INTRODUCTION

Cabbage (Brassica oleraceae var. Capitata L.) is a leafy vegetable belonging to the Brassicaceae family. It is among the vegetable crops widely grown by smallholder farmers as a food source and income generation (FAO, 1988). Cabbage production ranks the first among vegetables produced for increased returns to farmers. The production costs per head are generally low, allowing farmers a better gross margin over a relatively shorter period compared to the maize.

Cabbage head presents numerous health properties which includes high water content (92.8%), protein (1.4 mg), calcium (55.0 mg), and iron (0.8 mg); the leaves are eaten raw in salads or cooked (Adenji et al., 2010; Meena et al., 2010;Hasan and Solaiman, 2012). Cabbage is also known for its anti-inflammatory properties (Caunii et al., 2010). The red cabbage contains some oxidative compounds potentially beneficial in prevention of cancer (Kusznierewicz et al., 2008;Tendaj and Sawicki, 2012).

Cabbage varieties (cultivars) are generally classified according to season of maturity, leaf surface (smooth, savaged, or wrinkled), head shape (flattened, round, or pointed), and color (green or red). Varieties differ in their resistance to disease and in the tendency for heads to crack or split in the field. In Zimbabwe, poor yielding varieties coupled with inappropriate use of fertilizer limit production across many farming systems. Cabbage seed in Zimbabwe is imported and breeding programs are limited for vegetable crops. To sustain productivity and improve profitability, new and improved varieties with high genetic gains are required (Tanaka and Niikura, 2006). Important horticultural characteristics in cabbage are head size, shape, and firmness, taste, resistance to bolting, late flowering, and maturity.

Cabbage is a cool-season crop which grows best under cool, moist weather conditions (Thompson, 2002). It has not yet been studied whether Korean cabbage varieties can be adapted successfully in tropical conditions like Zimbabwe’s environments. Evaluating Korean cabbage varieties for adaptation to tropical conditions and agronomic performance relative to horticultural characteristics will help farmers, breeders, and seed companies in selecting and developing varieties for the local environment and markets.

The objective of the study is to evaluate the adaptation of the Korean cabbage varieties relative to horticultural characteristics and yield performance in order to select promising varieties for commercialization and possibly breeding lines to widen the scope of availability and enhance genetic gains and farm profitability.

MATERIALS AND METHODS

General information

The experiment was conducted at SIRDC farm between July and December 2017. The soil type is a red clay, namely a fersiallitic soil as documented in Nyamapfene (1992). Two Korean cabbage varieties ‘K8’ (white) and ‘K5’ (red) were evaluated in comparison with ‘Fabiola’ hybrid being an industry standard preferred for its high yield, earliness, and tenderness.

Experimental designs

A randomized complete block design with five replications was adopted for the study. The varieties were sown in 200-cell trays on 4 July 2017 and grown in floating beds in the green house. The seeds were fertilized with Kutsaga float-fert (20:10:20 NPK) soluble mix. On 19 August, following the hardening for 2 weeks, the seedlings were transplanted to open-field plots in raised beds and planted in double rows. At the time of transplanting the seedlings were 12 cm in height with 3 to 5 leaves. The inter row spacing was 30 cm and the distance between rows was 60 cm. A 50 cm path separated the double rows. Each cabbage variety treatment occupied a plot size was 4.90m × 1.80 m = 8.82 m².

There were 4 double rows in each plot (Fig. 1). The total number of plants per plot were 40. The two outer rows of each plot were treated as border while the remaining middle rows were regarded as experimental rows (20 plants). For the varieties to fully express their genetic potential in terms of growth and yield, the study adopted good farming practices which included a soil test based chemical fertiliser recommendation package to provide balanced nutrients for improving crop physiological development. This was a departure from the blanket fertiliser recommendation commonly used by smallholder farmers which is based on practical considerations for extension to communicate uniform messages rather than based on site-specific needs or the socio-economic circumstances of poorly resourced farmers. Blanket fertiliser recommendations ignores spatial variability of soils even in the same agroecological domain due to difference in soil fertility management. Consequently, all plots received a recommended fertilizer package comprising a basal fertilizer of ZFC vegetable blend (9:24:20 NPK + 9% S + 1% Zn, and 0.15% Boron) of 500 kg per hectare at planting. This was applied on-station in the planting hole using fertilizer cup number 22 delivering an equivalent of 500 kg/ha in one application plus nitrogenous fertilizer in the form of ammonium nitrate at 100 kg/ha applied in 2 splits at 3 weeks and 6 weeks after transplanting. The topdressing was applied using fertilizer cup number 8 and applied 3 cm from the plant. Drip irrigation supplied water throughout the growing season. Plant protection measures were taken against aphids and diamond back moth (DBM) pests upon manifestation. Dimethoate and Dichlorvos pesticides were alternated to control aphids every 14 days. A systemic pesticide, namely Belt was used to manage DBM and applied as a full cover spray at a rate of 4 ml in 15-liter knapsack. The application rates used for the pesticides were based on manufacturer’s recommendations (Dimethoate at 75 ml in 100liters of water and Dichlorvos at a ratio of 1:1 (pesticide:water).

