Introducing cactus-based agro-forestry practices to the drylands

Introduction

The drylands of Ethiopia, which constitute about 60% of the land, have poor vegetation cover, probably due to continuous cultivation of crops and free grazing. These areas have a high evapo-transpiration that exceeds precipitation. Livelihoods in such environments depend on resilience to cope with uncertainties in rainfall. The identification of suitable plant species that can thrive and produce yields and contribute to farming practices might add to this resilience, since the inability to identify such crops has often led to shortages of food.

One such plant species is the cactus pear, Opuntia ficus-indica (L.) Mill. Cactus pear is a native plant of Mexico but has been introduced to many parts of the drier world. It has tremendous potential of adaptation to drier areas and is consequently often considered as an invasive species. They are frequently mentioned in classical biological control programs (in Australia, South Africa and Madagascar) that involved the introduction of insects to control cactus invasions. These insects are now considered as pests because people have started farming cactus pear, for example in South Africa. Thus there has been a change in the value of cactus plants over time. This was also reflected when the International Biodiversity Day was celebrated on the 22nd of May 2006 that had the protection of biodiversity in drylands as its theme. The logo showed a cactus flower symbolizing courage that triumphs over appalling obstacles as the writer and photographer Randall Henderson said. The year 2006 had also been declared the International Year of Deserts and Desertification by the United Nations.

Fig 1. Mature cactus pear fruits ready for harvesting.

Cactus pear, Opuntia ficus-indica (L.) Mill, has adapted to many parts of Northern Ethiopia. In this area alone, natural cactus covered about 32000 ha of land. Cactus fruits are eaten fresh in the months of July to September (Fig 1). Cladodes (young stem segments) are used as livestock feed and for soil and water conservation purposes. Recently other uses have been introduced, including nopalitos (a vegetable prepared from cladodes), jam and carmine (a red pigment derived from cochineal insects living on the cactus plant). Cactus pear cultivars have total soluble solids (expressed as 0Brix) that reach up to 16.5 (Table 1) which makes it suitable for either sugar extraction or ethanol production, especially in areas where fruits are produced in excess. Currently, cactus is growing in the wild and thus it must be cultivated if the benefits from the plant are to be sustained. One likely option would be the introduction of cactus into farmlands as hedges or intercrops, of course with improved orchard management. This approach could have an additional advantage in mitigating the impacts of climate change. There are however no known agro forestry practices that involve cactus pear in Africa except the long rows of cactus pears planted in vast areas of grasslands in Tunisia, Algeria and in South Africa. In Northern Ethiopia, cactus live fences are also common surrounding farm fields though they are not managed properly.

Table 1. The total soluble solids of fruits of 13 local cultivars of cactus from Northern Ethiopia.

Cactus based agro forestry practices

Mekelle research centre conducted an observation on an orchard of 11 cactus pear cultivars. The spaces in between cactus pear plants were used for production of beans. Five cm deep trenches were dug in between the cactus plants to harvest water for use by both component crops. Reduced tillage was employed when planting beans (Fig 2a). The biomass of cactus cladodes, fruits and yield of haricot bean was then estimated.

Fig. 2. Cactus pear hedge-row intercropped with haricot bean: a. at planting haricot bean, b. bean nearing maturity. (Note rows used for planting beans and shallow trenches made in between the cactus pear plants).

Significant biomass of cactus pear cladodes (914.63 kg) and edible fruits (268.3 kg) was produced in addition to a significantly higher bean yield (1333.3 kg) per hectare (Fig 2b) in 8 months time. Because cacti are perennial plants, they will continue to grow and will yield more fruits and biomass in the following years. Bean plots with no cactus intercrops gave significantly lower yields (700 kg/ha). Intercrops had the additional benefits of trapping moisture in the trenches and this should have contributed to better use of the poor rains of the 2008 rainy season (375mm) in the area. Cactus does have the potential for hedge-row intercropping and the combination helps increase biomass produced per hectare besides increasing the vegetation cover. The latter is relevant to drylands where the land is bare for more than 7 months before the next crop is planted. Cactus-based agro forestry practice can therefore be considered as an adaptation option to climate change in the drylands.