Striga infestation in northern Cameroon: Magnitude, dynamics and implications for management

Abstract

Surveys of Striga (S. hermonthica (Del.) Benth.) infestation in northern Cameroon over the period 1987–2005 assessed Striga dynamics and evaluated its control strategies. In that period the percentage of Striga-infested fields increased in North and Far-North Provinces. Striga incidence increased more in maize fields than in the already heavily infested sorghum fields, where it remained almost constant. During the study period increased land pressure led to a reduction in the use of fallow and a higher frequency of cereal (mono-) cropping. Yields from farmers’ fields did not correlate with Striga incidence, confirming farmers’ prioritization of soil fertility, weeds, and labour for weeding as production constraints, rather than Striga. We discuss how conceptualization of Striga as a weed in the research arena may have led to a misunderstanding of farmers’ constraints. The decline of the cotton industry reduced farmers’ access to fertilizers, while access to organic manure remained limited, increasing the soil fertility constraint. We conclude that two decades of emphasis on Striga were unsuccessful. Enhanced crop yield through soil fertility management should be the entry point to tackle low yields and further worsening of the Striga situation.

Keywords:

• Crop production constraint
• Farmers’ priority
• Field survey
• Soil fertility
• Weeding

1 Introduction

Northern Cameroon, situated in the Sudano-Sahelian Zone, consists of North and Far-North Provinces and covers an area of 100,353 km² of which 85,000 km² represent almost the entire cotton belt of Cameroon [1]. About 82% of the area consists of arable land that could be cropped. Agriculture is the main source of subsistence for the local population and cereals are their source of staple foods.

Cotton (Gossypium hirsutum L.) is an important export crop of Cameroon and is the single most important cash crop in the two northern provinces. In the late 1980s the cotton production system enjoyed adequate technical and material support from the cotton development corporation (SODECOTON). Support to cotton production had its positive effect also on food crop production. Pesticides and fertilizers for the production of cotton were provided to farmers who subscribed to the prescriptions by SODECOTON at subsidized prices.

Traditionally, cotton in northern Cameroon was rotated with cereals to avoid a build-up of cotton pests and diseases. Farmers also used this rotation as an opportunity to exploit the residual fertility after a cotton crop for their cereal production. Otherwise, farmers hardly applied fertilizers to their principal cereal crop mainly because purchase costs were too high. Linkage between cotton production and food production weakened in the last decade due to the decline of the cotton sector, driven by lower cotton prices on the world market and increasing fertilizer prices (and decreasing fertilizer availability) as a consequence of structural adjustment programmes [2,3].

Soil fertility is an immense constraint to cereal production in the African savannah zone [4–7]. Apart from the direct negative effects of low soil fertility on agricultural production and food security, low soil fertility increases susceptibility to biotic pests. A major biotic pest in the savannah region is formed by parasitic plants of the genus Striga (Orobanchaceae). Most important are S. hermonthica (Del.) Benth., which is mainly found on cereals such as maize (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), pearl millet (Pennisetum glaucum (L.) R.Br.), and rice (Oryza sativa L.), and S. gesnerioides (Willd.) Vatke, found on the legume cowpea (Vigna unguiculata (L.) Walp.). Increased Striga infestations are associated with soil fertility decline [8], which in turn is often a consequence of increasing pressure on land resources. Increased land pressure, due to increasing population densities, forced farmers to shorten the fallow period necessary for soil fertility regeneration and to change cereal–legume rotations into systems of continuous cereal mono-cropping [9]. Both processes potentially increase infestation with S. hermonthica (hereafter referred to as Striga), setting into motion a downward spiral of agricultural productivity [9,10].

