Performance of Two Rape ( Brassica napus ) Cultivars under Different Fertilizer Management Levels in the Smallholder Sector of Zimbabwe

Crop response to fertilizer application depends on the physiological and morphological characteristics of the cultivars, thus causing cultivar differences in growth rate and final yield. A study was carried out in the Zvimba smallholder farming area of Zimbabwe, from November 2016 to February 2017, to investigate the performance of two rape (Brassica napus) cultivars under different nitrogen fertilizer management levels. An experiment was set up in a randomized complete block design, replicated three times, with two rape varieties, Hobson and English Giant, and 5 ammonium nitrate (AN) (34.5% N) fertilizer application rates (200 kg/ha, 300 kg/ha, 350 kg/ha, and 400 kg/ha) as factors. Nitrogen fertilizer rates had a significant (p< 0.05) effect on leaf length at 6 WAP and 7 WAP. Nitrogen fertilizer application levels had significant effects on both in-season and total fresh leaf yield. 'ere were no varietal effects on the leaf length (p> 0.05), in-season yield and total leaf yield (p> 0.05), and there were also no variety ∗ fertilizer application level interaction effects on leaf length, in-season yield and total leaf yield (p> 0.05). From the results of this study, English Giant rape would maintain a longer leaf length than Hobson up to the end of the season, but the two cultivars have similar yield response to fertilizers’ application rate. Both the English Giant rape and Hobson rape cultivars can, therefore, be recommended for production in the Mhondoro smallholder farming area of Zimbabwe and other areas with similar climatic and soil characteristics.


