Paraquat, diuron and atrazine for the renewal of chemical weed control in northern Cameroon

Herbicide treatment with a Handy® sprayer. PhotoJ. M artin

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.

J. MARTIN

CIRAD-CA, IRA-CRA M a ro u a , BP 3 3 , M a ro u a , C am eroon N e w address: CNRA/ISRA, BP 53, Bambey, Senegal

L. G A U D A RD

CFDT, S O D E C O T O N , BP 3 0 2 , G aroua , C am eroon

D

u ring the c r o p p in g season, farm ers in c o tt o n - g r o w in g regions devote most o f their tim e to weed control. In rainy years, weeds are definitely the most im por­ tant constraint on farms — d elaying a g r ic u ltu r a l w o r k and l im i t i n g the effectiveness of inputs. W hen w eed­ ing is postponed, cottonseed y ie ld losses due to weed c o m p etitio n are around 20 kg/ha/day after the o p t i­ m um w ee din g date, w h ic h is gene­ r a lly 1 0-1 5 days a fte r th e s o w in g d a te u n d e r ru ra l d r a u g h t fa r m i n g conditions. W eeding becomes more lab oriou s and takes longer beyond t h is id e a l d a te . T he r e g e n e r a t io n p o te n tia l o f some weeds increases c o n s id e ra b ly ( via cuttings or trans­ p la ntatio n) and the earliest species fruit. In addition to these problems of e a r ly w e e d c o m p e t i t i o n fo r lig h t, water and minerals, late competition (m ainly for water) can be a lim itin g fa c to r, e.g. o b s tr u c tin g h arv es tin g ( c lim b in g weeds) or c o n ta m in a tin g th e h a r v e s te d c r o p s (w e e d p la n t d e b r is and seeds). F i n a ll y , w e e d

cover can shelter crop pests (insects, diseases) o r e n e m ie s o f m an and domestic animals (snakes).

In n o r t h e r n C a m e r o o n , f a r m e r s p e r fo r m m o s t m a i n t e n a n c e tasks m a n u a l l y ( w e e d i n g , h o e in g ) o r m e c h a n ic a lly (tillin g , e arthin g-up ). However, they are generally unable to m a in t a in w e e d c o v e r at a c c e p ­ ta b le levels th ro u g h o u t th e ir cro p - fields. C h e m ic a l w ee d c o n tro l can thus be beneficial in tw o major ways: - better labour management (both on and o f f th e fa rm ) d u r in g in te n s iv e w o r k p e r io d s i n v o l v i n g t i l l i n g , sowing and the first weeding;

- lim ita tio n of early weed c o m p e ti­ tio n and easier subsequent m ainte ­ nance.

In a d d i t i o n to c h e m i c a l w e e d c o n tro l, the Société de d é v e lo p p e ­ ment du coton du Cameroun (SODE­ C O T O N ) rec o m m en ds c o n d u c tin g mechanical interrow weeding under d r a u g h t f a r m i n g c o n d i t i o n s . The c o m p a n y thus offers farmers w e e d ­ ing to o ls th a t are a da pte d to th e ir equipment and financial resources.

a n n u a l r a i n f a l l l e v e ls o f o v e r 1 000 mm, but it is still underpopulated and there are not many draught ani­ m als. H a n d sprayers are used fo r u lt ra l o w v o lu m e tr e a tm e n ts (10- 25 l/ha of spray mixture) - they are easy to h a n d le a nd re d u c e w a t e r supply problems.

T h e C a m e r o o n i a n I n s t i t u t de la recherche a gron om iqu e (IRA) regu­ l a r l y te s te d h e r b i c i d e s used to c o ntrol weeds in co tto n cropfields, b u t th e y had no data o r refe ren ce products concerning food cropfields. H e rb ic id e s w id e ly used by SODE- C O T O N were com m ercial fo rm u la ­ tions, m ainly binary mixtures, propo­ sed at relatively high doses (3-4 l/ha o f commercial product).

Table 1. Herbicides used for pre-emergence treatments of cropfields in northern Cameroon.

Herbicide Mode o f action Active ingredient dose (g/ha)

Advantage Treatment

stage diuron

cotton

root penetration 720 good efficacy -

on superficially rooting plants, beware of the phytotoxicity risks pre-emergence of weeds atrazine (maize)

root and leaf penetration

800 good efficacy

on superficially- rooting plants, selective for maize and sorghum

pre-emergence of weeds

paraquat contact 200 to 400 rapid effect

easy to use post-emergence of weeds, splitting recommended glyphosate non-selective systemic herbicide 1 440 useful in integrated control o f perennials post-emergence of weeds, slow action (2-4 weeks)

glufosinate non systemic 200 or 400 contact

action slower than with paraquat effective against Poaceae species and dicots pre-sowing, pre-tillage

Table 2. Binary mixtures used in pre-emergence treatments, formulated and marketed by several companies.

Crop Active ingredient or combination Active ingredient dose (g/ha)

Cotton flumeturon + prometryn 750 + 750

dipropetryn + metolachlor 720 + 480

terbutryn + metolachlor 500 + 1 000

dipropetryn 750

metolachlor 1 080

M aize atrazine + alachlor 1 000 + 1 000

atrazine + metolachlor 750 + 750

A gricultu re et développem ent ■ Special issue - M a y 1 9 9 7

Expansion of

chemical weed

control in cotton -

grow ing regions

In 1976, Cameroon was the first cot­ t o n - p r o d u c in g c o u n t r y in F rench- speaking Africa to use herbicides for w e e d c o n t r o l in c o t t o n and fo o d c r o p f ie ld s , i.e. m a iz e , g ro u n d n u t, sorghum and rainfed rice (Tables 1 a n d 2). S O D E C O T O N r e c o m ­ mended using pre-emergence herbi­ cides to enhance crop intensification in th e s o u th e rn part o f th e c o tto n belt. This in-m ig ra tion region has a h ig h a g r i c u l t u r a l p o t e n t i a l , w i t h

D u r in g th e 1980s, after the in itia l r a p id e x p a n s i o n , t h e h e r b i c i d e - t r e a t e d a re a r e m a i n e d s ta b le at 4 0 - 6 0 % o f th e o v e r a ll area u n d e r crops: 1 0 000-1 3 000 ha o f cotton cropfields (i.e. 40 to 60% o f the sur­ face) treated per year (10-15% of the area sown), and 2 0 00-3 000 ha o f intensively-grown maize cropfields. A fe w h un dred hectares o f rainfed r ic e , g r o u n d n u t a n d s o r g h u m c rop la nd were weeded c h e m ic a lly . P r e - e m e r g e n c e h e r b i c i d e s w e r e p a rtia lly subsidized in the first few years, b ut later became to o e x p e n ­ sive for farmers to purchase. In 1990, the average cost of a pre-emergence treatm ent against weeds in a cotton cropfield was the equivalent of tw o- t h i r d s o f t h e c o s t o f a p e s t i c i d e protectio n pro gram m e or o f 100 kg o f mineral fertilizer.

