditions are required for treat­ ments and the high dose can be split so as to conduct two treatments, which are more effective than a single treatment at 400 g/ha. The second treatment reaches the lower parts of the weeds, which become more accessible after the initial treat­ ment. In addition, any strips missed Ox-drawn tillage after paraquat treatment. in the first run can thus be covered. Photo P. Marnotte

Optimal treatment conditions (200 g/ha) have almost no toxic treated per run, spraying behind the Treatments done under the full sun impact. 1% paraquat spray mixtures operator, at a flow rate of about can wilt and completely dry out have the same risk level as products 20 l/ha of spray mixture. There is young annual plants within in a few classified as not dangerous, e.g. very little lateral and vertical drift hours. However, there are only par­ glyphosate and diuron. Users must under these conditions. tial and temporary effects on dense therefore be careful to keep contai­ plant stands composed of mature ners of paraquat (concentrated or Reducing the spray mixture annual or perennial plants, as diluted) away from children. concentrations regrowth can begin from the untou­ Long-term toxicity criteria should be ched lower plant parts or from To improve safety, the proposed taken into account with respect to underground reserves. measures involve reductions in the agricultural workers on industrial herbicide dose and operator exposu­ The treatment efficacy, even at high plantations. Moreover, these criteria re time. It would be unsuitable to doses, is limited by the fact that para­ should be considered in terms of reduce the spray mixture concentra­ quat reacts quickly under high light northern Cameroonian conditions, tion by increasing the volume, as this conditions. A treatment carried out where farmers carry out three to four would increase the problem of get­ in the evening or in overcast weather half-day herbicide treatments yearly ting water supplies at the side of the conditions will have a slower but (10 at most). Under these conditions, field and, in addition, farmers are greater effect, i.e. penetration is the properties of paraquat have a used to the convenience of simply improved through a cuticle that is relatively minor impact. filling the container with 5 I of spray more permeable due higher humidi­ mixture to treat quarter-hectare plots ty, and the herbicide is diffused bet­ in a single operation. On the other ter under low light conditions. The Treatment techniques hand, the dose can be reduced, cells are then destroyed when the For manual ultra low volume (ULV) while maintaining or increasing the light returns. treatments, spray mixture concentra­ herbicide efficacy, by conducting tions are high and the spray cloud treatments under optimal weather Addition of wetting agents generated through disk rotation can conditions or by splitting the Additives can be extemporaneously drift considerably. The risks depend treatments. mixed with the herbicide formula­ on the spraying technique and tion in some cases (CAUVRIT, constraints. Handy® sprayers are Splitting doses and optimal 1995). Ultra low volume paraquat used for herbicide treatments in nor­ treatments at 400 g/ha are not very thern Cameroon and other French- treatment conditions efficient against infestations of Tridax speaking countries of Africa. The procumbens, which has very downy Splitting doses herbicide spray mixture is gravity-fed leaves. The addition of tensioactive onto a rotating disk driven by a batte­ The paraquat doses used in northern wetting agents can enhance sprea­ ry-powered electric motor, thus for­ Cameroon (200-400 g/ha) are low ding of the herbicide film over the ming a low circular spray cloud of compared with those commonly leaf surface and between the leaf about 1 m diameter. One inter-row is used worldwide (600-800 g/ha). The hairs to the cuticle.