Harvest evaluation

For each variety, 20 cabbage heads were harvested from the two middle double rows of each plot. They were evaluated for head fresh weight, head width (diameter), and core length. Outer leaves were stripped from the head and the weights recorded. Finally, each head was cut in half to measure head width and core length, and pest damage (diamond back moth) severity was rated on a scale. Head width (diameter) was measured using a ruler across the horizontal section of the cabbage. Head length was measured longitudinally from the top of the cabbage to the bottom like Fig. 2. (Richardson, 2013). These measurements were taken from 20 heads and the average recorded.

Marketable yield was estimated from the yield per 20 heads which was corrected for the percentages of unmarketable rots, multiple head, off-types, and other large splits. Maturity date was selected based on marketable yield, number of splits and rots in the field and core splits and visual observation.

Statistical analysis

All data were analyzed by various techniques using GENSTAT statistical package.

RESULTS AND DISCUSSION

Soil Chemical Properties

The properties of the soil were analyzed as follows: pH 5.7 (CaCl₂), organic carbon 3.45 %, and mineral nitrogen 26 mg/kg. The exchangeable cations of Ca, Mg, K, and Na were 14.82, 9.66, 1.14, and 0.23 me/100 g, respectively.

Characteristics of cabbage head among varieties

Short days to maturity or earliness in cabbage is a desirable characteristic to meet early market demands. In order to address market demands, head shape, size, and density must correlate with earliness of head formation. Conversely, early maturing varieties are liable to bolt in the field and yield loss could be higher if market demand falls.

The obtained results suggest significant heterogeneity in the varieties as they differ in their development in the head formation and maturity (Table 1). The Fabiola developed mature heads during shorter period than the rest of the varieties. The actual days to maturity from transplanted seedlings were ranged between 85-90 days. However, K8 matured at 100-120 days and K5 at 120-140 days are categorized as mid-season to late maturing varieties. Results possibly suggested that Fabiola was bred to produce mature heads very early in the season enabling early harvesting shortly after the season begins. Further research will be required to validate findings across all seasons to determine appropriate planting time for optimum maturity dates and corresponding economic yields of quality.

Head core is important because it influences the amount of usable cabbage. It was noted that head size and shape can be affected by agronomic practices (Cervenski et al., 2011;2012). The K8 had bigger head weight than the Fabiola and K5, respectively (P<0.05). Interestingly, the head weight of K5 was 2 or 3 times less compared to the both varieties. The head width and length were variable, ranged from 12 to 18 cm. The core width of K8 was statistically different from that of the K5 (P<0.05), but not from the Fabiola. But the core length of K8 was significantly different compared to both Fabiola and K5, respectively (P<0.05).

Although, the K5 looks significantly lighter and smaller in size, but market analysis of similar varieties to K5 being sold in local retail shops such as Food Lovers, Pick and Pay, and OK Mart suggest that the reddish/purple types are generally of small size and are consumed as salads and cooked for health requirements especially minimizing effects of cancer (Kusznierewicz et al., 2008;Tendaj and Sawicki, 2012). It can be suggested that farmers could choose cabbage varieties for their purposes.

Post-harvest characteristics of the cabbage varieties

Post-harvest characteristics of the cabbages were summarized in Table 2. The internal color of the 3 cabbage varieties are as follows: white in Fabiola, reddish purple in K5, yellowish in K8, respectively.

Loose heads present an undesirable horticultural characteristic and is associated with a high incidence of cabbage head caterpillar, precisely diamond back moth (DBM), which occurs if cabbage is attacked at an early growth stage. In the present study, the K5 displayed low incidence of loose heads and also performed better for tolerance to bolting when compared with the Fabiola. The order of bolting in ascending order was K5 > Fabiola > K8. High incidence of bolting is associated with earliness and poor field storability, hence timing of planting responding to market demand will be required to reduce losses prior to harvest.

Growth and shape of cabbage head

Fabiola was the shortest variety with an average height of 22 cm, whilst K8 was at 27 cm tall (Fig. 3). The K5 was significantly different in plant height of 35 cm from the Fabiola (P<0.05).

Head shape is quantified by a difference between width and length of head. Positive values indicate that the heads are wider than tall. According to our data obtained, the shape was round in Fabiola, oval to round in K5, and slightly flattened to round in K8, respectively (Fig. 4). The closer the value is to zero (negative / positive) indicates the more perfectly round the head is. This was turned out to be true for the Fabiola and to some extent for K8.