In 1984, an assessment of crop damage in northern Cameroon revealed that 300,000 ha of maize were severely damaged by Striga, often with a near-nil crop yield [11]. Sorghum, which constitutes the major staple food for the population, was infested over an even larger area, although exact data are lacking, but reduction in grain yield was often less dramatic [11,12]. Based on these assessments, a technological package was proposed and implemented on farmers’ fields by FAO under an assistance programme to the Cameroonian government from 1987 under a pioneer Pan-African Striga control project. Components of the package included hand-pulling of mature Striga plants, killing adult plants before flowering by using herbicides, application of nitrogen fertilizer, using trap crops as intercrop or in rotation with crops that are hosts of Striga, sowing Striga-resistant or tolerant crop varieties, and using early-maturing crop varieties.

Besides the introduction of the Striga control package to farmers, a formal survey was carried out in farmers’ fields to measure the intensity and magnitude of the Striga problem. A decade after the FAO project on Striga control started, the situation did not seem to have really changed and there was still no simple and efficient Striga control method. An evaluation, which took place in 1999, put the Striga problem in the more specific context of crop production constraints. After this 1999 survey, a third, less extensive survey was done in 2005. This survey was a rapid appraisal of the Striga situation that looked only at numbers of infested fields and the level of infestation of host crops.

This paper reports the findings of these three surveys to analyse Striga dynamics between 1987 and 2005. It reflects on the very limited success of an approach that prioritized the scientific analysis of the Striga problem rather than the farmers’ analysis of production constraints. We conclude with an attempt to reconcile farmers’ perception of Striga in relation to other crop production constraints and approaches to Striga control.

2 Methodology

Three surveys were carried out in northern Cameroon, one in 1987 by an FAO Striga project (CMR/86/005), and two in 1999 and 2005 by a PRASAC (Pôle Régional de Recherches Appliqués au Développement des Savanes d’Afrique Central) and an ARDESAC (Appui à la Recherche Régionale pour le Développement Durable des Savanes d’Afrique Centrale) project, respectively.

The 1987 survey consisted of a detailed questionnaire. Interviews and field evaluation were done by two FAO experts. Villages were selected at random from a map of the region before each trip was undertaken. A total of 216 fields were visited, equally divided over North and Far-North Provinces, and distributed over 49 villages.

The surveys of 1999 and 2005 were carried out in two PRASAC benchmark territories: Mowo (Far-North Province) and Mafa-Kilda (North Province).These territories are representative of the two provinces surveyed in 1987. Mafa-Kilda, which is situated mid-way in North Province, is very accessible and lies in the River Benoué basin, at the frontier of farmland opened-up (between 1976 and 1982) by pioneer immigrant farmers from the densely populated Far-North Province [13]. Mowo is located in Far-North Province in a more densely populated and since long occupied area, where almost all soils are degraded and poor [14]. The climate in Far-North Province is cooler and drier than that in the North Province (see Fig. 1 for rainfall data).

Fig. 1 Rainfall per decade (days 1–10, 11–20 and 21–30/31 of each month) for the two benchmark sites Mowo and Mafa-Kilda in Far-North and North Provinces of Cameroon, respectively. Data represent long-term averages over the period 1984–2005 (drawn lines) and rainfall recorded during the three survey years 1987, 1999 and 2005 (bars).

In each benchmark territory, 60 fields with sorghum or maize were selected during the 1999 survey. The two territories were sub-divided on a map into equally sized areas in order to ensure an even distribution of the 60 sample fields over the entire territory. The choice of fields to be surveyed was made as follows. Trained field staff was guided by the actual area of the territory and by certain reference points such as footpaths, hills and valleys. The surveyor then walked, while sampling, through the territory in a regular pattern, to cover the entire sub-divided area. The field staff walked at an average speed of 1 m/s, and stopped after 5 min since the last sampled field in order to survey the nearest cereal field. After sampling, the original position and walking direction were resumed and from that point onwards the operation was repeated until the desired number of fields had been surveyed.

Prior to the survey, a team of five persons was trained in the way in which the survey was to be carried out. The field owners were interviewed with the help of a structured questionnaire. The questionnaire opened with introductory questions to describe field size, time since the field was first cropped, sowing dates, crop management, and yield estimates. The second part of the questionnaire focused on identifying crop production constraints and a detailed study of the Striga problem, in addition to observations at individual field level. A pre-test questionnaire on farmers’ production constraints showed a large array of difficulties, of which the lack of financial means to procure farm inputs was the most prominent one. Given the project objectives, we focused on constraints that directly hampered crop production.