Agriculture et développement ■ Special issue - May 1997 herbicides in Cameroon

Consequences of splitting is as effective as the same dose of herbicides that could be used in cot­ paraquat against Poaceae and more treatments ton and food cropfields, along with effective against dicots (LE BOUR­ non-selective herbicides. In 1990 Splitting treatments increases battery GEOIS et al., 1992). However, diffe­ and 1991, multi-site trials demons­ consumption and the demand for rent treatment conditions are neces­ trated the efficacy of two generic sprayers that SODECOTON supplies sary because of its slower action. molecules, i.e. diuron in cotton crop­ to farmers. This also increases the This is especially important for pre­ fields and atrazine in maize fields, workload at a point when the crop­ tillage treatments, as they must allow when treatments were conducted at ping calendar is quite full. However, for a 1 -2 week delay in the herbicide relatively low doses as compared to the additional treatment time is mini­ action. It could be classified among those used with binary mixtures sup­ mized when there is a sufficient moderately hazardous products, plied by the manufacturers. The stock of sprayers, i.e. 1 h to treat a although it is 4- to 8-fold less toxic by results helped define suitable doses, standard quarter-hectare plot, and ingestion than paraquat. The main the spectrum of activity, phytotoxici­ there is a long potential treatment factor that limits extension of this ty risks, impact on subsequent crops, period. Pre-emergence treatments product is its high cost. and treatment conditions adapted to can be carried out within the 4-day the farming environment. The tests period after sowing, and often even were carried out on split plots: later. This treatment modification is Glyphosate treated and non-treated. In 1992, thus quite easy to apply, especially following these tests, SODECOTON since some farmers already follow a Systemic non-selective herbicides recommended treatments with programme that includes pre-ti I ling such as glyphosate have almost no diuron at a dose of 720 g/ha and with and post-sowing treatments. shock effect. atrazine at 800 g/ha. Sometimes the pre-ti 11 i ng treatment Glyphosate treatment conditions dif­ can be advanced several days to fer from those of contact herbicides: reduce the herbicide quantity, they are more restrictive and require increase efficacy and obtain weed- General characteristics a new weed control approach invol­ free fields. ving a special cropping calendar These two molecules have many with treatment times that have to be common points. They were discove­ Recommendations for spray respected. red in the 1950s and developed in the 1960s, and were among the first mixture preparation This herbicide is of interest for targe­ selective pre-emergence herbicides To reduce user exposure to concen­ ted control of perennial weeds. available with persistent activity. trated paraquat, the spray mixture Moreover, the price of this com­ Maize-selective atrazine favoured should be prepared in the herbicide pound has dropped steadily since the the worldwide expansion of this shed, where safety gear is available expira tion of its patent in 1991 crop. Diuron was used for initial (soap, funnel, etc.), along with water (FEUILLETTE et ai, 1994; MARNOT- weed control treatments in tropical in case of accidents or splashing. A TE, 1994). In 1995, glyphosate began plantations, as well as in cotton crop­ deposit can form in preprepared mix­ being used widely in the cotton- fields, particularly in USA. tures and then redissolve when the growing region of northern Cameroon, container is shaken during transport with treatments at 1 440 g/ha They have quite high chemical stabi­ and treatment. Empty herbicide recommended against Imperata lity, with a few weeks to months per­ containers should be burned or cylindrica. sistence in the upper soil layers. buried after use, but in practice they Overall, they have the same herbici­ are often recovered and sometimes de performance, i.e. they have high sold. Such containers should be rin­ Conclusion efficacy against weeds with superfi­ sed at least three times, as recom­ There is presently no cost-effective cial root systems, destroying (from mended in northern Cameroon. and non-toxic herbicide available to the cotyledon leaf stage) weeds with replace paraquat which would be small seeds that germinate close to adapted to pre-emergence weed the soil surface. Features of other control in cotton-growing regions of These molecules are degraded by northern Cameroon. physico-chemical and microbiologi­ non-seledive herbicides cal processes, in relation to the bio­ logical soil activity and the affinity of Diuron and atrazine various types of microorganisms Glufosinate (SEVERIN & TISSUT, 1991). Diuron Glufosinate, a contact herbicide, can In 1989, IRA and CIRAD, in col­ belongs to the substituted urea fami­ replace paraquat in post-sowing laboration with several pesticide ly and atrazine to the chlorotriazine treatments. 15 days post-treatment, it manufacturers, renewed testing of family. Diuron and atrazine, like

Agriculture et développement ■ Special issue - May 1 997 herbicides in Cameroon

Draught tillage.

Ass-drawn weeding operation.

Light tractor tillage, ploughing in of Imperata cylindrica.

Ox-drawn earthing-up.

Grass stand scorched by paraquat.

Hoe weeding

Photos J. Martin

Agriculture et développement ■ Special issue - May 1 997 35 herbicides in Cameroon