Important weed control

innovations

In 1987, S O D E C O T O N introduced p a ra q u a t, a n o n -s e le c tiv e c o n ta c t h e r b i c i d e , s in c e p r e - e m e r g e n c e treatments are useless for controlling weeds in recolonised fields. Farmers were quick to adopt this new highly efficient, rapid-acting and relatively inexpensive product. In 1992, based on the w o r k o f IRA, S O D E C O T O N began p r o m o t i n g l o w - d o s e t r e a t ­ ments w ith tw o generic m olecules, d iu r o n to c o n tr o l w eeds in c o tto n c r o p f i e l d s and a t r a z in e in m a iz e fields. These herbicides replaced for­ m er b in a r y f o r m u la t i o n s , and the p e r f o r m a n c e : c o s t r a t io is a lm o s t threefold higher. Paraquat and pre­ emergence herbicides w ere shown to be quite c o m p le m e n ta ry . U nd er these suitable technical and e c o n o ­ m ic c o n d i t i o n s , and p a r t i c u l a r l y since tr a in in g , log istics and c r e d it w e r e p r o v id e d b y S O D E C O T O N , re n e w e d th e in te re s t in c h e m ic a l weed control.

Expansion

of herbicide-treated areas

1992 marked a reversal in the herbi­ cide treatment trend concerning cot­ ton and maize cropland. In 4 years,

herbicides in Cam eroon

Manual inter-row weeding after weeding with an ass-drawn plough. Photo J. M artin 4 0 3 0 -2 0 -Cotton

wnn

□

Herbicide blend: extemporaneous mixture of contact and pre-emergence herbicides Pre-emergence herbicide: since 19 9 2 , : | diuron at 7 2 0 g / h a on cotton, atrazine

at 8 0 0 g / h a on maize Contact herbicide: paraquat 2 0 0 -0 0 g / h a 1 9 8 8 I 1 9 9 0 I 1 9 9 2 I 1 9 9 4 I 1 9 8 9 1991 1 9 9 3 1 9 9 5 M a ize

M

U

hh

IU—I

1

1 9 8 9 1991 1 9 9 3 1 9 9 5 Year

Figure 1. Cotton and maize cropland treated with herbicides in northern Cameroon (source: SODECOTON).

th e re was a 4 - f o ld increase in the area under ch em ica l weed co ntrol, w h ile the area treated w ith pre-emer­ gence h e rb ic id e s increased 6 -fo ld over the same period (Figure 1 ).

In 1995, c h e m ic a l treatm ents were c a r r ie d o u t to c o n t r o l w e e d s on around 60 000 ha o f c ro p la n d , i.e. 42 000 ha of cotton and 16 000 ha of maize, along w ith 2 000 ha of sorg­ hum and groundnut. Between 1991 a n d 1 9 9 5 , t h e r e w a s a m a r k e d

increase in the surface area treated r e la t iv e to th a t s o w n : 1 2 - 2 8 % fo r c o tto n and 4 0 - 9 0 % for in te n s iv e ly cropped maize. Atrazine treatments were also started in fields o f " t ra d i­ tio na lly -grow n " maize (without ferti­ lizer): 3 700 ha in 2 years, or 13% of the area. There was also a modest, but defin itely significant increase in the use o f atrazine to control weeds in sorghum cropfields and diuron in g ro u n d n u t fields (1% o f the overall area).

The use of paraquat,

diuron and atrazine

Paraq ua t can be used a lo n e o r in extemporaneous mixtures w ith 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). H o w e v e r, there has been a red uctio n in its use in maize fields (less t h a n 5 0 0 ha in 1 9 9 5 ) . T h is reduction could be explained by the sp e c ta c u la r increase in treatm ents w i t h a t r a z i n e a lo n e , i.e . a lm o s t 5 0 0 0 ha in 1995, or + 2 50 % /y e a r since 1992. There was a more mode­ ra te in c r e a s e in t r e a t m e n t s w i t h d iu r o n a lo n e in c o tto n c r o p f ie ld s (+ 40%/year). Since 1992, combined t r e a t m e n t s ( p a r a q u a t + p r e ­ emergence herbicides) have increa­ sed substantially in cotton and maize cropfields, i.e. nearly 22 000 ha and 10 000 ha in 1995, or + 1 3 5% and 220%/year, respectively.

G e o g ra p h ic a lly , h e rb ic id e use has expanded considerably from the sou­ thern region (Touboro and, to a les­ ser e x te n t, C a r o u a regio ns) to the centre o f the co tto n -g ro w in g region ( G u i d e r r e g io n , w i t h a r o u n d 4 3 0 0 ha tre a te d , i n c l u d i n g m ore than 900 ha w ith food crops), and up to th e n o rth e rn part o f th e c o tto n - growing region (more than 2 600 ha treated in 1 9 9 5 , i n c l u d i n g a lm o s t 500 ha w ith food crops).

T h e c o t t o n - g r o w i n g a re a has generally increased due to the deva­ l u a t i o n o f t h e C F A f r a n c , th u s affecting all cotton-producing c o u n ­ tries of the franc zone. This change forced Cam eroonian farmers to use herbicides to a greater extent in their cotton and food cropfields (DUGUE 6 D O U NIAS , 1995).

Paraquat

treatments

Paraquat treatments were introduced in 1987, at a time w hen gramoxone was the o n ly fo rm u la tio n (200 g/l)

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Excellent efficacy of herbicides in a cotton field 20 days post-treatment. In the centre, the two untreated rows show a predominantly Poaceae plant population.

Photo J. M artin

m e e tin g th e Food and A g r ic u ltu r e O r g a n i z a t i o n / W o r ld H e a lth O rga - n i z a t i o n ( F A O / W H O ) s a f e t y s ta n d a rd s . T h is m o n o p o l y e n d e d recently. - tre a tm e n ts w it h p a ra q u a t a lo n e , w i t h o u t b e in g m ix e d w i t h a p r e ­ emergence herbicide; - p re -tilla g e tre a tm e n ts , to o b ta in weed-free fields, w ith a more sustai­ ned effect than w ith post-tillage treat­ m e n ts . P r e - t illa g e tr e a tm e n ts are sometimes advanced by several days in o r d e r to r e d u c e th e h e r b i c i d e dose.

It is im portant to note that paraquat enabled the d e v e lo p m e n t o f a new c ro p e s ta b lis h m e n t te c h n iq u e , i.e. d i r e c t - s o w i n g w i t h o u t t i l l i n g on w e e d m u l c h . In such c o n d it io n s , paraquat is often used in extempora­ neous mixtures with a pre-emergence herbicide, thus reinforcing its action and e n s u r in g a m o r e s u s ta in a b le clea rin g effect. This n ew te chn iqu e was applied on 1 5 000 ha of cotton cropfields in 1995, and has been fu l­ ly adopted in the southern part o f the cotton-grow ing region.

Limiting treatment risks

Paraquat, altho ug h n ot very hazar­ dou s in n o r m a l usage c o n d it io n s , is h i g h l y t o x i c w h e n s w a l l o w e d a c c id e n ta lly ( W H O , 1994). N e v e r­ theless, in hot climates, there is more r i s k o f i n t o x i c a t i o n t h r o u g h d e r m a l a b s o r p t i o n o r i n h a l a t i o n .