A targeted field observation throughout the entire cropping season with data collection on infestation levels and crop yield was carried out in 39 fields in Far-North Province and 27 in North Province. These fields, which were different from the ones surveyed earlier, were identified and selected at the start of the cropping season, based on the criterion of a long history of Striga infestation as known by local agricultural extension staff. At crop establishment, a sampling area was pegged out for monitoring Striga dynamics. In each field the first corner of the sampling area was taken as the fourth row from the border of the field at 2 m in the row from the edge of the field. From this point, 10-m plant rows were pegged out in rows 4, 6, 8, 10 and 12, the plants of which were used to determine Striga population density (severity, surface area 40 m²), and 6-m plant rows were pegged out in rows 5, 7, 9, 11, and 13, the plants of which were used to determine the number of cereal plants infested with at least one Striga plant (incidence, surface area 24 m²) and the total number of cereal plants. Plant distances for the cereal crop were 80 cm between and 50 cm within rows. Incidence and severity of Striga infestation were recorded every 2 weeks in each of the 5 rows. These plants were harvested at maturity. After air-drying for 2 weeks the maize ears and sorghum panicles were threshed and weighed and grain moisture content was determined. Grain yields were expressed on the basis of 12% moisture content.

In 2005, a rapid appraisal of Striga infestation was carried out by sampling 190 fields in Far-North Province (Mowo) and 60 fields in North Province (Mafa-Kilda).

The questionnaires for the field surveys and the observation sheets were manually coded and analysed statistically using the Statistical Analysis System (SAS) package. The General Linear Model was used to calculate correlations between grain yield and Striga infestation. Average percentages of crops infested by Striga and Striga incidence were used for analysis. Frequency tables were used to identify the changes over time in Striga infestation levels, Striga control options, and cropping systems.

3 Results

The dominant cereal crops cultivated in the study area, in order of decreasing importance, were sorghum, maize and pearl millet (Fig. 2). Maize and sorghum were of comparable importance in North Province, whereas sorghum was dominant in the drier Far-North Province. Pearl millet was sown in some years in some of the fields in Far-North Province.

Fig. 2 Importance of the three cereals grown in North and Far-North Provinces in each of the three survey years. Frequencies as percentage of all cereal fields surveyed.

During the 1999 survey, 120 farmers named at least three of the most important constraints to crop production. From their answers, 6 main constraints emerged (Fig. 3). Weed infestation of crops (and the need for weeding) was the major constraint at both locations, with low soil fertility coming second. In North Province, ravaging animals appeared an important constraint too, whereas labour scarcity was often mentioned in Far-North Province. Striga, which was a separate category from weeds, was frequently mentioned in Far-North Province, but hardly so in North Province.

Fig. 3 Importance of production constraints named by the farmers in North and Far-North Provinces. Farmers were allowed to name up to three major constraints.

When specifically asked, most farmers indicated that Striga was important. Between 1987 and 1999 the percentage of farmers who stated that Striga was important, rose from 86 to 94 in the North Province and from 82 to 94 in the Far-North Province. In order to appreciate farmers’ knowledge about Striga, their ability to diagnose Striga infestation before emergence of the parasite was investigated in 1999. Whereas the majority of farmers in Mowo claimed to be able to distinguish a Striga-infested crop from a healthy one before emergence of the parasite, the farmers in Mafa-Kilda were not (Table 1). Farmers named the following symptoms of Striga infestation: (1) leaf discolouring or yellowing (30 respondents), (2) reduced growth (21 respondents), (3) stunting (7 respondents), (4) wilting (3 respondents), and (5) root deformation (3 respondents). Out of the 45 respondents who said to be able to diagnose Striga infestation before emergence, one could not describe a single symptom, 27 described one and 17 two or more symptoms.