most herbicides used in pre-emer­ Efficacy of atrazine against gence treatments, are relatively Commelina benghalensis non-toxic to man and warm-blooded animals. Atrazine is of major interest because of its efficacy against C. benghalensis These two herbicides are used (MARTIN & GERARDEAUX, 1994). worldwide in integrated pest mana­ This weed becomes dominant after a gement programmes, alone or in few years of cropping, when it is dif­ combination or alternation with ficult to control because of regrowth. other active ingredients. It is highly competitive with crops, especially in the wetter southern parts of the cotton-growing region. It Weed control with germinates massively after any extent atrazine of surface tillage, and quickly affects cotton and maize cropping areas Atrazine is absorbed through the where these crops are sown after roots and leaves and has a broad rapid draught tillage (low coverage spectrum of activity in pre-emergence with little clearing). On sandy soils treatments (at less than 1 000 g/ha), with a low exchange capacity, which especially against Poaceae, annual is a very common situation in the Cyperaceae and Commelinaceae cotton-growing regions of Cameroon species (MARTIN & GERARDEAUX, High efficacy of atrazine in a maize and western Africa, post-sowing 1994). This product can be used in cropfield. treatments of maize cropfields with a post-emergence weed treatments, PhotoJ. Martin low atrazine dose (800 g/ha) general­ with high efficacy against broad-lea- ly keeps the cropfield weed-free for ved species (often dominant after more than a month. The young tillage), but with poor efficacy Poaceae species are generally abun­ weeds still present can then be against annual grasses, that are abun­ dant or dominant. Pre-emergence quickly and efficiently buried by an dant on new plots. It can also be treatments promote good initial plant earthing-up operation - along with a used in direct-sowing conditions growth and replace the first weeding, second urea application, when this is without tillage. which is generally a long operation. scheduled in intensive cropping situations. Sorghum is cropped on around Treatments adapted to maize 400 000 ha of land in northern The excellent efficacy of atrazine and sorghum crops Cameroon, including 40 000 ha against C. benghalensis is certainly planted with improved varieties, responsible for the increased use of Maize and sorghum very efficiently which are of considerable interest in chemical weed control in fields of metabolize and detoxify atrazine terms of weed control. maize, which is almost always sown (GAILLARDON, 1991). This herbici­ de can therefore be used in post­ emergence treatments on fields crop­ ped with these two cereals without any special precautions.

Atrazine does not have phytotoxic side-effects on subsequent crops (cotton or legumes). These risks have not yet been fully assessed on catch crops (cowpea, cover crops), although they seem low when the catch crop is sown 45 days after the pre-emergence treatment.

In 1992, atrazine began being used widely for pre-emergence treatments in sorghum cropfields, and this technique in now fully adopted. Improved red sorghum (Sorghum caudatum "Djigari") crops, sown Post-sowing treatment after tillage with a Handy® sprayer. w ithout tillage once the first rains Treatment with a pre-emergence herbicide and paraquat. occur, are soon overrun with weeds. PhotoJ. Martin

Agriculture et développement ■ Special issue - May 1997 herbicides in Cameroon

Diuron selectivity for cotton There are phytotoxicity risks of diu­ ron to cotton as this herbicide has position selectivity. Phytoxicity symptoms are often observed. They are limited to the cotyledons (white spots or plaques), sometimes rea­ ching the first or second true leaf (relatively discoloured intervein spaces), without harming subsequent development to a major extent. The herbicide concentration must be zero or very low in growth zone of the first roots, around 5 cm below the surface.

Severe phytotoxicity can occur in many cases: - on very poor or eroded soil, where herbicide migration is rapid, root growth is poor; - on shallow-sown plots or unclosed After weeding an intercropped field, the plants are piled up to hinder regrowth. seed holes; PhotoJ. Martin - after heavy rains, via rapid herbici­ de migration; - due to accidental overdoses or splitting of treatments. in tilled fields. Moreover, atrazine is especially if it rains between tillage The success of weed control depends sometimes persistent until harvest and sowing. Generally rapid and on regular sowing in closed seed under experimental conditions, shallow draught ploughing leads to holes, while complying to recom­ when weed control is very carefully rapid growth of weed cover (LE mended doses. performed - with cleaner tillage and BOURGEOIS & MARNOTTE, 1994). Groundnut and cowpea crops, very regular spraying. According to Cotton plantlets are challenged after which benefit from more careful farmers and extension agents, there a few days when cotton fields are sowing than cotton (1 seed per well- is lower weed infestation with cotton infested with C. benghalensis. Pre­ closed seed hole), can thus be when it is planted after atrazine- emergence treatments delay the treated with diuron. However, on treated maize, particularly when the emergence of C. benghalensis by 15- poor or eroded soils, the diuron dose field is infested with C. benghalensis. 20 days and promote the initial should be reduced by 25-50% or the growth of the crop. Thereafter, the treatment, which becomes dange­ advantage given to developing cot­ rous and not very efficient, could ton plants is quickly nullified, as the even be cancelled altogether. Diuron treatments density and growth rate of the of cotton cropfields C. benghalensis population are often markedly superior to those of the Complementarity of diuron In cotton cropfields, the persistent crop. Weeding is thus recommended activity of diuron (at 700-800 g/ha) is before the 30th day post-sowing, or atrazine with paraquat the same as that of widely used pre­ from the C. benghalensis 3-4 leaf emergence herbicides. However, its stage if possible, in order to lim it for controlling perennial persistence is twofold less than that regrowth via natural cuttings. Early weeds of atrazine, at a dose of 800-1 000 g/ha earthing-up should be done to hinder in pre-emergence treatments. C. benghalensis from re-emerging. In northern Cameroon, as in most cotton-growing regions, there are lar­ Limit weed competition There has been a marked increase in ge infestations of some perennial the use of diruon for weed control in weed species, against which diuron with cotton cotton cropfields, despite its fairly and atrazine are considered ineffe­ After shallow tillage, weeds grow short persistence. It is also starting to ctive: tuber-bearing Cyperaceae rapidly and regularly — before or at be used in groundnut cropfields, (Cyperus rotundus and C. esculentus) the same time as the cotton plants — with nearly 600 ha treated. or rh izome-bear i ng Poaceae