Users are less a w a re o f these risks and have no protective gear and c lo ­ thing (glasses, masks, boots, gloves, etc.), w h i c h are d i f f i c u l t to o b ta in a n d to w e a r u n d e r t r o p i c a l conditions.

Risks of oral intoxication

Reference to x ic o lo g ic a l levels offer o n l y a s ta rtin g p o i n t fo r assessing risks associated w it h th e h a n d lin g and spraying of commercial fo rm u la ­ tions (decanting and proportioning), s in c e t h e t o x i c i t y o f a m o l e c u l e d e p e n d s o n th e c o n c e n t r a t i o n . P a r a q u a t, at 2 0 % c o n c e n t r a t i o n (com mercial fo rm ulation), has been c la s s if ie d as h ig h l y h a z a rd o u s to dogs (BCPC, 1 9 8 7 ) and m an , i.e. sw allow ing a few drops can be fatal to s m a ll c h i l d r e n (SEVERIN & TISSUT, 1991 ; PAN & CTA, 1993). H ow e v er, there is very little risk of accidental ingestion, e.g. by c o n fu ­ s ion w i t h a d r i n k , b eca use o f th e h ig h ly repulsive effects o f fo r m u la ­ tions th at c o m p ly w it h F A O / W H O safety standards.

Suitable dilutions

Paraquat spray m ixtures d ilu te d to 2% (400 g/ha) are moderately hazar­ d o u s , w h i l e th o s e d i l u t e d to 1 %

Paraquat is a non-selective c o nta ct herbicide that is extensively used by fa rm e rs in n o r th e r n C a m e r o o n to facilitate the establishment o f rainfed crops, and to reinforce the action of pre-emergence herbicides in reinfes­ ted fields. It is highly useful when the rainy season is e arly, or s o w in g is extended or late.

Treatm ent c o nd itio ns ra p id ly beca­ me diversified, based on experience a c q u i r e d b y f a r m e r s a n d S O D E - C O T O N supervisory staff:

- trea tm en ts w it h d o u b le (or even t r ip le ) doses re la tiv e to th e i n it ia l 2 0 0 g /ha d ose, to d ea l w i t h h ig h weed conditions;

Insufficient weed control efficacy 30 days post-treatment in a field overrun with C. benghalensis. Manual weeding combined with herbicide treatment was too late. In the absence of herbicide, the cotton plants have practically disappeared

under C. benghalensis cover. Photo J. M artin

herbicides in C am eroon

Ox-drawn tillage after paraquat treatment. Photo P. M arnotte

(2 0 0 g /h a ) h a v e a lm o s t no t o x i c impact. 1% paraquat spray mixtures have the same risk level as products c la s s if ie d as n o t d a n g e r o u s , e.g. g lyphosate and d iu ro n . Users must therefore be careful to keep c o n ta i­ ners o f p a ra q u a t (c o n c e n tra te d or diluted) away from children.

Long-term toxicity criteria should be taken into a c c o u n t w it h respect to a g r ic u lt u r a l w o r k e r s on in d u s t ria l plantations. Moreover, these criteria s h o u ld be c o n s id e re d in te rm s o f n orthern C a m e ro o n ia n c o n d itio n s , where farmers carry out three to four half-day herbicide treatments yearly (10 at most). Under these conditions, the p ro p e rt ie s o f p a ra q u a t have a relatively m inor impact.

Treatment techniques

For manual ultra lo w v o lu m e (ULV) treatments, spray mixture concentra­ tions are high and the spray c lo u d generated through disk rotation can drift considerably. The risks depend on t h e s p r a y in g t e c h n i q u e a n d c o n s tr a in ts . H a n d y ® sprayers are used for herbicide treatments in nor­ thern C am eroon and other French- sp e a k in g c o u n t r ie s o f A f r ic a . The herbicide spray mixture is gravity-fed onto a rotating disk driven by a batte­ ry-powered electric motor, thus fo r­ m in g a lo w c irc u la r spray c lo u d of about 1 m diameter. One inter-row is

treated per run, spraying behind the o p e r a t o r , at a f l o w rate o f a b o u t 20 l/h a o f spray m ix t u r e . There is v e ry lit tle lateral and v e rtic a l d r if t under these conditions.

Reducing the spray mixture

concentrations

T o i m p r o v e s a fety , th e p ro p o s e d measures in v o lv e reductions in the herbicide dose and operator exposu­ re tim e . It w o u l d be u n s u it a b le to reduce the spray mixture concentra­ tion by increasing the volume, as this w o u ld increase the pro blem o f get­ ting water supplies at the side of the f ie ld and, in a d d it io n , farm ers are used to the c o n v e n ie n c e o f s im p ly fillin g the container w ith 5 I of spray mixture to treat quarter-hectare plots in a single o pe ration. O n the other h a n d , t h e d o s e c a n be r e d u c e d , w h ile m aintainin g or increasing the h e r b ic id e e ffic a c y , by c o n d u c tin g trea tm en ts u n d e r o p t im a l w e a th e r c o n d i t i o n s o r b y s p l i t t i n g th e treatments.

Splitting doses and optimal

treatment conditions

Splitting doses

The paraquat doses used in northern C a m e ro o n (2 0 0 -4 0 0 g/ha) are lo w c o m p a r e d w i t h th o s e c o m m o n l y used w o rld w id e (600-800 g/ha). The

" h i g h " d o s e ( 4 0 0 g /h a ) c a n be recommended when the weed cover is dense or there are tufts o f weeds partially buried by tilling. The " lo w " dose (200 g/ha) is recom m ended to a void treatm ents at c o n c e n tra tio n s above 1 %.

To obtain the same efficacy, optimal c o n d it io n s are r e q u ir e d f o r tr e a t­ ments and the high dose can be split so as to c o n d u c t t w o tr e a tm e n ts , w h i c h are m o r e e f f e c t i v e th a n a s in g le tr e a tm e n t at 4 0 0 g/ha. The second treatm ent reaches the low er parts o f the weeds, w h ic h beco m e more accessible after the initial treat­ ment. In additio n, any strips missed in the first run can thus be covered.

O p tim a l treatment conditions

Treatments done under the full sun c a n w i l t a nd c o m p l e t e l y d r y o u t young annual plants w ith in in a few hours. However, there are only par­ tial and tem porary effects on dense p la n t stands c o m p o s e d o f m a tu re a n n u a l o r p e r e n n i a l p la n t s , as regrowth can begin from the u ntou­ c h e d l o w e r p l a n t p a rts o r f r o m underground reserves.

The treatment efficacy, even at high doses, is limited by the fact that para­ quat reacts q u ic k ly under high light c o nd itio ns . A treatm ent carried out in the evening or in overcast weather c o n d it io n s w i l l have a s lo w e r b u t g re a te r e ffe c t, i.e . p e n e t r a t io n is im p ro v e d th ro u g h a c u t ic le th a t is more permeable due higher h u m id i­ ty, and the herbicide is diffused bet­ ter under lo w light c o n d itio n s . The cells are then d estroyed w h e n the light returns.