Table 1 Farmers’ response in the two territories to the question whether or not they were able to diagnose Striga infestation in their crops. Results of the 1999 survey. Four farmers did not respond. The difference between the two territories was highly significant (χ² = 11.1; p < 0.001).

Territory	No. of respondents	Ability to diagnose
		Not able	Able
Mafa-Kilda	31	25	6
Mowo	55	16	39
Total	86	41	45

Farmers were asked whether they perceived Striga infestation to be increasing or decreasing and what they considered the major cause for such a change. The majority of farmers (72%) indicated that Striga increased, and only 5 farmers (6%) thought that it decreased. The majority of farmers (59%) who perceived an increase had no idea about underlying causes, whereas 31% held a decline in soil fertility responsible for the worsening Striga situation. Three respondents thought that hand-pulling the infested plants affected Striga infestation: one stated that it caused a decrease whereas two said it did not affect Striga incidence (Table 2).

Table 2 Farmers’ perception of changes in the Striga situation over time and their causal explanation for observed dynamics. Results of the 1999 survey, one farmer did not respond.

Status of Striga infestation	Farmers’ reasons for perceived changes
	None	Hand-pulling	Fallow	Soil fertility	Flooding	Total
Increasing	38	0	4	20	2	64
No change	10	2	0	7	1	20
Decreasing	0	1	0	2	2	5
Total	48	3	4	29	5	89

Farmers were asked if they were able to control Striga in their fields. In Far-North Province more farmers answered this question in the affirmative than in North Province. Whereas in Far-North Province the percentage of farmers who claimed to be able to control Striga remained constant (around 80%) over the period 1987–1999, the percentage of farmers in North Province who made the same claim decreased over that period (from 70 to 35). Among Striga control options (Table 3), hand-pulling was mentioned frequently in both surveys, but was not practised anymore in North Province in 1999. Several farmers mentioned weeding separately from hand-pulling. Abandoning infested fields, which was practised in 1987, was no option any longer in 1999. Some farmers in North Province mentioned the sowing of a cereal they called fonio as an option for the control of Striga. However, the corresponding crop was pearl millet (P. glaucum) and not fonio or hungry rice (Digitaria exilis (Kippist) Stapf). In 1999 few farmers mentioned the use of inorganic fertilizers or organic manure as Striga control method. Availability of organic (mainly animal) manure was higher in Far-North Province than in North Province. No farmer named the use of herbicides, trap crops or specific varieties as management options.

Table 3 Numbers of farmers in Far-North and North Provinces who named different Striga management practices, in 1987 and 1999.

Striga management practice	Far-North Province		North Province
	1987	1999	1987	1999
Hand-pulling	47	38	47	0
Weeding	29	2	2	0
Crop rotation	0	18	0	0
Field abandonment	26	0	9	0
Fertilization (organic and inorganic)	0	9	0	7
Pearl millet cropping	0	0	0	6

Striga incidence was higher in sorghum fields than in maize fields. The percentage of Striga-infested maize fields increased between 1987 and 2005, while the percentage infested sorghum fields remained invariably high (Fig. 4).

Fig. 4 Frequency with which surveyed maize and sorghum fields were infested with Striga, as recorded in each of the three survey years.

There was no correlation between cereal grain yield and Striga population density (Fig. 5). High Striga counts always implied low yields, but low Striga counts were often accompanied by low grain yields. In other words, low yields were not necessarily caused by high Striga incidence.

Fig. 5 Scatter diagram of maize (triangles) and sorghum (circles) grain yields plotted against maximum Striga incidence. Combined data for the two provinces surveyed in 1999. The vertical and horizontal lines represent average maximum Striga counts and average maximum grain yields for sorghum (solid lines) and maize (dotted lines), respectively.

In several fields, Striga occurred in patches. The patchy distribution could be explained by the relative position of trees in the fields and by the presence of termite mounds. Close to trees and termite mounds Striga infestation was very low or absent. Tree species were not always identified, but many trees belonged to the species Faidherbia albida (Delile) A. Chev. Termite mounds were usually located under or close to trees. Soil characteristics also affected Striga infestation: the number of infested stands was low on clay soils and very high on infertile sandy soils (Table 4).