Agriculture et développement ■ Special issue - May 1 997 herbicides in Cameroon

(Imperata cylindrica). Moreover, ments of pendimethalin combined In 1 995, pen dim e th alin (at paraquat has only a temporary and with diuron or systemic anti- 1 000 g/ha) combined with diuron (at partial effect on perennials and well- Cramineae products. 720 g/ha) was recommended for developed annuals. controlling R. cochinchinensis in cotton cropfields. When these weeds do not cover the Resistance and vigour soil, combined diuron + paraquat of R. cochinchinensis The use of this product is limited, in treatments in cotton cropfields, or spite of good results, by its relatively This annual large-seed Poaceae spe­ atrazine + paraquat in maize fields, high price. Although pendimethalin cies, with rapid initial growth, can show an interesting synergistic effi­ is not very hazardous, it has a poor efficiently compete with crops, parti­ cacy, and the effect of paraquat is reputation because it can deeply dye cularly on rich well-watered soils. Its boosted. clothes and skin yellow. There are seed can germinate very deeply, thus other chemical solutions, such as providing it with a good resistance to Some growers claim that diuron post-emergence treatments of cotton herbicide treatments. treatments facilitate weeding of fields fields with slow-acting, systemic gra- infested with I. cylindrica. Progressive minicides, which are very selective absorption of small quantities of Variable susceptibility for dicots, but are still too expensive. herbicide slows plant growth and to pendimethalin weakens its root system. The slight but prolonged effect of this root- R. cochinchinensis is susceptible penetrating photosynthesis inhibiting to pendimethalin at a dose of New chemical herbicide can add to the shock effect 1 000 g/ha in pre-emergence treat­ of paraquat, particularly when the ments (LE BOURGEOIS et ai, 1992). weed control weed roots are mostly superficial, However, the results of experiments which is often the case with tufts of conducted since 1992 have high­ possibilities weeds disturbed by tillage but not lighted considerable variability in the ploughed under. efficacy of pendimethalin against Since 1992, SO DECO TO N has R. cochinchinensis. This could be extended the use of diuron (at 720 attributed to weather conditions g/ha) and atrazine (at 800 g/ha), Treatment programmes during treatments (promoting volati­ replacing the binary formulations lity and photodegradation losses), used in pre-emergence treatments. against Rottboellio to soil porosity (influencing vapour The reasons behind the current cochinchinensis dispersal in the soil) and to the depth increase in chemical weed control at which the seeds are buried, since are technical, economic and linked Diuron and atrazine are ineffective they can only germinate in the top with organization, in addition to the against R. cochinchinensis. Presently 8 cm soil horizon (LE BOURGEOIS fact that it can promote an increase proposed solutions involve treat­ & MERLIER, 1995). in cotton cropland.

Table 3. Variation in the cost of supplying farmers or farmers' organizations with established or recently recommended herbicides from SODECOTON in northern Cameroon (source: SODECOTON, agricultural production directorate). Crop Herbicide treatment Active ingredient 1991 1992 1993 1994 1995 dose g/ha

Price of pre-emergence treatment CFA franc/ha

Cotton binary 400 SC(1) 1 200 10 000 4 000 Cotton diuron 800 WP(1> 720 4 000 4 000 5 200 5 600 Maize binary 400 SC 1 600 10 000 6 000 Maize atrazine 500 WP 800 6 000 4 000 5 200 5 600

Cotton pendimethalin 500 SC 1 000 16 000 Cotton chlortoluron 500 SC 1 000 5 600

Price of non-selective herbicide treatment CFA franc/l

paraquat 200 SL 200-600 2 000 2 000 2 100 3 000 3 700 glyphosate 360 SL 1 080 5 000 4 500 (1): SC = suspension concentrate, WP = wettable powder.

