Dissertation
Climate change has put a strain on water resources in most parts of the world. Water, on
the other hand, is a critical input for agricultural production and plays a critical role in
food security. As a result, the risk of various forms of malnutrition has increased,
potentially leading to under nutrition as well as overweight and obesity, which is likely to
worsen further due to the health and socioeconomic effects of Covid-19. Thus, the
importance of producing food to improve nutrition security is obvious, as it is a promising
approach to combating malnutrition. However, improving food production is faced with
the challenge of water scarcity, especially in sub-Saharan Africa. Methods that enhance
efficiency in water use are promoted to improve food production. Deficit irrigation (DI)
strategies in pressurized irrigation systems are beneficial in reducing applied irrigation
water and nutrient loss through leaching, as well as crop vegetative vigour. This strategy
is useful for dealing with water scarcity and improving water productivity. This study
used field experimentation to assess the effects of deficit irrigation on orange-fleshed
sweet potato storage root yield, water productivity, and nutritional water productivity, as
well as the financial implications in Tanzania. Orange fleshed sweet potato (OFSP)
varieties are documented to offer a balanced nutritional composition, mature early and
can withstand marginal conditions where other crops fail.
To achieve the intended goal, three irrigation rates were used for scheduling: 100 per cent
(Full irrigation (FI)), 60 per cent (60 DI), and 30 per cent (30 DI) of total available water
(TAW) in a factorial arrangement of treatments laid out in a randomized complete block
design (RCBD), with Naspot-13 and Jewel of orange-fleshed sweet potato crop as test
varieties. A 3 x 2 factorial design with three replications was used. When 20 per cent of
the total available water (TAW) was depleted, the FI treatment was irrigated to 100 per
cent field capacity, whereas the 60 DI and 30 DI treatments received 60 per cent and 30
per cent of the amount applied in the FI treatment, respectively. A drip irrigation system
was used to apply irrigation water using a continued deficit irrigation strategy. The
strategy was implemented eight days after the plants were planted. Analysis of Variance
was performed on response variables data and treatment means separated using Tukey’s
multiple range test at p<0.05.
The first goal was to evaluate the yield, water productivity, harvest index, and dry matter
content of OFSP varieties subjected to deficit irrigation. The fresh yields were harvested
120 days after planting then weighed and weight converted to tons per hectare. In this study, only marketable storage roots were considered. Marketable storage roots were
defined as storage roots weighing 100 g or more and free of defects caused by diseases or
mechanical damage. The dry matter content of the test varieties was determined by drying
peeled and sliced OFSP in a solar drier for 48 hours at a temperature of 62.4 O C.
The second goal was to evaluate the nutritional root quality parameters and nutritional
water productivity of OFSP varieties in response to deficit irrigation. To obtain an
analytical sample, four medium-sized orange-fleshed sweet potato varieties were
homogenized in a food processor. The analytical sample was extracted for 1 minute with
methanol: tetrahydrofuran (THF) (1:1) and then filtered into petroleum ether (PE), as
described by Rodriguez-Amaya and Kimura (2004). An Atomic Absorption
Spectrophotometer (ASS) was used to determine the iron and zinc micronutrient content
of a portion of the analytical sample, as described by AOAC (2002).
The third goal was to evaluate the production economics of drip-irrigated OFSP varieties
subjected to deficit irrigation. The benefit-cost ratio (BCR) analysis is a financial measure
of the value of a project. The economic return analyses of the production system of OFSP
varieties under each irrigation regime were calculated using BCR in this study. However,
BCR components including the capital cost of a drip irrigation system and the gross and
net return of each irrigation regime were estimated assuming a field area of one hectare
and flat land.
According to the results of the first objective, the Naspot-13 variety demonstrated a high
yielding ability to water stress, recording 17 t/ha, whereas the Jewel variety recorded 9
t/ha of storage root yield. The Jewel variety produced the most storage root yield, with the
highest at 25 t/ha during the first season, while the Naspot-13 variety performed poorly.
This could be attributed to the rain effect that occurred during the first season. Under
deficit irrigation, the varieties' water productivity (WP) and harvest index (HI) increased.
The varieties' WP ranged from 1.8 to 11.0 kg/m 3 , and their HI ranged from 52 to 58 per
cent. Both varieties' dry matter content ranged from 23 to 31 per cent and was statistically
similar. The fresh above-ground biomass decreased significantly as the amount of water
applied decreased. According to the findings of the study, the Naspot-13 variety can be
strategically used as a feasible crop for producing more food with less water.
In the second objective, it was discovered that the beta-carotene content of the Jewel
cultivar was 27 per cent higher than that of the Naspot-13 cultivar. Iron and zinc
concentrations in both cultivars were found to be sufficient and minimal respectively.
Otherwise, when deficit irrigation was compared to full irrigation, neither cultivar's root quality changed. All the same, a typical serving (125g) of test OFSP varieties provides
more than 100% of the vitamin A, sufficient iron, and minimal zinc dietary requirements
for 4-8-year-old children and 10-50-year-old pregnant women. In general, the nutritional
water productivity of root quality parameters increased as water application decreased.
Finally, a positive water-food-nutrition nexus was established under the Naspot-13
variety, which is capable of alleviating Vitamin A, and iron as well as minimising zinc
deficiency health-related problems, particularly in children and pregnant women living in
areas with limited water resources.
The results of the third objective show that a net positive return using the benefit-cost
ratio (BCR) was observed, with the highest net financial return (6516 US $ per ha)
observed for the Naspot-13 variety at the 60 DI deficit strategy. As a result, for Naspot-13
cultivation using drip irrigation in sandy-clay soils, the 60 DI strategy is a more cost-
effective option than full irrigation. However, in the case of water-limiting situations, the
highest net financial return was observed under Jewel variety at 30DI treatment (8404 US
$).In summary, the findings in this study indicated that the production of OFSP for deficit
irrigation is economically feasible.
The East African Community (EAC) scholarship program, which was funded by KFW
and implemented by the Inter-University Council for East Africa (IUCEA)