Introduction
e current food production in Southern Africa and specifically in Zimbabwe is inadequate.Zimbabwe is currently importing much of its food, and most of the basic commodities and agriculture produce are imported and yet there are vast tracts of land in farms and small suitable pockets in the smallholder sector where food production can be practiced.ere is need to find new efficient ways to meet these growing food needs without undermining our futures.With the population of 17 million, expected to double in 2050, the demand for food will more than double as a result of urbanization [1].
ere are, however, opportunities in increasing agricultural output and profitability through promoting the use of improved and adaptable hybrids and through the application of site-specific rates of nutrient sources.Such gains are likely to catalyze economic growth and subsequent improvement in livelihoods, food security, and employment and thus, meet the Sustainable Development Goals which aim to end poverty and hunger, among other objectives.
Vegetable production in Zimbabwe is a fast expanding enterprise because of the increasing demand resulting from the rapid increasing urban populations [2].Diversification in crop production by small holder farmers in Southern Africa has the capability of creating employment and increase in income.is, in turn, allows for the purchase of food to increase food security [3].Vegetables provide nutrients such as vitamins, minerals, and roughage which are essential for a balanced diet [4].Brassica napus and other leaf vegetables are a profitable agricultural enterprise in both local and export markets [2,5].
Rape is one of the primary vegetable crops in Zimbabwe, grown for its leaves [4], which are rich in vitamin A, thiamine, and ascorbic acid.Rape seed oil is used for baking bread, illumination, lubrication, and the manufacture of soaps [6].e vegetable is grown all year round in Zimbabwe and is used as a source of income through the selling of the leaf produce [7].
Rape (Brassica napus) has a high demand for water due to extensive leaf area and thus needs regular intervals of irrigation.e critical period for rape is the stem-elongation stage when the crop builds the branching structure and strong stems, then produce high yields [8].Rape is sown for later transplanting at the age of 4-6 weeks after sowing, but the crop can also be grown in situ [6].e crop has a short life span as compared to other leafy vegetables, making it the most preferred leaf vegetable grown by smallholder farmers in Zimbabwe [9].
According to Musara and Chitamba [10], there are many rape cultivars available in Zimbabwe but the most popular ones are English Giant rape and Hobson cultivars.e characteristics of the two cultivars were summarized by Muchecheti et al. [9].e English Giant rape cultivar has dark green leaves and medium branches and has an immense growth rate.e English Giant rape has large broad leaves, with a yield potential of the cultivar of 25-40 tons/ha.e cultivar has large broad leaves and is normally preferred for its hardness.e cultivar cooks quicker than its competitor, Hobson.e English Giant rape germination takes 5-10 days and also takes 90 days to mature [11].
Another variety is Hobson rape, which is an improvement of the English Giant rape variety. is variety can be grown throughout the year and is reported to be a high yielding, with the potential of producing 25-40 tons/ha during its production period [12].e Hobson rape cultivar has a rapid growth and responds well to regular cuttings [13].e cultivar has pale green leaves and is a very palatable vegetable.It is an excellent variety with a wide sowing window.
Both Hobson and English Giant rape varieties are heavy feeders and respond well to applications of farm yard manure or mature compost, which should, however, be supplemented with either a compound or straight fertilizer [6,14].A recommended good practice is to collect soil samples from the land and send for analysis before planting in order to apply the exact amount of fertilizer and manure required by the crop [15].e condition of the crop will dictate whether further top dressings are needed [11].
e production of leaf vegetables all year round requires land use intensification, and the enterprise is only viable and profitable when nutrients from the soils which are depleted during crop production are replenished [16,17].Continuous cropping in most soils in sub-Saharan Africa has led to the deficiency in nitrogen (N), phosphorus (P), or both [18].e negative nutrient balances have been attributed by the removal of crop residues from the fields, coupled with lower rates of macronutrients applications compared to losses [19].
Rape grows on a wide range of soils, provided the inherent drainage and fertility problems associated with clays and sandy soils are alleviated [11,20].e improvement and maintenance of soil fertility is the major constraint to sustainable crop production in the communal smallholder farming areas of Zimbabwe.From a regional assessment carried out in the Chinamhora and Mhondoro communal areas of Zimbabwe, it was revealed that over 40% of the soils had phosphorus deficiency, while 82% had very low nitrogen [21].In addition, communal area fields show wide ranges in the values of soil properties [21], and this coupled with continuous cropping has led to the deficiency in nitrogen (N), phosphorus (P), or both [18].e implications are that sustainable crop yield can only be achieved through determination of site-specific variety and nutrient requirements instead of the generalized crop and fertilizer recommendations, an approach that is currently used in Zimbabwe.
e rate of fertilizer use by famers generally depends on various factors, including arable land size, household income, and distance to input purchasing point, fertilizer price, and transport cost [22,23].Findings by Murwira [24] were that smallholder communal farmers in Zimbabwe applied an average of 18 kg/ha of Compound D (8:14:7) fertilizer, which was below the recommended rate of 300 kg/ ha.Such application of lower than the required amount of fertilizer by the smallholder farmers could be attributable to the blanket recommendations that made by research and extension services on fertilizer application rates [25].e cost of fertilizer is prohibitively high, difficult to transport, and often in short supply, forcing many farmers to apply less than the recommended rates which eventually leads to low yields [26].Smallholder farmers usually apply these nutrients, not at site-specific rates but using the general recommendations from agronomic guidelines [25,27,28].Most countries in the region's mineral recommendation rates are geared towards large-scale commercial farmers and thus overlooked the spectrum of farming objectives and returns and investment that typifies smallholder farming systems [27].Such imperfect fertilizer management has always resulted in the inconsistent and inappropriate application of fertilizer in agricultural production, with consequent environment risks [29].
e generally recommended fertilizer application rates for both English Giant rape and Hobson varieties 600 kg/ha Compound C fertilizer (6N 15P 2 O 5 12 K 2 O) applied as basal before or during transplanting.Ammonium nitrate (AN) is applied as a topdressing at the rate of 200 kg/ha (NTS, 2010).Since the crop is harvested continuously, topdressing is usually split with the first application at three weeks after transplanting and every three weeks, thereafter, as long as picking is continuous [30].
If appropriate fertilizer inputs are combined with suitable varieties, larger increases in crop yields could be achieved on a sustainable basis.It is hypothesized in this study that the two rape varieties, English Giant and Hobson, have similar growth and yield response to varying AN fertilizer application levels.e results of the experiment are useful in minimizing the cost of fertilizers through 2 International Journal of Agronomy application of site-specific requirements, instead of using the general recommendation in the general rape agronomy guide.

Description of the Study Area.
e experiment was carried out from November 2016 to February 2017 in a communal area, in Ward 4 of Chegutu District in the Mashonaland West Province of Zimbabwe, where the main economic activity is agriculture.Ward 4 of Chegutu District is located 95 km southwest of the city of Harare, about 60 km southeast of Chegutu, at an altitude of 11280 m [31].
e experimental site lies in the natural Agro-Ecological Region 2b of Zimbabwe, where the climate is characterized by an annual rainfall that ranges from 750 to 1000 mm in the months of November to March and a mean annual rainfall of 819 mm [31].e study area has unreliable rainfall pattern which is not evenly distributed and hence is characterized by seasonal dry spells.
e area records minimum and maximum temperatures for the site area at 2 °C and 32 °C, respectively.e coldest and the hottest months for the area are July and October, respectively [32].e main agricultural system in the research area is mainly subsistence farming [33].Local people rely mainly on agriculture to make their living, cultivating subsistence crops chiefly maize, groundnuts, pulses, tobacco, and horticultural crops and keeping cattle.
e soils are mainly granitic sandy soils.e predominant soils in the research area are granitic sand soils which are high in quartz and are light colored.e soils have a low inherent fertility.
e soils have low available water-holding capacity and poor structure; this makes them to have high leaching losses as when compared to other soils [33].