Agriculture et développement ■ Special issue - M ay 1 997 herbicides in Cameroon

Cost-effective Sufficient organization of around 10 farmers with joint res­ ponsibility, thus ensuring repayment. pre-emergence treatments Extension of any herbicide product Herbicides, batteries, sprayers and involves training and monitoring gear are supplied to village stores The use of diuron and atrazine has activities, as well as adapted mana­ between cropping seasons. lowered the average cost of pre­ gement of supplies and credit. emergence treatments by threefold Hundreds of demonstration plots, (Table 3). There are three compo­ with cotton and food crops, have nents involved in this reduction: Economic impact of weed been set up yearly since 1992 throu­ price, dose and formulation. ghout cotton-growing regions. control Since the patents for these molecules Technical datasheets designed for The costs of the different herbicide expired, they have become generic extension agents have been updated. products and expenses associated herbicides used worldwide. They are IRA researchers conduct sessions with their use have been assessed: produced and marketed by many intended for SODECOTON trainers. labour savings for weeding, crop competing companies. Conversely, Follow-up is provided by SODE­ production increases, treatment previously recommended binary for­ COTON staff, who are experienced costs, and supplementary labour mulations contain active ingredients and highly present in the field during costs due to increased yields. that are more expensive and less crop establishment. The herbicide widespread. supply costs include battery An optimal maintenance programme Currently recommended active consumption and depreciation of involves two weedings and an ear- ingredient doses are about twofold SODECOTON spraying equipment. thing-up before the weed cover lower since binary formulations have Herbicides for cotton and food crops develops beyond the economic been abandoned. The herbicide are supplied on credit and paid for damage threshold, e.g. on days 14, combinations previously recommen­ when the cotton is sold. The "input" 28 and 42 after sowing (GABOREL, ded for maize all contained atrazine credits are granted to organizations 1989). In practice, maintenance at similar doses. The reduction in the herbicide dose can be especially attributed to the absence of a second The economic impact of chemical weed active ingredient, belonging to the chloro-acetamide family (metola- control in 1995 chlor or alachlor). It was estimated that chemical weed control in cotton cropfields enables a labour Diuron was not included in pre- gain of 12 days/ha. The herbicide can reduce period of excessive weed competition viously-recommended herbicide by 10 days for cotton, i.e. equal to 15 kg/day of seed cotton. The estimated crop-yield formulations for cotton. gain following herbicide treatment is 150 kg/ha for cotton. The financial assessment was carried out as follows (Table 4): The high active ingredient concen­ - one day's income is 500 CFA francs, i.e. 6 000 CFA francs for 12 days of work; tration in wettable powders reduces - the price of seed cotton in 1995 was 160 CFA francs/kg; transport costs and these form ula­ - the costs associated with cotton harvesting (picking and marketing) were tions have a long shelf I ife of many 30 CFA francs/kg; years, thus minimizing losses. - the cost of herbicide treatments with an extemporaneous mixture of a pre-emergence herbicide + paraquat at low close was 5 600 + 3 700 = 9 300 CFA francs/ha. Total savings obtained through chemical weed control were evaluated at more than Adaptation of formulations 16 000 CFA francs/ha for cotton. These economic benefits are incurred with increa­ sed crop production, estimated at 6 3001 of seed cotton for the entire cotton-growing and packaging region, and 7 0001 of maize seed. Herbicide spray preparations from wettable powders (WP) of diuron Table 4. Estimated seed cotton and maize production gains (CFA francs) and atrazine are highly practical, following herbicide treatments. especially concerning single dose Budgetary terms Cotton Maize packets for quarter-hectare treat­ Savings on weeding + 6 000 FCFA + 9 000 FCFA ments. This type of packaging is not (18 days) as expensive as containers required Value of supplementary production + 24 000 FCFA + 20 000 FCFA for suspension concentrate (SC) (50 FCFA per kg formulations. Powder formulations Cost of harvesting supplementary - 4 500 FCFA have a long shelf I ife, while suspen­ production - 1 500 FCFA sion concentrates often form relati­ (5 FCFA per kg) vely unmixable sediments in the Cost of herbicide treatment - 9 300 FCFA - 9 300 FCFA containers. Total + 16 200 FCFA + 18 200 FCFA