A dd itio n of w etting agents

Additives can be extemporaneously m ix e d w it h the h e rb ic id e f o r m u la ­ t i o n in s o m e cases ( C A U V R I T , 1995). U ltra lo w v o lu m e paraquat treatments at 400 g/ha are not very efficient against infestations o f Tridax

procumbens, w hich has very dow ny

leaves. The addition o f tensioactive w e ttin g agents can enhance sprea­ ding o f the h e rb ic id e film over the lea f surface and b e tw e e n the leaf hairs to the cuticle.

C onsequences of splitting

treatments

Splitting treatments increases battery c o n s u m p t io n and th e d e m a n d fo r sprayers that SO DECOTON supplies to farm ers. This also increases the w orkloa d at a p oint w he n the c ro p ­ ping calendar is quite full. However, the additional treatment time is m in i­ m iz e d w h e n t h e r e is a s u f f i c i e n t stock o f sprayers, i.e. 1 h to treat a standard q u a rte r-h e c ta re plo t, and th ere is a lon g p o te n tia l tre a tm e n t p e rio d . P re -em ergence trea tm en ts can be carried o ut w it h in the 4-day period after sowing, and often even later. This treatm ent m o d ific a tio n is thus quite easy to apply, especia lly since some farmers already f o llo w a pro gram m e that includes pre-ti I ling and p o s t - s o w i n g t r e a t m e n t s . Sometimes the pre-ti 11 i ng treatm ent can be a d v a n c e d s e veral d ays to r e d u c e t h e h e r b i c i d e q u a n t i t y , increase efficacy and obtain w eed- free fields.

Recommendations for spray

mixture preparation

To reduce user exposure to concen­ trated paraquat, the spray m ix tu re should be prepared in the herbicide shed, w here safety gear is available (soap, funnel, etc.), along w ith water in case o f accidents or splashing. A deposit can form in preprepared m ix ­ tures and then redissolve w hen the container is shaken durin g transport a n d t r e a t m e n t . E m p ty h e r b i c i d e c o n t a i n e r s s h o u ld be b u r n e d o r buried after use, but in practice they are often recovered and sometimes sold. Such containers should be rin­ sed at least th ree tim es, as re c o m ­ mended in northern Cameroon.

Features of other

non-seledive herbicides

Glufosinate

Glufosinate, a contact herbicide, can r e p la c e p a r a q u a t in p o s t - s o w i n g treatments. 15 days post-treatment, it

is as e ffe c tiv e as the same dose o f paraquat against Poaceae and more e ffe ctive against d ic o ts (LE B O U R ­ GEOIS et al., 1992). However, diffe­ rent treatment conditions are neces­ sary because o f its s lo w e r a c tio n . This is especia lly im portant for pre­ tillage treatments, as they must a llo w for a 1 -2 week delay in the herbicide action. It c ould be classified among m o d e r a t e l y h a z a r d o u s p r o d u c t s , although it is 4- to 8-fold less toxic by ingestio n than paraquat. The main fa c to r th a t lim its e x te n s io n o f this product is its high cost.

Glyphosate

System ic n o n -s e le c tiv e h e rb ic id e s such as glyphosate have alm ost no shock effect.

Glyphosate treatment conditions d if­ fer from those o f contact herbicides: they are more restrictive and require a new weed control approach inv o l­ v in g a s p e c ia l c r o p p i n g c a le n d a r w ith treatment times that have to be respected.

This herbicide is of interest for targe­ te d c o n t r o l o f p e r e n n i a l w e e d s . M o r e o v e r , th e p r ic e o f t h is c o m ­ pound has dropped steadily since the e x p i r a t i o n o f its p a t e n t in 1991 (FEUILLETTE et a i , 1994; M A R N O T - TE, 1994). In 1995, glyphosate began b e in g used w i d e l y in th e c o t t o n - growing region of northern Cameroon, w i t h t r e a t m e n t s at 1 4 4 0 g /h a r e c o m m e n d e d a g a in s t I m p e r a t a

cylindrica.

Conclusion

There is presently no cost-effective and non-toxic herbicide available to re p la c e p a ra q u a t w h i c h w o u l d be a d a p te d to p r e - e m e r g e n c e w e e d control in cotton-grow ing regions of northern Cameroon.

Diuron and atrazine

In 1 9 8 9 , IR A and C IR A D , in c o l ­ l a b o r a t io n w i t h several p e s t ic id e m an ufa c tu re rs , ren ew ed te sting o f

herbicides that could be used in cot­ ton and fo od cropfields, along w ith n o n -s e le c tiv e h e rb ic id e s . In 199 0 and 1991, m ulti-site trials d em on s ­ trated th e e f f ic a c y o f t w o g e n e ric molecules, i.e. diu ron in cotton crop­ field s and atrazine in m aize fields, w hen treatments were conducted at relatively low doses as compared to those used w ith binary mixtures sup­ p li e d b y th e m a n u f a c t u r e r s . T he results helped define suitable doses, the spectrum o f activity, p hy to to x ic i­ ty risks, impact on subsequent crops, and treatment conditions adapted to the fa rm in g e n v iro n m e n t. The tests w e r e c a r r i e d o u t on s p l i t p lo t s : trea te d and n o n -tr e a te d . In 1 99 2, fo llo w in g these tests, SO DECO TO N r e c o m m e n d e d t r e a t m e n t s w i t h diuron at a dose o f 720 g/ha and with atrazine at 800 g/ha.

General characteristics

These t w o m o l e c u l e s h ave m a n y c om m on points. They were discove­ red in the 1950s and d e v e lo pe d in the 1960s, and were among the first selective pre-emergence herbicides a v a ila b le w it h p e rs is te n t a c tiv it y . M a iz e - s e le c tiv e a tra z in e fa v o u re d t h e w o r l d w i d e e x p a n s io n o f th is c r o p . D iu r o n w as used f o r i n i t i a l weed co ntrol treatments in tropical plantations, as w ell as in cotton c rop ­ fields, particularly in USA.

They have quite high chemical stabi­ lity, w ith a few weeks to months per­ s is te n c e in th e u p p e r s o il layers. Overall, they have the same herbici­ de performance, i.e. they have high efficacy against weeds w ith superfi­ cial roo t systems, d es troy in g (from the cotyledon leaf stage) weeds with small seeds that germinate close to the soil surface.

These m o le c u le s are d eg ra de d by physico-chemical and m ic ro b io lo g i­ cal processes, in relation to the b io ­ logical soil activity and the affinity of v a r io u s ty p e s o f m ic r o o r g a n i s m s (SEVERIN & TISSUT, 1991). D iu ron belongs to the substituted urea fam i­ ly and atrazine to the chlorotriazine f a m i l y . D iu r o n and a tra z in e , lik e

Hoe weeding Draught tillage.

Light tractor tillage, ploughing in of Imperata cylindrica.

herbicides in Cameroon

Photos J. M artin

Ass-drawn weeding operation.

Ox-drawn earthing-up.

Grass stand scorched by paraquat.

m ost h e rb ic id e s used in p re -e m e r­ g e n c e t r e a t m e n t s , a re r e l a t i v e l y

non-toxic to man and warm -blo oded animals.

