- Auteur(s) : Roland Dumont, Philippe Letourmy, Amani Michel Kouakou
, CIRAD-CA, DR Système de cultures, BP 5035, 34032 Montpellier cedex, France, CIRAD-CA, UR Biométrie et informatique, BP 5035 34032 Montpellier cedex, France, IDESSA, BP 635, Bouaké, Côte d’Ivoire.
- Page(s) : 107-14
- Année de parution : 1997
An opinion that is widely held by African farmers is that chemical fertilization jeopardizes yam storage, increasing losses due to spoiling. This problem has not yet been seriously addressed, so the IDESSA outpost in Bouaké decided to study it more closely through a series of repeated tests during four successive campaigns from 1985 to 1988. In 1988, the study became more complex by including yams from two different planting periods. Two species that are widely cropped in Côte d’Ivoire were studied simultaneously, Dioscorea alata (cvs Yellow Lisbon and Florido) and D. cayenensis rotundata (cv Krenglé). The yam cropfields were fertilized with mineral manure, i.e. 75 N-54 P2O5-94 K2O/ha, and the yams produced were compared with those obtained without fertilizer. Observations in each case concerned a sample of about 220 yams, which were numbered and weighed individually at the start of the trial. This provided baseline numbers and weights that were used to evaluate subsequent losses. The different samples were observed monthly. In most cases, observations began at the end of January and continued through July. Two categories of losses were evaluated at each control: damage loss, corresponding to the number of altered yams in each sample (qualitative variables), and physiological loss, corresponding to the weight loss of yams that remained undamaged until July (quantitative variables). The types of yam involved in the trial were treated separately, while the two loss categories each underwent a specific statistical analysis. Damage loss results (Tables 1, 2 and 3) were analyzed in two ways. First, losses were assessed after 6 months of storage by taking the effect on individual yam weight into accound. This was done separately for each trial, and then globally for all trials with the same yam species. Second, loss variations over time were analyzed by distinguishing different weight groups. For physiological losses, weight loss patterns were studied. To compare treatments and trials, the linear component for each yam was calculated over time and analyzed in relation to treatment and weight group factors. Mean weight losses were expressed as a percentage of total production. Losses due to damage varied markedly and were often linked with chemical fertilization, but in most cases this connection was only apparent after a long storage period. For physiological losses (Tables 4, 5 and 6), the negative effect of chemical fertilization on storage was more prevalent, and significant in 75% of the trials involved. D. cayenensis rotundata seemed to be considerably more affected, as noted previously in Nigeria and Côte d’Ivoire. High nitrogen input accelerated the metabolic activity of stored yams and increased weight loss . In addition, metabolic degradation of starch was far more rapid in D. cayenensis rotundata due to a two enzyme System (amylase and phosphorylase), as opposed to amylase only in D. alata . Yam unit weight seemed to have a bearing on loss during storage, especially with cv Florido (D. alata), where losses tended to decrease as yam unit weight increased. In 1988 only, the influence of planting date on storage losses was measured. Late planting led to reduced losses during storage. This is in line with Côte d’Ivoire farmers’ technique of planting D. alata late, thus enabling longer produce storage and ensuring food reserves. The overall results highlighted that chemical fertilization may have a negative effect on yam preservation. However, this does not always occur, and the effects are never as catastrophic as farmers daim. Our conclusions show that there is a complex relationship between fertilization and storage capacity. This could explain the contrasting views that have been expressed on this topic based on previous scientific research. We stress the need to conduct further research in this specific field, as mineral fertilizers are essential for sedentary and intensive yam production.