Agriculture et développement ■ Special issue - May 1997 herbicides in Cameroon

work is always carried out later than cancelled. Split treatments are more GRARD et al., 1995), full and accu­ the optimal weed treatment dates difficult to carry out in cotton cropfields. rate knowledge of available herbi­ (DUGUE & DOUNIAS, 1995), thus cides is required for the purposes of substantially increasing the duration New solutions are being investigated rational control. and laboriousness of weeding opera­ in order to diversify the range of pro­ tions (GABOREL, 1989), reducing ducts available and weed control the efficacy of earthing-up and lea­ programmes. Treatment recommen­ ding to crop-production losses due to dations concerning glyphosate, pen- Bibliography weed competition. Under field dimethalin and chlortoluron (a conditions, one herbicide treatment substituted urea herbicide) should be BCPC, 1987. The pesticide manual. A reduces the weeding time and the drawn up. In trials currently under world compendium. Highth édition. period when weed cover is invasive, way, treatments with chlortoluron (at W ORTHINC C.R. & BARRIE WALKER S. (eds). The British Crop Protection Council. which enables labour savings and 1 000 g/ha) were found to be as Thornton Heath, UK, 1 081 p. yield gains. efficient as those with diuron (at BEKOLO M., CAU D AR D L., 1995. 720 g/ha). Rapport semestriel, mai à octobre 1995. In Benin, in a region of similar ecolo­ SODECOTON, direction de la production gical conditions, 175 tests were car­ agricole, Cameroon, 43 p. ried out in collaboration with local Increased logistics and DUGUE P., DOUNIAS I., 1995. farmers (GABOREL & FADOE- Intensification, choix techniques et stratégies paysannes en zone cotonnière du Cameroun. GNON, 1991). The results were as training needs Le cas des systèmes de culture des zones follows: An increased range of herbicides and d'installation des agriculteurs migrants. In - the use of herbicides led to a yield weed control combinations implies Succès et échecs des révolutions vertes. CIRAD, 6 September 1995, Montpellier, increase of 240-600 kg/ha of seed complex supply organization and France. cotton and 550-650 kg/ha of seed reduced training needs. Regular FEUILLETTE B., MARNOTTE P., LE maize; efficient organization, involving BOURGEOIS T., 1994. Fiche technique - labour time was reduced by extension and follow-up, logistics mauvaise herbe. La lutte contre Imperata 1 7-19 days/ha for both crops. These and credit, is an essential prerequi­ cylindrica. Agriculture et développement 3 : 47-48. gains are the equivalent of weeding site for rational weed control. GABOREL C., 1989. La nuisibilité des one highly weed-infested hectare of The control strip method — with two adventices en culture cotonnière au Bénin. cropland or two lightly infested hectares. Relation entre le niveau d'enherbement et les or three lines left untreated for a few temps de travaux. In Actes de la metres — provides a good weed 1re conférence de la recherche cotonnière control demonstration. This tech­ africaine, 31 January-2 February 1989, Lomé, Togo. CIRAD-IRCT, Montpellier, France, nique can enhance and accelerate The future of chemical vol. 1, p. 385-394. training, especially if undertaken by weed control in cotton- GABOREL C., FADOEGNON, 1991. Le many neighbouring users: highligh­ désherbage chimique du cotonnier et du maïs growing regions of Africa ting variability in the effects of au Bénin. Acquis et proposition de la treatment and their causes through recherche. In réunion de coordination de recherche phytosanitaire cotonnière, discussions between neighbours, or The herbicide treatment experience commission régionale de défense des cultures acquired in northern Cameroon, with the help of a trainer, and d'Afrique de l'Ouest (CIRAD-IRCT, cotton using paraquat, atrazine and diuron, gaining knowledge on the diversity Network CORAF, INERA), 7, 26-31 January of effects of different products under 1991, Ouagadougou, Burkina Faso, TOE A., will be useful for other African coun­ NIBOUCHE S., PINCHARD V. (Eds). CIRAD- various treatment conditions. tries. Indeed, chemical control could IRCT, Paris, France, p. 135-151. complement current (mainly GAILLARDON P., 1991. La sélectivité des manual) weed control techniques. traitements herbicides. In Les herbicides, All treatment possibilities have not mode d'action et principes d'utilisation, Conclusion SCALLA R. (edit.), INRA éditions, Paris, yet been fully explored, e.g. interac­ France, p. 237-264. tions between paraquat and pre­ A "list of essential commonplace GAUVRIT C., 1995. Les adjuvants : emergence herbicides, apart from herbicides" would be highly useful mirifiques, illusoires, ou dangereux ? the complementarity noted above. for savanna farmers, such as that Phytoma. La défense des végétaux, 470 : Pre-emergence herbicide doses recommended by WHO for health 17-22. could be reduced by 25-50% for purposes and applied to many GRARD P., LE BOURGEOIS T., MERLIER H., 1995. Adventrop. Les adventices poor soils with a low exchange capa­ African countries. Nevertheless, trai­ tropicales d'Afrique soudano-sahélienne. city, while being increased for soils ning, follow-up, herbicide supply Flore + CD Rom multimédia, CIRAD-CA, with high clay content, organic mat­ and credit (and repayment sche­ Montpellier, France. ter or organic mulch levels. In maize dules) still have to be organized. In LE BOURGEOIS T., GERARDEAUX E., cropfields, atrazine treatments can addition to having an overall unders­ BEIX Y., 1992. Rapport d'activité 1991-1992. Section malherbologie. Institut de la be split into two treatments, or the tanding of weeds that infest crop­ recherche agronomique, Garoua, Cameroon, post-sowing treatment could even be fields (LE BOURGEOIS, 1 993; 16 p.

