What role can planted fallows play in the humid and sub-humid zone of West and Central Africa?

Abstract

Crop management without fertilizer input, which is commonly practiced by most farmers in the humid forest zone of West and Central Africa, requires soil fertility replenishment during a fallow period. Hypothetical relationships between fallow length and crop yields assume, that after the cropping phase replenishment starts with high annual increments, leading to an early recovery of most soil fertility, then slowly approaching a maximum level. The few available empirical data, however, indicate that this assumption is wrong. Within the first 8 years of fallow, biomass and nutrient accumulation is either progressive (low initial increments) or linear. Planted fallows are supposed to replenish soil fertility faster or to higher levels than natural regrowth and should thus lead to higher crop yields. Two major types of planted fallow are distinguished: tree-based and herbaceous fallows. Data from West and Central Africa do not confirm that tree based fallows are generally capable of attaining higher crop yields than natural regrowth or other planted fallows. The majority of experiments with tree-based fallows showed no differences to the control (60.0%). Crop yield declines were found in 15.7% of cases, and only 24.3% resulted in significant yield increases. Changes in soil properties were more frequently positive (34.3%) than negative (9.8%), yet, most often (55.9%), there was no effect. Herbaceous fallow had dominantly positive effects on crop yields (52.5% of cases), with only 3% of cases in which significant reductions were observed. Positive features of some herbaceous fallows, such as easy establishment, rapid weed suppression, and labor efficient slash-and-burn crop establishment make the technology more likely to be accepted and adopted by farmers. It appears that fallows have to be specifically designed for responsive crops, i.e. maize. It is unlikely that one type of fallow can serve the multitude of crops and intercrops grown in the region. Depending on the major constraints to crop production or income generation, planted fallows have to be specifically designed to address these constraints. This may require de-emphasizing the soil fertility aspect and focusing on marketable fallow by-products, weed and pest suppression and reduced labor requirements. Thus, future impact through research on planted fallows will depend on exact targeting of specific fallow types and species to the most responsive crops and to explicit farmer circumstances.Deceased.

Broussonetia papyrifera (paper mulberry): its growth, yield and potential as a fallow crop in slash-and-burn upland rice system of northern Laos

“…These results are similar to others showing no improvement over a bush fallow of legume tree fallows (Leucaena and pigeon pea) in northern Laos (Roder and Maniphone 1998;Roder et al 1998;Saito et al 2008). A review article on planted tree fallows in West Africa also reported that in 60% of cases, crop yields following tree-based fallows were not different from those of the control (Hauser et al 2006). Good management during the rice growing season involves maintaining a low stem density at less than 0.22 stems m -2 and pruning branches (Fig.…”

“…It is a multipurpose tree with the inner bark used to make paper, the stems for firewood, and the leaves for animal feed. The recent development of domestic processing industries and improved market channels have encouraged farmers to grow paper mulberry as a cash crop (Hamman 2001). …”

Meta-analysis of maize yield response to woody and herbaceous legumes in sub-Saharan Africa

“…In parts of West, Central, East and southern Africa, where the fallow periods have been shortened and cultivation has been extended for more than 2 years, maize yields generally decline rapidly (Drechsel et al 1996;Hauser et al 2006;Mafongoya et al 2006). Although use of inorganic fertilizer can overcome the problem, most smallholder farms use little or no mineral fertilizer (Mwangi 1999).…”

Influence de la composition chimique du sol sur la teneur en éléments nutritifs et le rendement du manioc (<i>Manihot esculenta</i> Crantz, Euphorbiaceae) dans deux zones agro-écologiques du Cameroun

“…L'analyse du sol (Tableau 1) montre d'ailleurs que la matière organique du sol est plus élevée à Ekona qu'à Ongot. Bien que le manioc soit connu pour croître dans les sols très pauvres (Hauser et al, 2006 ;Echebiri et Edaba, 2008), un sol riche en éléments minéraux favorise sa croissance.…”