Table 4 Effect of soil type on proportion of crop stands infested with Striga (±standard deviations).

Soil type	Sample size (n)	% crop stand infested
Clay	5	17.7 ± 10
Fertile degraded land	5	40.8 ± 24
Infertile degraded land	3	48.7 ± 33
Fertile sand	5	54.0 ± 41
Infertile sand	10	85.7 ± 22

An analysis of the yearly cropping sequence as recalled by farmers over the 4 years preceding the 1987 and 1999 surveys in Far-North Province revealed a consistent alternation (rotation) on part of the fields between cotton and cereals (Table 5). In North Province a comparable pattern was observed for the 4 years preceding 1987, but not for the 4 years preceding 1999. This cotton–cereal rotation is practised by a majority of the farmers in the cotton belt of northern Cameroon. Both cowpea and groundnut are less prominent in the cropping system than cotton, especially in Far-North Province, but for groundnut a comparable rotation was observed as for cotton in North Province in the years preceding 1987. Legumes are usually intercropped as insurance against cereal crop failure. Fallow was practised fairly often in 1987 but was much less common in the later 1990s, more specifically in Far-North Province.

Table 5 Crops and frequency (%) of being planted in the 4 years preceding cereal cropping in 1987 (1983–1986) and 1999 (1995–1998) in North and Far-North Provinces of Cameroon as recalled by farmers in the 1987 and 1999 interviews.

	1987 interviews				1999 interviews
	1983	1984	1985	1986	1995	1996	1997	1998
North Province
Sorghum	43	16	51	17	5	7	5	2
Maize	24	4	31	6	23	47	35	30
Pearl millet	–	–	–	–	2	–	–	–
Cotton	11	46	7	60	27	20	20	48
Groundnut	4	24	7	12	13	10	35	15
Cowpea	–	–	–	–	–	–	–	2
Tuber crops	1	1	1	1	–	–	–	–
Fallow	9	7	3	5	–	3	2	3
No data^a	8	2	–	–	30	13	3	–
Total	100	100	100	100	100	100	100	100
Far-North Province
Sorghum	67	52	66	52	72	25	70	28
Pearl millet	22	18	20	16	12	3	12	3
Cotton	4	22	6	25	17	72	18	67
Groundnut	1	4	2	1	–	–	–	–
Cowpea	1	–	–	–	–	–	–	–
Fallow	5	4	6	6	–	–	–	2
Total	100	100	100	100	100	100	100	100

a Farmers did not remember.

4 Discussion

4.1 The Striga situation in northern Cameroon

Striga infestation in Far-North Province over the period 1987–1999 remained invariably very high (from 84 to 91%), but that in North Province increased (from 67 to 88%). Similarly, infestation of sorghum (the main crop in Far-North Province) and maize (together with sorghum the main crops in North Province) with Striga increased from 90 to 95% and from 46 to 81% between 1987 and 2005, respectively (Fig. 4). These trends confirm farmers’ perception of increased Striga infestation: 72% of the interviewed farmers claimed that Striga infestation levels had increased over that period (Table 2). Apparently, two decades of emphasis on Striga in research and extension have not resulted in an improvement of the Striga situation. The question can thus be posed whether the proposed techniques were technically inadequate or whether adoption of in principle effective techniques is the major constraint. We conclude that Striga research and the techniques it has provided largely fail to address the heart of the problem, i.e., declining soil fertility. Science generally communicates with farmers about Striga as a weed, but this does not correspond with farmers’ perceptions and management. This mismatch is not specific for the study area, because similarly high and persistent infestation levels (80–90% of farmers’ fields) have been reported for sorghum and maize in northern Nigeria [15] and are probably characteristic for cereal cropping systems of resource-poor farmers in the Sahel.