Agriculture et développement ■ Special issue - M ay 1 997 herbicides in Cameroon

LE BOURGEOIS, 1993. Les mauvaises MARTIN J., GERARDEAUX E., 1994. SEVERIN F., TISSUTM., 1991. Principes herbes dans la rotation cotonnière au L'expérimentation sur le désherbage chimique d'utilisation des herbicides. In Les herbicides, Cameroun (Afrique). PhD thesis, USTL, du maïs en zone cotonnière au Nord- mode d'action et principes d'utilisation, Montpellier II, France, 249 p. Cameroun. In Progress in food grain research SCALLA R. (éd.), INRA éditions, Paris, France, LE BOURGEOIS T., MARNOTTE P., 1994. and production in semi-arid Africa. SAFGRAD p. 281-325. Fiche technique mauvaise herbe. La lutte internet work conference, Niamey, Niger, TOURNEUX H., 1994. L'interprétation contre Commelina benghalensis. Agriculture 3-7 March 1991. SAFGRAD, Ouagadougou, paysanne des pictogrammes phytosanitaires. et développement 2 : 53-54. Burkina Faso, p. 513-522. Agriculture et développement 1 : 39-42. MARNOTTE P., 1994. Fiche technique PAN, CTA, 1993. Pesticides et agriculture WHO, 1994. The W HO recommended mauvaise herbe. La lutte contre Cyperus tropicale : dangers et alternatives. Pesticides classification of pesticides by hazard and rotundus. Agriculture et développement 1 : action network et Centre technique de guidelines to classification 1994-1995. 57-58. coopération agricole et rurale, C. DUMMLER International programme on chemical safety. MARNOTTE P., 1995. Utilisation des (Ed.). Verlag J. MARGRAF, Scientific books, United Nations Environment Programme herbicides : contraintes et perspectives. Tropical agroecology, 3. Weikersheim, International Labour Organization, World Agriculture et développement 7 : 12-21. Germany, 281 p. Health Organization. WHO/PSP/94.2, 64 p.