T h e s e t w o h e r b i c i d e s are used w o r ld w id e in integrated pest mana­ g e m e n t p ro g r a m m e s , a lo n e o r in c o m b i n a t i o n o r a l t e r n a t i o n w i t h other active ingredients.

W eed control with

atrazine

A t r a z i n e is a b s o rb e d th r o u g h the roo ts and leaves and has a b ro ad spectrum o f activity in pre-emergence treatments (at less than 1 000 g/ha), especially against Poaceae, annual C y p e ra c e a e and C o m m e lin a c e a e species (MARTIN & GERARDEAUX, 1994). This p ro d u c t can be used in p o s t-e m e rg e n c e w e e d trea tm en ts , w ith high efficacy against broad-lea- ved species (often d o m in a n t after t i l l a g e ) , b u t w i t h p o o r e f f i c a c y against annual grasses, that are a bun­ d a n t on n e w p lo ts . It can also be used in d ir e c t - s o w in g c o n d it io n s w it h o u t tillage.

Treatments a d a p te d to m aize

and sorghum crops

M aize and sorghum very efficien tly m e t a b o liz e and d e t o x i f y a tra z in e (GAILLARDO N, 1991). This herbici­ de can t h e r e f o r e be used in p o s t­ emergence treatments on fields crop­ ped w it h these t w o cereals w it h o u t any special precautions.

A tra z in e does n ot have p h y to to x ic s id e -e ffe c ts on s u b s e q u e n t c ro p s (cotton or legumes). These risks have not yet been fu lly assessed on catch c r o p s ( c o w p e a , c o v e r c r o p s ) , a lth o u g h th e y seem l o w w h e n the catch crop is sown 45 days after the pre-emergence treatment.

In 1992, atrazine began being used w id e ly for pre-emergence treatments in s o r g h u m c r o p f i e l d s , a n d t h is t e c h n i q u e in n o w f u l l y a d o p t e d . Im p r o v e d red s o rg h u m (S o rg h u m

c a u d a t u m " D j i g a r i " ) c ro p s , s o w n

w i t h o u t tilla g e o n c e the first rains occur, are soon overrun w ith weeds.

High efficacy of atrazine in a maize cropfield.

PhotoJ. M artin

Poaceae species are generally abun­ d a n t o r d o m in a n t . Pre -e m e rg en ce treatments promote good initial plant growth and replace the first weeding, w h ic h is generally a long operation. S o r g h u m is c r o p p e d on a r o u n d 4 0 0 0 0 0 ha o f la n d in n o r t h e r n C a m e r o o n , i n c l u d i n g 4 0 0 0 0 ha p la n t e d w i t h i m p r o v e d v a r ie t ie s , w hic h are of considerable interest in terms o f weed control.

Efficacy of atrazine against

C om m elina benghalensis

Atrazine is o f major interest because of its efficacy against C. benghalensis (MARTIN & GERARDEAUX, 1994). This weed becomes dom inant after a few years of cropping, w hen it is d if­ fic u lt to control because of regrowth. It is h ig h ly c o m p e titiv e w it h crops, e s p e c ia l ly in th e w e t t e r s o u th e rn parts o f the cotton-growing region. It germinates massively after any extent of surface tillage, and q uic k ly affects c o tt o n and m a iz e c r o p p i n g areas w h e r e these c ro p s are s o w n after rapid d raught tillage (lo w coverage w it h little clearing). O n sandy soils w ith a low exchange capacity, w hich is a v e ry c o m m o n s itu a tio n in the cotton-growing regions o f Cameroon a nd w e s t e r n A f r i c a , p o s t - s o w i n g treatments of maize cropfields w ith a lo w atrazine dose (800 g/ha) general­ ly keeps the c ropfield weed-free for m o r e t h a n a m o n t h . T h e y o u n g w e e d s s t i l l p r e s e n t c a n t h e n be q u ic k ly and efficiently buried by an earthing-up operation - along w ith a second urea application, when this is s c h e d u le d in in t e n s iv e c r o p p i n g situations.

T he e x c e lle n t e ff ic a c y o f a tra z in e against C. benghalensis is c e rta inly responsible for the increased use of c h e m ic a l w eed c o n tro l in fie ld s of maize, w h ic h is almost always sown

Post-sowing treatment after tillage with a Handy® sprayer. Treatment with a pre-emergence herbicide and paraquat. PhotoJ. M artin

herbicides in C am eroon

After weeding an intercropped field, the plants are piled up to hinder regrowth. PhotoJ. M artin

in tille d fields. Moreover, atrazine is s o m e tim e s persistent u n til harvest u n d e r e x p e r i m e n t a l c o n d i t i o n s , whe n weed control is very carefully performed - w ith cleaner tillage and very regular spraying. A c c o rd in g to farmers and extension agents, there is low er weed infestation w ith cotton w h e n it is p la n t e d a fte r a tra z in e - treated maize, particularly when the field is infested w ith C. benghalensis.

Diuron treatments

of cotton cropfields

In c o tto n c ro p fie ld s , the persistent activity of diuron (at 700-800 g/ha) is the same as that of w id e ly used pre­ emergence herbicides. However, its persistence is tw o fo ld less than that of atrazine, at a dose of 800-1 000 g/ha in pre-emergence treatments.

Limit w e e d com petition

w ith cotton

A f te r s h a llo w tilla g e , w e e d s g r o w rapidly and regularly — before or at the same time as the cotton plants —

especially if it rains between tillage and s o w in g . G e n e r a l ly ra p id and sh a llo w draught p lo u g hin g leads to r a p id g r o w t h o f w e e d c o v e r (LE BOURGEOIS & MARNOTTE, 1994). Cotton plantlets are challenged after a fe w days w h e n c o tto n fie ld s are infested w it h C. benghalensis. Pre­ e m e r g e n c e t r e a t m e n t s d e l a y th e emergence of C. benghalensis by 15- 20 d a y s a n d p r o m o t e th e i n i t i a l g ro w th o f the crop. Thereafter, the advantage given to d ev e lo pin g c o t­ ton plants is q u ic k ly nullified, as the d e n s i t y a n d g r o w t h ra te o f th e C. benghalensis population are often m a r k e d ly s u p e rio r to th ose o f th e crop. W eeding is thus recommended b e fo re th e 3 0 th d a y p o s t - s o w in g , fr o m th e C. b e n g h a le n s is 3 -4 leaf stage if p o s s ib le , in o r d e r to l i m i t re g ro w th via natural cuttings. Early earthing-up should be done to hinder C. benghalensis from re-emerging. There has been a marked increase in the use o f diruon for weed control in co tto n c ro p fie ld s , despite its f a ir ly short persistence. It is also starting to be used in g r o u n d n u t c r o p f i e l d s , w ith nearly 600 ha treated.

Diuron selectivity for cotton

There are p h y to to x ic ity risks o f d iu ­ ron to c o tto n as this h e rb ic id e has p o s i t i o n s e l e c t i v i t y . P h y t o x i c i t y symptoms are often observed. They are lim ited to the cotyledons (white spots o r pla qu es), s o m e tim e s rea­ c h in g th e firs t o r se con d tru e leaf ( r e l a t i v e l y d is c o l o u r e d i n t e r v e i n spaces), w ith o u t harming subsequent d evelopm ent to a m ajor extent. The h e r b i c i d e c o n c e n t r a t i o n m u s t be zero or very lo w in g ro w th zone o f the first roots, around 5 cm b elow the surface.

