Agricultural potential of sweet potato (Ipomoea batatas, L. (Lam)) for forage production

Vo Lam

Angiang University, Faculty of Agriculture and Natural Resources, Vietnam
vlam@agu.edu.vn

Abstract

Three varieties of sweet potato, Trang giay, Duyen ngoc and Hshinchu, were grown for forage production with harvests at 15 to 20 day intervals during a period of 125 days. Two kinds of fertiliser were used, chemical fertiliser (160 kg N, 60 kg P and 30 kg K per ha), and goat manure with a similar amount of N. The control treatment was without fertiliser.

Dry matter (DM) yield of sweet potato vines was similar for both sources of fertilizer and was some 30% better than in the control with no fertilizer. The highest DM content was in the control treatment with no fertiliser applied and the lowest in the treatment with goat manure. The sweet potato vines fertilised with chemical fertiliser had the highest CP content. Ash content of sweet potato vines fertilised with goat manure was higher than on the two other treatments. Sweet potato for forage production should be replanted after four months and/or be given fertilizer to prolong the harvesting period.

Key words: Duyen ngoc, goat manure, Hshinchu, Trang giay

Introduction

Sweet potatoes are grown throughout the tropics for their edible tubers, which are ·an important source of food in many developing countries for both humans and livestock. In the Mekong River Delta region, sweet potato is well adapted to all the agro-ecological zones and has been grown for a long time in the area (Kim 2000). Most of the sweet potato is grown in the rice based cropping systems, which are generally practised in areas characterised by fertile and sandy soils and can thus produce a considerable amount of tubers (Binh and Toca 1997).

The aerial part of sweet potato, the vines, may be utilised as an animal feed in smallholder animal production systems (Phuc 2000). The productive potential of different varieties of sweet potato can vary from 4.3 to 6.0 tonnes dry matter (DM) of vines per ha and crop (Dominguez 1992). When sweet potato was planted as a perennial crop and the vines were cut at intervals of 15 to 20 days large amounts of green feed for animals were produced (Son 1992). Recent research on the expanded use of sweet potato vines as a forage/feed for ruminants has shown that there is a good potential, both for agro-biological and economical reasons. The most important constraint for using sweet potato vines is that the largest amount of sweet potato vines is available during a short period of time from seasonal cropping of the tubers. Ruiz et al (1981) showed that some varieties of sweet potato can be grown two or three times per year and yield up to 125 tons of fresh biomass per ha, out of which 64% constitutes the aerial part, and can be used as forage. Binh (1997) reported that almost all farmers used chemical fertiliser for their sweet potato in the Mekong River region and fertiliser rates per ha were relatively high (250 kg urea, 250 kg P, and 50 kg K). Animal manure was not commonly used in the area (Binh and Tacao 1997). However, the use of goat manure for tuber crops has been reported to increase DM production and improve soil fertility and water holding capacity. Goat manure can substitute part of the N, P, and K fertiliser (Duarsa et al 1996). However, little information is available on the effect of goat manure on forage yield from sweet potato. The objective of the experiment was to determine the aerial biomass production of different varieties of sweet potato and the effect of goat manure in comparison to chemical fertiliser on forage yield in the Mekong Delta of Vietnam.

Materials and Methods

Location

The experiment was conducted from May to October, 2002 at the Research farm of Can Tho University, Can Tho city, Vietnam.

Experimental design

The experiment was a 3 x 3 factorial experiment with four replicates (four blocks) in a Randomised Complete Block Design. The treatments were:

Variety:
Trang giay
Duyen ngoc
Hshinchu

Fertilizer:
Chemical NPK
Goat manure
None

The rate of fertiliser application was 60 kg N, 60 kg P and 30 kg K per ha for chemical fertiliser and 60 kg N for goat manure. The control treatment was without fertiliser.

The selected land was demarcated by thirty-six equal subplots, 5.6m wide and 2.6m long in size and nine subplots were grouped into a block.

Plant material, planting and management

Of the three varieties of sweet potato used in the experiment, two are typical for tuber production: Trang giay (trifoliate, 3 points/leaf) and Duyen Ngoc (palmate, 5 points/leaf). The third variety (Hshinchu) had entire, 1 point/leaf and big area leaf. Sweet potato stems were brought from farmers' fields and home gardens in the area and the stem cuttings were used for planting. The selected stems were of similar size in diameter and number of exposed nodes.

Two raised beds at the same site on arable land in the experimental farm were selected to conduct the experiment. The size of each bed was 6 m wide and 30 m long with a relatively uniform vegetation cover and even surface. To determine the homogeneity of the soil, the soil was sampled before the start of the experiment and at the last harvest. For the sampling before the start six sites within the two selected beds were randomly sampled. For the sampling at the end of the experiment, 10 points at 0- 30 cm of depth within a subplot were taken and pooled to a general sample.