Experimental Design.
A two-factor experiment was set up in a randomized complete block design and was replicated 3 times.Factor one was rape variety (English Giant and Hobson varieties), and the second factor was ammonium nitrate (34.5% N) fertilizer application rate (200 kg/ha, 300 kg/ha, 350 kg/ha, and 400 kg/ha).Each plot consisted of three 1 by 10 m beds (30 m 2 ), and experimental assessments were done in the central bed (10 m 2 ).
e experimental treatments were allocated randomly to the treatment plots.
e seedlings for the experiment were raised from the seed that was purchased from the National Tested Seeds, a company that sells seed for horticultural crops in Zimbabwe.Basal compound (NPK) and topdressing fertilizer as well as lime were applied in accordance with the recommendations made when the soil was sent for analysis (Figure 1).e basal fertilizer used in the experiment was Compound C (6% N; 15% P; 12% K).
e Compound C fertilizer was applied during transplanting at the rate of 600 kg/ha.Ammonium nitrate (AN) (34.5% N) was used as a nitrogen source, and it was applied as a top dressing.Ammonium nitrate fertilizer was applied in two equal parts, with the first application done at 2 weeks after transplanting, while the second was done at 5 weeks after transplanting.e standard requirement was 200 kg AN/ha, as recommended by the result of soil analysis (Figure 1).e experimental plots were hand weeded regularly.Insect pests were controlled to minimize economic losses.During the course of the experiment, the plots were watered 2 times a week at the rate of 30 mm, and this depended on the crop requirements and weather conditions.

Harvesting.
Harvesting commenced on the first week after the second topdressing application.Two parameters, the longest leaf length and the fresh leaf mass, were measured at 5, 7, 9, and 11 weeks after transplanting (WAP).For each treatment, data were collected from 10 plants systematically selected from the central bed and tagged for repeated measurements.At every harvesting period, the longest leaf was measured, and the harvested leaves from the sampled 10 plants were weighed on a digital scale.

Data Analysis.
e statistical package, GenStat Discovery 14th Edition, was used to analyze the data.All data were subjected to an analysis of variance (ANOVA), and treatment means were separated using the least significance difference (LSD) test at p < 0.05.

Results
ere were significant (p < 0.05) AN fertilizer application rate effects on the longest leaf length at 5, 7, 9, and 11 weeks after transplanting (Table 1).All the tested AN application rates (300 kg/ha, 350 kg/ha, and 400 kg/ha) had a significant greater (p < 0.05) longest leaf length than the standard (200 kg/ha) at all the assessment times (5, 7, and 9 WAP).Generally, longest leaf dimensions increased with the increasing levels of AN fertilizer from 200 to 400 kg/ha.ere were no significant (p > 0.05) longest leaf length differences between the 350 kg/ha and 400 kg/ha treated plants at 5, 7, and 11 WAP.e two rape varieties, Hobson and English Giant, had no significant (p > 0.05) effect on the longest leaf length differences at 7 and 9 WAP.e English Giant rape had a significantly higher (p < 0.05) longest leaf at 9 and 11 WAP.ere were no significant variety × fertilizer application level interaction effects (p > 0.05) on the longest leaf length at all the assessment times (5,7,9,and 11 WAP).
e AN fertilizer application rate effects on the rape fresh leaf yield were significant (p < 0.05) at all the assessment times (5, 7, 9, and 11 WAP) (Table 2).All the tested AN application levels (300 kg/ha, 350 kg/ha, and 400 kg/ha) had a significantly greater (p < 0.05) in-season fresh yield than standard treatment (200 kg/ha) at all the assessment times (5, 7, 9, and 11 WAP).Generally, in-season fresh leaf yield increased with the increasing levels of fertilizer from 200 to 400 kg AN/ha.In-season fresh leaf yield for 300 kg and 350 kg/ ha AN-treated plants was statistically similar at 5, 7, and 9 WAP, but lower than that from the 400 kg/ha AN-treated plants.e 200 kg/ha, 300 kg/ha, 350 kg/ha, and 400 kg/ha AN/ha treated International Journal of Agronomy plants had statistically similar in-season fresh leaf yield at 11 WAP.
ere were no signi cant (p > 0.05) in-season fresh leaf mass di erences between the two rape varieties, Hobson and English Giant, and there were also no signi cance (p > 0.05) on the variety x fertilizer level interaction e ect on in-season fresh leaf yield all the assessment times (5, 7, 9, and 11 WAP).
ere was a signi cance (p < 0.05) of AN fertilizer application rate e ects on the total rape fresh leaf yield (Figure 2).All the AN application rates (200 kg/ha, 300 kg/ha, 350 kg/ha, and 400 kg/ha) had a signi cant greater (p < 0.05) total fresh yield than the negative control treatment (0 g Nm −2 ).Total fresh leaf yield for 300 kg/ha and 350 kg/ha AN-treated plants had statistically similar fresh leaf yield with the standard (200 kg/ha) (Lsd 2.643), while fresh leaf yield from the 400 kg/ha AN-treated plants was signi cantly greater than all the tested AN fertilizer rates (Lsd 2.646).
ere were no signi cant (p > 0.05) total fresh leaf mass di erences between the two rape varieties, Hobson and English Giant, and there were also no variety x fertilizer level interaction e ects on total fresh leaf yield.

