Black wattle (Acacia mearnsii) and the fever tree (Acacia xanthophloea) in alleycropping systems.
Alleycropping is an agroforestry technology of planting crops between rows of trees, preferable legumes to promote an interaction among them with positive benefits in terms of improving soil fertility and hence good crop yields. The technology has been tested with a variety of trees/shrubs species in association with crops (alleycropping) or with grasses (alleygrazing), sometimes with encouraging results and sometimes not, in a wide range of environmental conditions around the world. Research in alleycropping started in late 1970s and sinc~ then many publications have been released. However, little or nothing has been reported about this technology using black wattle and the fever tree, two nitrogen-fixing trees common in South Africa and reported as fast-growing species which produce a considerable amount of biomass within a short period of time. Due to that fact, a two-year trial was established in 2003 at the Ukulinga research farm, Pietermaritzburg, South Africa to evaluate the potential of the two tree species under alleycropping with maize and cowpeas as joint intercrops, under alleycropping with pumpkin, and under alleygrazing with Eragrostis curvula and with Panicum maximum. The trial assessed the crop yields and the biomass production from all the components, and their fodder digestibility using Neutral Detergent Fibre (NDF) and Acid Detergent Fibre (ADF) determinations. Additionally the changes in tree grovlth vaa."'i.ables (difu~eter, total height, total \lollhl1e and biomass) were mortitored to produce regression equations to predict those variables, one from another, using regression analysis. The diameter was taken at ground level (dgl) and at the height of 1.3 m, normally called diameter at breast height (dbh). The results showed that tree growth and biomass production were better in black wattle alleycropping than in association with the fever tree. The average dgl of black wattle after 12 months was 48mm and the average dbh was 36mlll. Over the same period the total tree height was about 406cm. A tree pruning was done to one-year old black wattle in the whole trial and the prunings produced about 5.6t/ha of fresh foliage biomass in the association with maize and cowpea and 4.5t/ha in the association with pumpkin. In alleygrazing the growth variables were similar to those obtained in alleycropping but the biomass production was considerably different. The prunings produced about 7.66t/ha of fresh foliage biomass. The dry matter biomass from the prunings was 1.96t/ha, 1.58t/ha and 2.68t/ha in the association with maize and cowpeas, pumpkin and E. curvula respectively. The dry matter was obtained from 4days- oven-dried samples and it was 35% of the fresh foliage biomass and 60% of the fresh woody biomass. The fever tree did not grow significantly during the study period and due to that fact, the species was discarded from the study. Similarly, because after several endeavours using different seed lots, P. maximum had germinated very unevenly, and this grass was excluded from the experimentation. Values ofNDF and ADF less than 35% are considered good, between 35% and 60% fair and poor if greater than 50%. Using is classification the NDF and ADF values from this study were good in pumpkin (30.5% and 29.9%) and cowpeas (36.5% and 46.9%) biomass, fair in E. curvula (41.9% and 39.9%) and maize stover (53.6% and 42.1%) and poor in black wattle (76.58% and 68.1%) foliage. If black wattle is to be used as fodder, it must be mixed with highly digestible fodder like P. maximum, Digitaria sp., and other legume plants, to increase animal intake and to avoid any risk of it becoming an animal hazard due to tannin effects. The regression equations produced linear relationships between dgl and age, and biomass and dbh. The other interactions were not linear. The best equations were obtained in the interaction between dgl and age (dgl= 4.8*Age -7.03; R2 =0.86; SE= 6.6), dgl and height (h= -0.03dg12 + 10.5dgl - 21.25; R2= 0.96, SE= 42.9, h= height), biomass and dbh (lny = 2.409*lndbh; R2=0.99, SE=O.O, Y= tree foliage biomass). During the study, monkeys, cattle, birds and bushbucks posed a threat to the success of the study due to damage they caused to the crops. It was possible to keep the damage below the critical levels, although at high cost.
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