Abstract... Résumé... Resumen

J. MARTIN, L. GAUDARD — Paraquat, diuron and J. MARTIN, L. GAUDARD — Paraquat, diuron J. MARTIN, L. GAUDARD — Paraquat, diurón y atrazine for the renewal of chemical weed et atrazine pour renouveler le désherbage atrazina para renovar la escardadura química en control in northern Cameroon. chimique au Nord-Cameroun. el norte de Camerún. In Northern Cameroon, based on the recommendations of Au Nord-Cameroun, le paraquat est vulgarisé depuis En el norte de Camerún, el paraquat está vulgarizado SODECOTON and tests carried out by IRA in collaboration 1987 et le diuron et l'atrazine depuis 1992, à la suite des desde 1987 y el diurón y la atrazina desde 1992, with CIRAD, paraquat has been used widely since 1987, recommandations de la SODECOTON et des tests réalisés consecutivamente a las recomendaciones de SODECOTON and diuron and atrazine since 1992. Paraquat shows par l'IRA en collaboration avec le CIRAD. Le paraquat y a las pruebas realizadas por el IRA en colaboración con rapid and highly effective activity. To limit the toxicity montre une efficacité importante et rapide. Pour limiter el CIRAD. El paraquat ofrece una eficacia elevada y effects and optimize this treatment, it is recommended les risques de toxicité et optimiser ce traitement, il est rápida. Para limitar los riesgos de toxicidad y optimizar to split the treatments into 200 g/ha doses, with conseillé de fractionner les doses en applications de este tratamiento, se aconseja fraccionar las dosis en treatments preferably in the evening or under cloudy 200 grammes par hectare, à réaliser de préférence le soir aplicaciones de 200 gramos por hectárea, realizadas conditions. Treatments conditions are varied (pre­ ou par temps couvert. Les conditions d'emploi sont preferentemente al caer la tarde o en tiempo cubierto. Las ploughing treatment, high-dose treatment in case of high diversifiées (traitement de pré-labour, traitement à forte condiciones de empleo son diversificadas (tratamiento weed populations, etc.). The recommendations must be dose en cas de végétation importante d'adventices). Les previo a la labranza, tratamiento a dosis fuerte en caso de followed for the spraying preparation. Glyphosate and recommandations doivent être suivies pour la préparation gran vegetación de adventicias). Deben seguirse las glufosinate can be used in pre-sowing treatments, but des bouillies. Le glyphosate et le glufosinate sont recomendaciones para la preparación de los caldos. El their mode of application is more restrictive. In cotton- utilisables en traitement de pré-semis, mais leur mode glifosato y el glufosinato son utilizables en tratamiento growing regions, weed control recommendations also d'application est plus contraignant. Dans la zone previo a la siembra, pero su modo de aplicación es más include the use of diuron at 720 g/ha in cotton cropfields cotonnière, les conseils de désherbage comprennent aussi restrictivo. En la zona algodonera, los consejos de and atrazine at 800 g/ha for maize. Atrazine is a very l'emploi du diuron à 720 grammes par hectare sur escardadura incluyen también el empleo de diurón a interesting herbicide for the control of C. benghalensis in cotonnier et l'atrazine à 800 grammes par hectare sur 720 gramos por hectárea en el algodón y la atrazina a maize fields and for pre-emergence treatments in maïs. L'atrazine est un herbicide très intéressant pour 800 gramos por hectárea en el maíz. La atrazina es un sorghum fields. Diuron is less effective on C. benghalensis lutter contre C. benghalensis dans les cultures de maïs et herbicida muy interesante para luchar contra but its use is recommended in cotton fields. In addition, en traitement de pré-levée des sorghos. L'effet du diuron C. benghalensis en los cultivos de maíz y en tratamiento diuron and atrazine are associated with paraquat for the sur C. benghalensis est moindre, son emploi est previo al brote de los sorgos. El efecto de diurón en control of perennial weeds. The pre-emergence recommandé en culture cotonnière. En outre, le diuron et C. benghalensis es menor, recomendándose su empleo en treatments recommended in this cotton-growing region l'atrazine peuvent être associés au paraquat pour lutter cultivo algodonero. Además, el diurón y la atrazina were shown to be cost-effective thanks to the gains contre les vivaces. Les traitements de pré-levée préconisés pueden asociarse al paraquat para luchar contra las obtained in working time and production. Logistics and dans cette région cotonnière se révèlent rentables grâce viváceas. Los tratamientos previos al brote de los sorgos training support are provided by SODECOTON. aux gains de temps de travail et de production obtenus. en esta región algodonera resultan rentables gracias a los Un appui de logistique et de formation est assuré par la ahorros de tiempo de trabajo y producción obtenidos. Keywords: maize, cotton, sorghum, weed, herbicide, SODECOTON. SODECOTON aporta un apoyo de logística y formación. diuron, atrazine, paraquat, dose, toxicity, Cameroon. Mots-clés : maïs, cotonnier, sorgho, mauvaise herbe, Palabras clave: maíz, algodón, sorgo, maleza, herbicida, herbicide, diuron, atrazine, paraquat, dose, toxicité, diurón, atrazina, paraquat, dosis, toxicidad, Camerún. Cameroun.

Agriculture et développement ■ Special issue - M ay 1997