At the same time, our surveys and field observations provided a paradoxical picture. We documented very high (Far-North Province) or increasing (North Province) percentages of infestation with Striga. On the other hand, a large majority of farmers in Far-North Province and as many as ca. 35% of the farmers in North Province claimed to be able to control Striga. Many farmers stated to apply hand-pulling, but evidence indicates (Table 2) that very few farmers considered hand-pulling effective. And finally, among the prioritized constraints Striga was not very high on the list (compared with weeds or soil fertility), but when asked further, almost all farmers suggested that Striga was an important problem.

Comparable contradictions have also been encountered in Ghana and Nigeria. In a survey in Ghana it was found that 80% of the farmers with Striga in their fields adopted mechanical weeding or hand-pulling. However, at the same time the people that carried out the survey mentioned apparent ignorance of farmers concerning the effects of Striga on crops [16]. Although 70% of the farmers in a different study in northern Nigeria mentioned weeding as a local Striga control method, the study also reported that an integrated Striga project increased Striga weeding from 4 to 82% [17]. However, the study did not specify the form(s) of weed management, but an earlier study in the same area and from the same research group reported that hand-pulling Striga is decreasing (and ranked by farmers as very costly), while hoe-weeding is increasing and universally applied [18].

We propose two explanations for this paradox. Inconsistencies between surveys and field observations may be partly the result of the questionnaires (and interviewing staff) being too much geared towards Striga. With the benefit of hindsight it is also evident that projects that tackle single aspects of farming systems are bound to have little effect. This is also true for Striga where it has become almost a mantra to state that only integrated Striga control measures will be effective [17,19,20]. We think that very few farmers apply Striga control measures, they rather manage their cropping systems in ways that also affect Striga [21]. Nevertheless the need for agro-ecosystem management leaves open the question whether the entry points for such integrated packages are to be found in Striga, in any other constraint prioritized in surveys, or in the farming system at large.

Second, farmers’ priorities were taken insufficiently into account due to the fact that researchers conceptualized Striga differently from farmers, thereby misinterpreting their constraints. In retrospect, the importance of the relation between yield and Striga incidence (Fig. 5) has been initially overlooked by us. Whereas a first inspection (and conventional interpretation) of the graph in Fig. 5 suggests a negative correlation between Striga incidence and crop yield for both sorghum and maize (and hence can be read as implying that increasing Striga numbers cause lower crop yields), dissection of that graph (especially the left part of it) suggests room for an alternative interpretation. That interpretation states that in the absence of Striga, crop yields vary substantially, suggesting that other constraints are more important than Striga. The questionnaire (Fig. 3) did reveal such constraints. Weeds and soil fertility rather than Striga were farmers’ main constraints. Mentioning weeds implies labour as a constraint (Fig. 3), because in the area weeding consumes 30–60% of farmer's time spent on their agricultural operations [22]. Fig. 5 also indicates that Striga incidence may vary widely without much relation to yield. This leaves open the question whether reducing Striga by management would lead to returns on investment through improved yield. Fig. 5 therefore also supports farmers’ claims that there is little effect of proposed Striga management.

One reason why we misinterpreted the significance of weeds (and labour for weeding) stems from our conceptualization of Striga as weed [23]. From a scientific perspective a weed is a plant that grows at a place where humans do not want it to grow (mainly because its presence negatively affects the yield of the desired crop). From that perspective, Striga is clearly a weed, albeit a special one. Farmers conceptualized weeds differently from scientists, as is clear from their mentioning both weeds and Striga (in Far-North Province) or only weeds (in North Province) as constraints (Fig. 3). Although the scientists considered Striga in their questionnaire as a weed category different from other weeds, this does not necessarily imply conceptualizing this difference in the same way as farmers. From a farmer perspective weeds are plants that emerge together with crops and that require labour to suppress them during early stages to allow the crops to perform, as the weeds cause low yield and grow better on more fertile soils. In their view, Striga is a symptom of poor soil fertility rather than a cause of low yields [22,24]. A major consequence of this differential conceptualization is that weeding, when mentioned by farmers, is likely interpreted by scientists as Striga management. This interpretation is in accordance with the observation that weeding was mentioned by many farmers in the study area (Table 3) and also in several countries of West Africa [16–18]. Hand-pulling Striga, which is recommended to prevent seed set and seed dispersal, does not directly affect crop yield and is therefore not effective in the short term [25]. It is therefore not part of farmers’ conceptualization of weeding. Conventional hoe-weeding is even counter-productive as it improves conditions for Striga during its below-ground stage [26]. Lack of effectiveness of weeding (and more generally lack of effectiveness of human labour to control Striga in the short term) makes Striga essentially different from weeds and thus confirms that Striga is not a weed.