Discussion
Rape growth increased as levels of increased nitrogen concentration was increasing with the increase in the nitrogen rate from 0 to 400 kg/ha in spite of the time of application showing that there are additional yield bene ts that could be realized if higher levels of N are applied to both varieties.According to Wang and Li [34], vegetable yields are increased continuously to a certain threshold rate, and any excess input of fertilizer more or less reduced plant growth, leading to yield decline harvests.As argued by Kant et al. [35], the increases in rape leaf yield with nitrogen availability showed that nitrogen favoured more vegetative growth producing plants with a higher leaf count and subsequently higher total green leaf area per plant.Low nitrogen fertilizer of ammonium nitrate treatments might have negatively a ected the photosynthesis process leading to reduced growth rate and consequently yield [36], while high rate of nitrogen increasing leaf area development increases overall crop assimilation [37].while high rate of nitrogen increasing leaf area development increases overall crop assimilation [38] According to Öztürk [38], International Journal of Agronomy ammonium nitrate at optimal rates of 100 to 150 kg/ha was more appropriate on rape than comparative rates using ammonium sulphate and urea, thus highlighting the practical importance of adequate fertilizer application and even suitable source in both seed and leaf yield.However, the accumulation of nitrates is one of the major disadvantages of high-level fertilizer application in rape [39].However, NO 3 -N concentration increased with supply under drought but was una ected by supply under irrigation [40], making the practice of optimum water application a necessity.e Giant rape and Hobson rape had similar leaf length response to AN application rate under the subtropical Zvimba District conditions.As argued by Bacha et al. [41], the genotype × environment × E interaction e ects make it di cult to select the best performing as well as the most stable genotypes, and so, its e cient interpretation is an important issue in plant improvement.is similarity of the two rape cultivars to application rate could be an indication of similarity in the breeding history, may be with a common parentage.
e general decline trend in rape leaf yield in all treatments from the rst to the last harvest may be attributed by changes in B napus developmental stages and declining in nutrient supply [4].
e general yield potential of both varieties is 24-40 tons/ha (National Tested Seeds, Zimbabwe).However, the average yield produced from the research was 19.3 tons/ha far below the expected yield probably to the excessiveness of 920 mm of rainfall and the above 22 °C received during the nine weeks of the experiment.e excessive rains received could have caused the leaching of nitrogen fertilizers; as a result, this reduced the yield of rape [42].

Conclusion.
e results showed that the highest vegetative growth rates, as measured by the longest leaf, as well as the highest fresh leaf yield were obtained from the 400 kg/ha AN-treated plots. is drives home the point that the higher growth rate and yield are obtained when nitrogen fertilizer is applied in large quantities.Also, the results showed no cultivar e ect on growth and yield of rape vegetable.However, English Giant rape growth rate started on higher threshold levels and then subsequently declined as the harvest progressed, whereas Hobson rape showed the opposite results, it started at low levels and then increased as the harvesting progressed.

Recommendations.
It is recommended that farmers should be encouraged to grow either of the two varieties as they showed that they have no e ect on the growth rate and yield of rape.Both varieties indicated that they perform very well under high nitrogen levels.It also recommended that further research should be done to ascertain the hazards of nitrates in vegetables to human health and their e ect to the environment.Also, another research to assess the chemical content and the taste of rape vegetables after application of higher nitrogen fertilizer levels needs to be investigated.

Figure 1 :
Figure 1: Site-speci c results of soil analysis.

Figure 2 :
Figure 2: Average yield (t/ha) for the di erent fertilizer application levels.

Table 1 :
E ect of variety and AN fertilizer application rate on rape longest leaf lengths at 6, 7, 8, and 9 weeks after transplanting.