Severe p h y t o t o x ic i t y can o c c u r in many cases:

- on very poor or eroded soil, where h e r b ic id e m ig r a t io n is ra p id , ro o t growth is poor;

- on shallow-sown plots or unclosed seed holes;

- after heavy rains, via rapid herbici­ de migration;

- d u e to a c c id e n t a l o v e rd o s e s or splitting of treatments.

The success of weed control depends on re g u la r s o w in g in c lo s e d seed holes, w h i l e c o m p ly i n g to re c o m ­ mended doses.

G r o u n d n u t a n d c o w p e a c r o p s , w h i c h b e n e f i t f r o m m o r e c a re fu l sowing than cotton (1 seed per w ell- c l o s e d seed h o l e ) , c a n t h u s be treated w it h d iu r o n . H o w e v e r , on poor or eroded soils, the diu ron dose should be reduced by 25-50% or the trea tm en t, w h i c h b ecom es d a n g e ­ rous and n o t v e ry e f f ic ie n t, c o u ld even be cancelled altogether.

Complementarity of diuron

or atrazine with paraquat

for controlling perennial

weeds

In n o rth e rn C a m e ro o n , as in most cotton-growing regions, there are lar­ ge in fe s ta tio n s o f som e p e r e n n ia l weed species, against w h ic h diuron and atrazine are considered ineffe­ c t i v e : t u b e r - b e a r i n g C y p e r a c e a e

(Cyperus rotundus and C. esculentus)

o r rh i z o m e - b e a r i n g P o a c e a e

(Im p e ra ta c y l i n d r i c a ) . M o r e o v e r ,

paraquat has o n ly a te m p ora ry and partial effect on perennials and w ell- developed annuals.

W hen these weeds do not cover the s o il, c o m b in e d d iu r o n + p a ra q u a t trea tm en ts in c o tto n c r o p fie ld s , or atrazine + paraquat in maize fields, show an interesting synergistic effi­ ca cy, and the effe ct o f p araq ua t is boosted.

S o m e g r o w e r s c l a i m t h a t d i u r o n treatments facilitate weeding o f fields infested with I. cylindrica. Progressive a b s o r p t i o n o f s m a ll q u a n t i t i e s o f h e r b ic id e s low s p la n t g ro w t h and weakens its roo t system. The slight b u t p r o lo n g e d e ffe c t o f th is ro o t- penetrating photosynthesis inhibiting herbicide can add to the shock effect o f paraquat, p a r t ic u la r ly w h e n the w ee d roots are m o s tly s u p e rfic ia l, w h ic h is often the case w ith tufts of w eeds d is tu rb e d by tilla g e b u t n ot ploughed under.

Treatment programmes

against Rottboellio

cochinchinensis

D iu ro n and atrazine are ineffective against R. cochinchinensis. Presently p ro p o s e d s o lu t io n s i n v o l v e t r e a t ­

ments o f p e n d im e th a lin c o m b in e d w i t h d i u r o n o r s y s t e m i c a n t i - Cramineae products.

Resistance and vigour

of R. cochinchinensis

This annual large-seed Poaceae spe­ cies, w it h rapid initia l g ro w th , can efficiently compete w ith crops, parti­ cularly on rich well-watered soils. Its seed can germinate very deeply, thus provid ing it w ith a good resistance to herbicide treatments.

V ariable susceptibility

to pendim ethalin

R. c o c h in c h in e n s is is su s c e p tib le

to p e n d i m e t h a l i n at a d o s e o f 1 000 g/ha in pre -emergence treat­ ments (LE BOURGEOIS et a i , 1992). However, the results o f experiments c o n d u c t e d since 199 2 have h ig h ­ lighted considerable variability in the e f f ic a c y o f p e n d i m e t h a l i n a ga in st

R. c o c h in c h in e n s is . This c o u ld be

a t t r ib u t e d to w e a t h e r c o n d i t i o n s during treatments (promoting v o la ti­ lit y and p h o to d e g r a d a tio n losses), to soil porosity (in flu e n c in g va po ur dispersal in the soil) and to the depth at w h ic h the seeds are buried, since they can o n ly g erm inate in the top 8 cm soil h orizo n (LE BOURGEOIS & MERLIER, 1995).

In 1 9 9 5 , p e n d i m e t h a l i n (at 1 000 g/ha) c ombined w ith diu ron (at 7 2 0 g/ha ) w a s r e c o m m e n d e d f o r c o n t r o l l i n g R. c o c h i n c h i n e n s i s in cotton cropfields.

The use o f this product is limited, in spite of good results, by its relatively high price. A lthough pendim ethalin is not very hazardous, it has a poor reputation because it can deeply dye clothes and skin y e llo w . There are o th e r c h e m ic a l s o lu tio n s , such as post-emergence treatments of cotton fields w ith slow-acting, systemic gra- m inicides, w h ic h are very selective for dicots, but are still too expensive.

New chemical

weed control

possibilities

S in c e 1 9 9 2 , S O D E C O T O N has extended the use o f d iu ro n (at 720 g /ha ) and a t r a z i n e (at 8 0 0 g /h a ), r e p la c in g th e b in a r y f o r m u la t io n s used in pre-em ergence treatm ents. T h e re a s o n s b e h i n d th e c u r r e n t increase in c h e m ic a l w eed co n tro l are technical, e c o n o m ic and linked w ith organization, in addition to the fact that it can promote an increase 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

dose g/ha

1991 1992 1993 1994 1995

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.

herbicides in Cam eroon

Cost-effective

pre-emergence treatments

The use o f d iu ro n and atrazine has l o w e r e d th e a v e ra g e c o s t o f p r e ­ emergence treatm ents by th reefold (Table 3). T here are th ree c o m p o ­ n ents in v o l v e d in th is r e d u c t i o n : price, dose and formulation.

Since the patents for these molecules expired, they have become generic herbicides used w orldw ide . They are p r o d u c e d and m a rk e te d by m a n y c o m p e tin g companies. Conversely, previously recommended binary for­ mulations contain active ingredients t h a t are m o r e e x p e n s iv e and less widespread.

C u r r e n t l y r e c o m m e n d e d a c t i v e ing re die nt doses are a bout tw o fo ld lower since binary form ulations have b e e n a b a n d o n e d . T he h e r b i c i d e combinations previously recomm en­ ded for maize all contained atrazine at similar doses. The reduction in the h e r b i c i d e dose can be e s p e c ia lly attributed to the absence of a second a ctive ing re die nt, b e lo n g in g to the c h lo r o - a c e t a m id e f a m i l y (m e to la - chlor or alachlor).

D i u r o n w a s n o t i n c l u d e d in p re- v i o u s l y - r e c o m m e n d e d h e r b i c i d e form ulations for cotton.