Some farmers in North Province mentioned rotation with pearl millet as a Striga control strategy. Resistance of this crop to Striga was observed repeatedly in the 2005 survey. We observed no pearl millet crop stand that was infested with Striga, and even where this crop was intercropped with sorghum only the sorghum plants were infested. In areas of Benin, where pearl millet is not commonly grown, farmers reported that this crop was not attacked by Striga and sometimes the rotation with pearl millet was used as a Striga control method [27].

Because Striga forms separate host races on sorghum and millet, initial Striga resistance after cereal switches are not infrequent. However, because these host races retain interfertility and because Striga is an obligate out-breeder, such resistance is rapidly broken [28]. In the drier Far-North Province, where pearl millet occurs more frequently than in North Province, no such effect of this crop on Striga control was mentioned, which is in line with the fact that Striga is a major problem in pearl millet in for instance Mali and Niger [8,29].

4.2 Why has the Striga situation worsened?

Soil fertility constraints were prioritized by farmers, and farmers were aware of the link between soil fertility decline (and hence cereal yield decline) and an increase in Striga incidence (Fig. 5). However, along the pathway of soil fertility decline, Striga does not always increase and soils can become too impoverished to maintain high Striga population levels. Such soils would be in the lower left part of the graph, with low Striga numbers and low yields. Under such soil fertility conditions Striga management would initially always seem to be a failure because reducing Striga incidence will not increase yields and will therefore not be attractive to labour- and cash-constrained farmers. Soil fertility improvement, however, would initially result in somewhat higher Striga numbers [30]. Yet, we suggest that improving soil fertility as primary constraint constitutes the better way [25]. A study in Benin also demonstrated that effective Striga management will only be of interest to farmers if soil fertility exceeds a threshold value. If not, Striga management will absorb farmers’ precious resources without any yield increase [21].

Soil fertility improvement in the area had not occurred over the last two decades. The decline of the cotton industry [2,3], driven by low world market prices for cotton and increasing fertilizer costs due to structural adjustment programmes, has reduced the availability of fertilizer for cereals. The collapse of the cotton sector rather than Striga seems ultimately responsible for the agricultural stagnation and lack of success of a Striga programme that lasted for two decades.

Agricultural stagnation resulted in the lack of appropriate measures to maintain or improve soil fertility and hence resulted in soil fertility decline. In most of the benchmark territory in Far-North Province there has been increased pressure on available arable land due to the rapidly growing human population. Far-North Province (Mowo territory) saw a 67% population increase between 1992 and 1996 [14,31]. Increased pressure on the land reduced the fallow period in crop rotations, and increased cereal mono-cropping over cereal–legume rotations (Table 5).

Field abandonment or fallowing, which used to be an option for Striga management mentioned by farmers in 1987, was not mentioned in 1999 (Table 3). The demographic pressure forced many farmers to migrate (organized migration) between 1974 and 1986 from Far-North Province to North Province [22,32] where land was still available, demographic pressure lower, soils more fertile or less degraded, and rainfall slightly more favourable. On this (virgin) land Striga was still largely absent. Under these new agro-ecological conditions, a cereal-rich cropping pattern was again favoured, with maize partly replacing sorghum (Table 5). However, although Striga started off as less of a problem, the introduced cropping pattern and gradual increase in land pressure led to a mounting Striga incidence. The rise in Striga infestation in maize between the 1999 and 2005 surveys (Fig. 1) may also be further explained by the reduced access to inorganic fertilizer by farmers.

