Climate change vagaries have prompted the need for farmers to adopt climate smart crops that help mitigate against the effects of climate change. The false starts to the seasons, recurrence of mid-season droughts, heat waves, floods, persistent rains and the premature termination of the rainfall season over the years, are factors that require farmers to diversify cropping programmes so as to spread risk and increase chances of increased productivity. 

Crop diversification

Diversification promotes farm viability through improving income streams and liquidity. The good recent rainfall activity recorded across the country brought a sigh of relief to farmers across the country. This has enabled farmers to consider growing lucrative crops like sugar bean. 

Sugar bean is one of the crops that is well within its planting season as it can be established from January to mid-February depending on the predicted rainfall forecast of a farming region. Farmers who intend to establish rain-fed crops should constantly refer to seasonal forecasts to enable them to make informed decisions. 

Economic importance of 

the sugar bean crop 

Sugar bean is a crop of economic importance with a potential Return on Investment (ROI) of $2, 50 to $4 for every dollar invested. This makes sugar bean a lucrative cropping venture to consider especially if yield levels are optimised. Sugar beans is a crop with a wide market range making it profitable to grow for commercial use, provided the yield levels are optimised. In addition to this, sugar bean like most legumes is a rich source of protein (15 – 25 percent). This is an important attribute given the growing percentage of health-conscious consumers. The sugar bean crop has significant importance in the drive towards sustainable agriculture because of its nitrogen fixing ability which enhances soil fertility. It is important for farmers to incorporate crop residue after harvesting or to use it as ground cover (mulch) to align with one of the key principles of conservation agriculture of having (30 percent) ground cover.

Sugar bean variety selection 

Sugar bean variety selection as is the case in any profitable cropping venture should be guided by market research, expected yield levels, disease tolerance and growth habit in relation to season length to ensure that productivity is optimised. Farmers are encouraged to grow certified seeds from reputable suppliers. 

Variety preferences include speckled beans like SC Bounty and SC Sharp and SC Bonus indeterminate varieties with a yield potential of one and a half to two and a half tonnes per hectare under good management. Ukulinga and Gadra are the new sugar bean varieties on the market with a yield potential of up to three tonnes per ha under optimum management. Certified sugar bean varieties have a good disease tolerance package which saves on the cost of buying fungicides, thereby increasing profit margins. The days to maturity for sugar beans range between 80 to 110 days, depending on variety, altitude and management.

Agronomic requirements

Profitable sugar bean production hinges on the ability to couple seed selection and Good Agronomic Practices (GAPs). It is important for farmers to observe planting dates so that critical growth stages such as flowering and grain filling do not occur during periods of moisture stress or frost. This is because sugar bean thrives in warm climates with an optimal temperature range of 18-24 degrees Celsius. For rain-fed crops, the ideal amount of rainfall is in the ranges of (450mm to 650mm). 

Sugar bean grows best in well drained sandy loam or clay loam soils. This promotes good seed to soil contact which is key in the achievement of the desired crop stand and plant population. Soil pH level affects crop growth in most cropping programmes. In sugar bean production, the ideal pH level is in the range of 5,5 – 6,5 (Calcium chloride scale). It is important to promote good drainage of excess water in the field as water logging conditions tend to affect germination and effective crop growth.

Planting guidelines

Yield per plant and yield per unit area is a direct function of the plant population. The recommended seed rate for sugar bean production is 80kg to 100kgs/ha. The interrow spacing is (45cm – 50cm) while the inrow spacing is (4cm – 10cm) to achieve a plant population of between 220 000 to 330 000 plants per hectare. Some farmers are establishing at higher rates of about 400 000 plants per hectare to increase productivity. The planting depth (2, 5 – 4cm) is also key in ensuring effective germination and crop establishment. At planting, land preparation should aim to achieve a fine tilth for a good seed to soil contact to enable sugar bean to germinate well above the ground (epigeal germination). 

Agronomic principles

The basal fertiliser recommended for sugar bean is compound D (7:14:7) at a rate of 200 to 300 kg/ha or specialty fertilisers like Compound D or cereal blends at rates of 150 to 200kg/ha. The recommended rate for Ammonium Nitrate (AN) fertiliser ranges between 100kg/ha to 150kg/ha. 

However, some farmers may opt to use rhizobium bacteria to boost nitrogen fixation. When purchasing rhizobium, it is important to specify that the rhizobium bacteria needed is for sugar beans and not for soya beans as they are different. The amount of rhizobium required for sugar bean is about 100g to treat 25kg of sugar bean seed. 

As a result a farmer would need four sachets to treat 100kg of seed enough to plant one hectare. Farmers should mix 100g of rhizobium with 25kg of seed in 250mls of water and add two to three tablespoons of sugar. The use of seed dressing chemicals for the management of fungal diseases is also recommended at planting.

Weed control should be guided by the weed spectrum, time of application and rotation plan. Insect pest control should be done based on effective systematic scouting to determine insect pest levels before economic threshold levels are reached. However a pest of economic importance in sugar bean production is the Bean stem maggot which should be managed effectively within the first month of crop emergence.

Crop husbandry

It is recommended for farmers to apply basal fertilisers when planting sugar beans; quantities applied must however, be guided by soil analysis and previous cropping programmes. Sugar bean, being a leguminous crop, fixes nitrogen in the presence of nitrogen fixing bacteria. This process can be amplified by the application (coating) of an inoculant bacterium (rhizobium) on the seed before planting. 

It is imperative to note that rhizobium meant for soya beans is not interchangeable with that of sugar beans. As a living organism, rhizobium is sensitive to a number of conditions including direct exposure to fertiliser, heat, and large quantities of water. Rhizobium should be kept in a cool environment and direct contact with fertiliser at planting should be avoided. 

Rhizobium inoculant for sugar beans should be applied at a rate of about 100g (one satchet) to treat 25 kg of seed, hence four sachets are required to treat a single hectare. The water required during mixing is 250mls of water and three tablespoons of sugar. If the process of root nodule-colonising is successful, the rhizobium eliminates the need for top dressing with ammonium nitrate, resulting in the reduction of production cost and consequently increase in profit margins. 

If the crop appears pale there might be need to apply ammonium nitrate as it may be a failure of rhizobium to fix nitrogen. It is encouraged for the farmer to contact agronomists for consultancy as this colour may be from other causes rather than just the lack of nitrogen. 

Effective pest control can be achieved if regular scouting is religiously followed so that insect pests are controlled before they reach optimum threshold levels. In sugar bean production the bean stem maggot is a pest of economic importance especially in the first month of production and as such farmers should adequately prepare for it. 

The adult bean stem maggot moth (fly) lays its eggs inside the stem. When the eggs hatch, the larvae start feeding on assimilates generated by the plant and block the vessels, hence cutting water and nutrient supply leading to wilting and eventually death of the plant. 

Farmers should spray recommended insecticides at least four times in the first month of crop emergence for effective control of bean stem maggot. Other pests such as cutworms, aphids, boll worms and leaf hoppers must be controlled before they reach economic threshold levels that can cause significant yield losses. 

Note: Having received significant rains across the country, farmers should establish their sugar bean crop as soon as possible to reduce chances of crop failure. Crop diversification plays a pivotal role in climate smart mitigation initiatives. Remember farming is a potentially lucrative business that requires good genetics and strict adherence to Good Agronomic Practices (GAPs).

◆ Wendy Madzura is head of agronomy for Seed Co Zimbabwe.