The high active ingredient c o n c e n ­ tration in wettable powders reduces tra n s p o rt costs and these f o r m u la ­ tions have a long shelf I ife o f many years, thus m in im iz in g losses.

A daptation of formulations

and packaging

H e r b ic id e spray p re parations from w e t t a b le p o w d e rs (WP) o f d iu r o n and a tra z in e are h ig h l y p r a c tic a l, e s p e c ia lly c o n c e r n in g s ing le dose pac k e ts f o r q u a r te r - h e c t a r e tr e a t ­ ments. This type of packaging is not as expensive as containers required f o r s u s p e n s io n c o n c e n t r a t e (SC) fo rm u la tio n s . Pow der fo rm u la tio n s have a long shelf I ife, w h ile suspen­ sion concentrates often form relati­ v e ly u n m i x a b l e s e d im e n t s in th e containers.

Sufficient organization

Extension o f any h erbic ide p ro d u c t in v o lv e s t r a i n i n g and m o n it o r i n g activities, as w e ll as adapted mana­ g e m e n t o f s u p p li e s a n d c r e d i t . H u n d re d s o f d e m o n s tr a tio n plots, w it h c o tto n and fo o d c rop s, have been set up yearly since 1992 th ro u ­ g h o u t c o t t o n - g r o w i n g r e g io n s . T e c h n ic a l datasheets designed for extension agents have been updated. IRA researchers c o n d u c t sessions intended for SO DECO TO N trainers. F o l l o w - u p is p r o v id e d b y S O D E ­ C O T O N staff, w h o are experienced and highly present in the field during c ro p e s tab lish m e nt. The h e rb ic id e s u p p l y c o s ts i n c l u d e b a t t e r y c o n s u m p t io n and d e p r e c ia tio n o f S O D E C O T O N spraying equipment. Herbicides for cotton and food crops are supplied on c re d it and paid for when the cotton is sold. The "in p u t" credits are granted to organizations

of around 10 farmers w ith jo in t res­ ponsibility, thus ensuring repayment. H erbicide s, batteries, sprayers and gear are s u p p lie d to v illa g e stores between cropping seasons.

Economic impact of weed

control

The costs o f the differen t herbicide p ro d u c ts and expenses associated w it h th e ir use have been assessed: l a b o u r s a vin gs fo r w e e d in g , c r o p p r o d u c t i o n in c r e a s e s , t r e a t m e n t costs, and s u p p le m e n t a r y l a b o u r costs due to increased yields.

An optimal maintenance programme involves t w o w eedings and an ear- t h i n g - u p b e f o r e t h e w e e d c o v e r d e v e lo p s b e y o n d t h e e c o n o m i c damage threshold, e.g. on days 14, 28 and 42 after sowing (GABOREL, 1 9 8 9 ) . In p r a c t i c e , m a i n t e n a n c e

The economic impact of chemical weed

control in 1995

It was estimated that chemical weed control in cotton cropfields enables a labour gain of 12 days/ha. The herbicide can reduce period of excessive weed competition by 10 days for cotton, i.e. equal to 15 kg/day of seed cotton. The estimated crop-yield gain following herbicide treatment is 150 kg/ha for cotton. The financial assessment was carried out as follows (Table 4):

- one day's income is 500 CFA francs, i.e. 6 000 CFA francs for 12 days of work; - the price of seed cotton in 1995 was 160 CFA francs/kg;

- the costs associated w ith cotton harvesting (picking and marketing) were 30 CFA francs/kg;

- 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

16 000 CFA francs/ha for cotton. These economic benefits are incurred with increa­ sed crop production, estimated at 6 3 0 0 1 of seed cotton for the entire cotton-growing region, and 7 0 0 0 1 of maize seed.

Table 4. Estimated seed cotton and maize production gains (CFA francs) following herbicide treatments.

Budgetary terms Cotton Maize

Savings on weeding

Value of supplementary production

Cost of harvesting supplementary production

Cost of herbicide treatment Total + 6 000 FCFA + 24 000 FCFA - 4 500 FCFA - 9 300 FCFA + 16 200 FCFA + 9 000 FCFA (18 days) + 20 000 FCFA (50 FCFA per kg - 1 500 FCFA (5 FCFA per kg) - 9 300 FCFA + 18 200 FCFA

w o rk is always carried o ut later than th e o p t im a l w e e d tr e a tm e n t dates (D U G U E & D O U N IA S , 1995), thus substantially increasing the duration and laboriousness of weeding opera­ tio n s (GABOREL, 1989), r e d u c in g the efficacy o f earthing-up and lea­ ding to crop-production losses due to w e e d c o m p e t i t i o n . U n d e r f i e l d conditions, one herbicide treatm ent reduces the w e e d in g tim e and the period when weed cover is invasive, w h i c h enables la b o u r savings and yield gains.

In Benin, in a region of similar e c olo ­ gical conditions, 175 tests were car­ ried o ut in c o lla b o ra tio n w ith local fa r m e r s ( G A B O R E L & F A D O E - G N O N , 1991). The results w ere as follows:

- the use o f herbicides led to a yield increase o f 2 4 0 -6 0 0 kg/ha o f seed c o tto n and 5 5 0 -6 5 0 kg/ha o f seed maize;

- l a b o u r t i m e w a s r e d u c e d by 1 7-19 days/ha for both crops. These gains are the equivalent of w eeding one highly weed-infested hectare of cropland or two lightly infested hectares.

The future of chemical

weed control in cotton-

grow ing regions of A frica

The herbicide treatm ent experience a c q u ir e d in n o r th e r n C a m e r o o n , using paraquat, atrazine and diuron, w ill be useful for other African co un ­ tries. Indeed, chemical control could c o m p l e m e n t c u r r e n t ( m a i n l y m an ua l) w eed c o n tro l te c h n iq u e s . A ll treatm ent possibilities have not yet been fu lly explored, e.g. interac­ t io n s b e tw e e n p a r a q u a t and p r e ­ e m e rg en c e h e rb ic id e s , a pa rt fro m the c o m p le m e n ta r ity noted above. P r e - e m e r g e n c e h e r b i c i d e d oses c o u l d be r e d u c e d by 2 5 - 5 0 % fo r poor soils w ith a low exchange capa­ city, w h ile being increased for soils w ith high clay content, organic mat­ ter or organic mulch levels. In maize c ro p fie ld s , atrazine treatm ents can be sp lit into tw o treatm ents, or the post-sowing treatment could even be

cancelled. Split treatments are more difficult to carry out in cotton cropfields.

N ew solutions are being investigated in order to diversify the range o f pro­ ducts a v a ila b le and w e e d c o n tr o l programmes. Treatment reco m m en ­ dations concerning glyphosate, pen- d i m e t h a l i n and c h l o r t o l u r o n (a substituted urea herbicide) should be d ra w n up. In trials c u rre n tly under way, treatments w ith chlortoluron (at 1 0 0 0 g /h a ) w e r e f o u n d to be as e f f i c i e n t as th o s e w i t h d i u r o n (at 720 g/ha).