A cropping system based on a high frequency of cereals as Striga-host with limited legumes in the rotation and in combination with little or no fallow and very limited use of inorganic fertilizer and organic manure for fertility restoration over the years worsened the Striga situation. Cereal–legume intercropping and to a lesser extent rotating cereals with legumes, are important in reducing the seed bank of Striga and its effect on the host [29,33–35].

4.3 The way forward

With current soil fertility and crop management, densities of Striga will remain (very) high. Fig. 5 shows that good yields were not obtained when Striga incidence was high. So it is likely that average yields will decline as the best yields will be obtained from fewer and fewer fields if Striga continues to increase. Improved management of cropping systems that would simultaneously lead to a reduction in Striga would have to start from those constraints that farmers see as their priorities (Fig. 3). The role of scientists in this would be to investigate what interactions could make different possible approaches more or less likely to lead to effective improvement in farming systems income. Where labour (for weeding) is scarce, increases in yield [2,22] would improve productivity, but only if such changes do not demand large additional labour inputs. Since soil fertility decline is linked with the build-up of Striga, an approach that looks only at controlling Striga while neglecting the need for improving soil fertility would do little to restore on-farm productivity and make farming sustainable. Fig. 5 predicts that without soil fertility management farmers would attain low yields even at low Striga levels.

Joint experimentation in farmers’ fields is therefore needed to see how the negative spiral of low yields leading to low labour and land productivity – leading to low investing capacity – leading to soil degradation and high Striga infestation – leading to low yields, etc. can be broken. Where Striga scientists have often advocated actions based on their understanding that attacking the Striga problem would be the best entry point, actions based on farmer's priorities seem a better approach. The motor for improvement of soil fertility leading both to higher yields and reduced losses from Striga could be provided by a cash crop for which the market is well organized. In the area of research, cotton has played this role with varying success [2].

However, cotton production in the region has a complex history. Kossoumna Liba’a and Havard [2] recognized three stages in the period 1951–2002: (1) 1951–1974, when cotton was an obligatory crop imposed on farmers by the colonial government, (2) 1975–1994, when farmers could not do without cotton, because it was a source of obtaining credit facilities, farm-input subsidies and community infrastructure development; during this period, the area under cotton increased substantially compared with the first period, (3) 1994–2002, when the uncertainty with respect to the future of the cotton industry was high, and the area under cotton decreased and production stagnated. Nevertheless, inorganic fertilizer use in the period when the cotton area increased did not lead to an overall decrease in the presence of Striga. In other words, the affordability of inorganic fertilizers is not enough for cotton growers to tackle the fertility and Striga problem, and more integrated soil fertility management options that include a combination of organic and inorganic fertilizers will be needed [36].

5 Conclusions

Between 1987 and 2005, Striga infestation in northern Cameroon increased despite 20 years of project support, of which the FAO (1987–1990, 1994 and 1995) provided the more Striga focused support. Increased pressure on the land, leading to a reduction in the use of fallow and to a cereal mono-cropping system, is responsible for the Striga situation worsening. For farmers, Striga was not a priority-one-problem because their cereal yields could not be directly related to Striga incidence. Soil fertility management is a crucial entry point. The low use of organic manure and decreased use of inorganic fertilizers after the collapse of the cotton sector indicate that this option may be difficult to attain. Yet, improved management of cropping systems that would lead to a reduction in Striga has to start from those constraints (soil fertility, labour) that farmers identify as priorities.

Acknowledgments

The first author thanks the PRASAC and ARDESAC project, which largely financed the surveys, and the defunct FAO Striga project for permission to use some of the survey data. We are grateful to Dr. Venasius W. Lendzemo and Mr. J.-P. Olina Bassala for their permission to use a small part of the 2005 ARDESAC survey data. We owe Mr. Michel Havard (CIRAD French Technical Cooperation with IRAD) a great debt of gratitude for the information from his work under the PRASAC project.