Damage characteristics of the cabbage varieties

The three varieties presented distinct characteristics (Fig. 5). K5 showed the longest wrapper leaves, followed by K8. The K8 and Fabiola presented a uniformity in size; in terms of uniformity in size, Fabiola was the best in the trial, followed by K8. K5 had the poorest uniformity. K5 was the only variety in the trial that had no thrips feeding injury. K8 had the highest thrips damage injury depth of 3.8, severity of 6.3. Fabiola had notably above average depth of thrips damage, but below average severity of damage. K8 had 8.5% incidence of feeding damage in the heads at harvest while Fabiola and K5 had 5.7% and 2% incidence of feeding damage, respectively (data not shown).

Estimated marketable yield

Cabbage yield and yield characteristics are determined by several factors, such as variety, plant spacing, environmental conditions, and market requirements, among others. Marketable yield was estimated from the total yield per 10 heads which was corrected for the percentages of unmarketable rots, multiple heads, and other unmarketable off-types and large head splits but not for small splits or sunscald (top wrapper leaf breakdown). The K8 variety had higher estimated marketable yield at 35t/ha on November 14 comparable to Fabiola (25t/ha) on October 16 and to K5 (18t/ha) (Fig. 6).

On the other hands, the percentage of unmarketable yield includes rots, large splits, and multiple heads and other unmarketable off-types. Unmarketable heads occurred mostly due to splits. K8 had unmarketable yield of 9.5% and this was mostly due to large splits (7.9%) and rots (1.8%). The unmarketable heads increased dramatically after 118 days from sowing. Fabiola had low incidence of unmarketable heads (7.1%) largely due to splits at day 95 after sowing and 1.2% rots and splits 5.9% (data not shown). Unmarketable heads present a challenge to the grower which impinges on and robs potential profits. Splitting and bolting have been reported in other studies reducing and affecting cost best ratio and consequently investment returns. The unmarketable heads increased dramatically after 118 days from sowing. Fabiola had low incidence of unmarketable heads (7.1%) largely due to splits (5.9%) at day 95 after sowing and rots (1.2%) (data not shown). Unmarketable heads present a challenge to the growers which impinges on and robs potential profits. Splitting and bolting have been reported in other studies reducing and affecting benefit-cost ratio and consequently investment returns. To minimize deterioration of cabbage value due to splitting, growers must secure markets prior to cultivation and ensure guaranteed markets perhaps through contract farming and also engage in low cost post-harvest processing to add value. K5 was the only variety in the trial which did not have splits. However, off-types amounted to 13% of unmarketable yields and no rots were observed. The high incidence of off-types suggests the need to determine the optimum planting time through assessment.

Overall, the yields from the three improved varieties were superior to cabbage grown under smallholder farming systems. The head weights and other yield characteristics of the varieties in this study are consistent with Greenland et al., (2000) and Adeniji et al., (2010) who determined that cabbages for the fresh market must have high head densities, small to medium-sized heads, and good appearance. The results also suggest that K8’s outcome is competing with 2010 global yield estimates of 27.8 t/ha (FAOSTAT, 2010). This reflects the potential of this variety to sustainably improve yields and/or augment breeding programs in developing countries particularly in the wider Southern Africa where cabbage hybrid seed is imported and sold at unaffordable prices for poor farmers.

CONCLUSION

Sufficient genetic variation exists among the varieties. The study has shown that the two Korean varieties (K8 and K5) can adapt satisfactorily to Zimbabwean and the tropical conditions. This determination is based on market requirements which has a high demand for both red and white cabbage types. The K8 presents an added advantage with its superior head weight, head width, head length and best yield potential. In general, the three varieties displayed good to excellent quality characteristics. To reduce the risk of splits, earlier harvesting is recommended consequently minimizing exposure to insect pest and disease problems. Some niche markets favor smaller and larger sized cabbages and farmers may select from any of these three varieties. This implies that the two Korean varieties have a potential to be alternative varieties to existing industry standard “Fabiola”.

적 요

양배추는 짐바브웨 소규모 농가의 음식, 영양 및 소득원으 로서 가장 중요한 엽채류 중의 한가지이다. 본 연구에서는 2 종의 한국산 양배추 품종(K5 적보라색과 K8 녹색)이 짐바브 웨의 열대성 기후에 적응이 가능한지 확인하고자 현지 생산 품종인 Fabiola과 생산성(yield), 조숙성(earliness) 및 연도 (tenderness) 등의 특성 비교 실험을 실시하였다. 본 실험은 2017년 짐바브웨 과학산업연구개발센터(SIRDC, Scientific Industrial Research and Development Center)의 시험포장에서 격리 상태로 5반복 실험으로 진행되었다.

이상의 결과를 종합해보면 K8과 K5 두 한국 양배추 품종 의 생산성, 재배기간 및 품질 특성 등을 현지 품종인 Fabiola 와 비교한 결과 열대성 기후인 짐바브웨에서도 재배가 가능하 며 상품성 또한 가지고 있다고 사료된다.