Increased logistics and

training needs

An increased range of herbicides and w eed co ntrol co m b in a tio n s implies c o m p le x s u p p ly o r g a n iz a t io n and r e d u c e d t r a i n i n g n ee ds . R e g u la r e f f i c i e n t o r g a n i z a t i o n , i n v o l v i n g e x te n s io n and f o l lo w - u p , log istics and credit, is an essential p rerequi­ site for rational weed control.

The control strip method — w ith tw o or three lines left untreated for a few m e tre s — p r o v id e s a g o o d w e e d c o n t r o l d e m o n s tr a tio n . This t e c h ­ n iq u e can enhance and accelerate training, especially if undertaken by many n eighbouring users: h ig h lig h ­ t i n g v a r i a b i l i t y in th e e ffe c t s o f trea tm en t and th eir causes through discussions between neighbours, or w i t h th e h e l p o f a t r a i n e r , a n d g aining k n ow led ge on the diversity o f effects of different products under various treatment conditions.

Conclusion

A " l i s t o f essen tial c o m m o n p l a c e herbicide s" w o u ld be h ig h ly useful f o r sa van na fa rm e rs , such as th a t re c o m m e n d e d by W H O fo r health p u r p o s e s a n d a p p l i e d to m a n y African countries. Nevertheless, trai­ n in g, f o l l o w - u p , h e r b ic id e s u p p ly and c r e d i t (and r e p a y m e n t s c h e ­ dules) still have to be organized. In addition to having an overall unders­ ta n d in g o f w eeds th a t infest c r o p ­ f i e l d s (LE B O U R G E O I S , 1 9 9 3 ;

GRARD et al., 1995), full and accu­ rate k n o w le d g e o f a v a ila b le h e r b i­ cides is required for the purposes of rational control.

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Abstract... Résumé... Resumen

J. MARTIN, L. GAUDARD — Paraquat, diuron and atrazine for the renewal of chemical weed control in northern Cameroon.

In Northern Cameroon, based on the recommendations of SODECOTON and tests carried out by IRA in collaboration with CIRAD, paraquat has been used widely since 1987, and diuron and atrazine since 1992. Paraquat shows rapid and highly effective activity. To lim it the toxicity effects and optimize this treatment, it is recommended to s p lit th e tre a tm e n ts in to 2 00 g / h a doses, w ith treatments preferably in the evening or under cloudy cond itio ns. T reatm ents co nditions are v a rie d ( p re ­ ploughing treatment, high-dose treatment in case of high weed populations, etc.). The recommendations must be followed for the spraying preparation. Glyphosate and glufosinate can be used in pre-sowing treatments, but their mode of application is more restrictive. In cotton- growing regions, weed control recommendations also include the use of diuron at 720 g /h a in cotton cropfields and atrazine at 800 g /h a for maize. Atrazine is a very interesting herbicide for the control of C. benghalensis in m aize fie ld s and fo r p re -e m e rg en ce tre a tm e n ts in sorghum fields. Diuron is less effective on C. benghalensis

but its use is recommended in cotton fields. In addition, diuron and atrazine are associated with paraquat for the co n tro l o f p e re n n ia l weeds. The p re -e m e rg e n ce treatments recommended in this cotton-growing region were shown to be cost-effective thanks to the gains obtained in working time and production. Logistics and training support are provided by SODECOTON. Keywords: maize, cotton, sorghum, weed, herbicide, diuron, atrazine, paraquat, dose, toxicity, Cameroon.

J. MARTIN, L. GAUDARD — Paraquat, diuron et atrazine pour renouveler le désherbage chimique au Nord-Cameroun.

Au Nord-Cameroun, le paraquat est vulgarisé depuis 1987 et le diuron et l'atrazine depuis 1992, à la suite des recommandations de la SODECOTON et des tests réalisés par l'IRA en collaboration avec le CIRAD. Le paraquat montre une efficacité importante et rapide. Pour limiter les risques de toxicité et optimiser ce traitement, il est conseillé de frac tion ne r les doses en applications de 200 grammes par hectare, à réaliser de préférence le soir ou par temps couvert. Les conditions d 'e m p lo i sont diversifiées (traitement de pré-labour, traitement à forte dose en cas de végétation importante d'adventices). Les recommandations doivent être suivies pour la préparation des b o u illie s . Le g lyph os ate et le g lu fo s in a te sont utilisables en traitement de pré-semis, mais leur mode d 'a p p lic a tio n est plus c o n tra ig n a n t. Dans la zone cotonnière, les conseils de désherbage comprennent aussi l'e m p lo i du diuron à 7 20 gram m es par hectare sur cotonnier et l'atrazine à 800 grammes par hectare sur maïs. L'atrazine est un herbicide très intéressant pour lutter contre C. benghalensis dans les cultures de maïs et en traitement de pré-levée des sorghos. L'effet du diuron sur C. b e n g h a le n s is est m o in d re , son e m p lo i est recommandé en culture cotonnière. En outre, le diuron et l'atrazine peuvent être associés au paraquat pour lutter contre les vivaces. Les traitements de pré-levée préconisés dans cette région cotonnière se révèlent rentables grâce aux gains de temps de travail et de production obtenus. Un appui de logistique et de formation est assuré par la SODECOTON.

Mots-clés : maïs, cotonnier, sorgho, mauvaise herbe, herbicide, diuron, atrazine, paraquat, dose, toxicité, Cameroun.

J. MARTIN, L. GAUDARD — Paraquat, diurón y atrazina para renovar la escardadura química en el norte de Camerún.

En el norte de Camerún, el paraquat está vulgarizado desde 1987 y el d iu ró n y la a tra z in a desde 199 2, consecutivamente a las recomendaciones de SODECOTON y a las pruebas realizadas por el IRA en colaboración con el CIRAD. El p araquat ofrece una eficacia elevada y rápida. Para limitar los riesgos de toxicidad y optimizar este tratam iento, se aconseja fraccionar las dosis en aplicaciones de 200 gramos por hectárea, realizadas preferentemente al caer la tarde o en tiempo cubierto. Las condiciones de empleo son diversificadas (tratamiento previo a la labranza, tratamiento a dosis fuerte en caso de gran vegetación de adventicias). Deben seguirse las recomendaciones para la preparación de los caldos. El glifosato y el glufosinato son utilizables en tratamiento previo a la siembra, pero su modo de aplicación es más re strictiv o . En la zona algo do ne ra , los consejos de escardadura incluyen tam bién el empleo de diurón a 720 gramos por hectárea en el algodón y la atrazina a 800 gramos por hectárea en el maíz. La atrazina es un h e rb ic id a m uy in te re s a n te para lu ch a r contra

C. benghalensis en los cultivos de maíz y en tratamiento previo al brote de los sorgos. El efecto de diurón en

C. benghalensis es menor, recomendándose su empleo en cultivo algodonero. Además, el diurón y la atrazina pueden asociarse al paraquat para luchar contra las viváceas. Los tratamientos previos al brote de los sorgos en esta región algodonera resultan rentables gracias a los ahorros de tiempo de trabajo y producción obtenidos. SODECOTON aporta un apoyo de logística y formación. Palabras clave: maíz, algodón, sorgo, maleza, herbicida, diurón, atrazina, paraquat, dosis, toxicidad, Camerún.