Land for the trial was cleared and the soil prepared to clods of 3-5 cm in diameter before demarcating and planting . After land preparation, low-lying beds, 30 cm high, 5.6 m wide and 2.6 m long, were raised. Stem cuttings with a length of 30 cm (5 to 8 nodes) were used as planting material and were placed in rows after each other with a space of 35 cm between rows. The cuttings were covered with soil between the nodes, which were at a distance of 6 cm for Hshinchu variety and 4 cm for Trang giay and Duyen ngoc varieties.

The plants were watered two times per day in the dry season, and weeding was done manually every week and pest prevention was done when the pest was evident.

Data collection and chemical analysis

The first young shoots of the sweet potato vines were cut at 20 days, and the first harvest was performed at 45 days. The following harvests were done at 65, 80, 95 and 110 days after planting, respectively, and the last harvest was at 125 days after planting. One row around the subplots was left when harvesting and all harvesting was done during dry days. Incidences of diseases and pests were monitored during the period of the experiment.

The soil samples were analysed at the laboratory of the Soil Science Department, College of Agriculture, Cantho University. Total N, P, pH, organic matter (OM) and bulk density were analysed according to the Walkley Black procedure (Walkley et al, cited by Begheijn 1980). Exchangeable K was analysed according to Houba et al. (1995).

Fresh weight of the sweet potato vines of each harvest was recorded immediately after cutting and 2 kg per subplot was randomly collected for chemical analysis. The samples of sweet potato vines were analysed for DM, ash, crude protein (CP), neutral detergent fibre (NDF) and acid detergent fibre (ADF). The DM, CP and ash were determined according to AOAC (1990). CP content was analysed by the Kjeldahl method as N * 6.25 and ash was assayed by incinerating samples at 600°C. The contents of NDF and ADF were determined according to Van Soest and Robertson (1985).

Statistical analysis

The data were analysed by an analysis of variance using the General Linear Model (GLM) procedure of Minitab Statistical Software Release version 12.21 (Minitab 1998). When the differences between treatment means were significant at the probability level of P<0.05, the means were compared using Tukey's pair wise comparison test. The yields of DM and CP in kg per ha are the sum of all harvests during the period of the experiment. The statistical model used was Yijk = µ+ bi + Fi +VJ< + FVik + eii where Yijk = observed yield for treatment j in block i, µ = overall mean, bi = mean yield for block i, Fi =fertiliser effect on yield for treatment j, Vk = variety effect on yield for treatment k; FV.ik = interaction between fertiliser and variety, and eii =represents the random unit variation within a block.

Results and Discussion

The total N, P and exchangeable K contents of the soil were 0.20, and 0.27% and 0.32Meq/100mg , respectively (Table 1). The OM content in the soil was relatively high.

The nutrient content of goat manure showed a large variation between samples taken during the experiment. The content of total N was low (2.20% to 3.83% in DM) while P and K content were quite high, 1.4 and 1.3%, respectively.

The data in Table 1 show the homogeneity of soil in the experiment. This soil is characteristic for most of the arable land of the Mekong Delta region. Tiem et al (2000) reported that most arable soils along the Tien and Hau rivers contained more than 0.18% total N and were rich in P (>0.1%) and OM (4 to 5%). These findings are also supported by Ung (1974) and Tiem et al (2000) as levels suitable for the growth of sweet potato, though the pH value was slightly low at 5.51. Danh (1998) grew soybean at the same site and recorded the same pH value and confirmed that the pH value did not affect the vegetative development and yields of many annual crops like cassava and sweet potato. 

Goat manure had a positive effect on the soil characteristics as bulk density was decreased, which indicated increased water holding capacity and content of OM (Duarsa 1996; Martin 2003). The total N, and P and exchangeable K of the treatments fertilised by goat manure were 0.29, and 0.32%, and 0.53 Meq/100mg, respectively, and were significantly different to the treatments with chemical fertiliser (0.19, and 0.24% and 0.18 Meq/l00mg, respectively). The N, P, K, and pH values of soil fertilised by goat manure were also higher and significantly different to the soil fertilised by chemical fertiliser or without fertiliser. Duarsa (1996) showed that goat manure can substitute part of N P K fertiliser and had a good effect on soil properties. Peacock (l 996) stated that soil benefits from the release of nutrients in goat manure, and the build-up of organic matter improves the structure of the soil.

There was no interaction between varieties and fertilisers, and there was no difference between the three varieties of sweet potato (Table 2). The DM, CP and NDF yields of the sweet potato vines fertilised by goat manure were not different to the sweet potato vines fertilised by chemical fertiliser. The yields of sweet potato vines in the control plots were lower than in the fertilised plots. There was no relationship between growth period and yield (R² = 0.014) during the time interval studied. Yields of sweet potato vines started to decline at the sixth cutting at 125 days (Figure 1).

Figure 1. The relationships between the growing time of plants and yield.

The highest DM content was obtained in the treatment without fertiliser, significantly different to the treatment with goat manure . The sweet potato vines fertilised by chemical fertiliser had the highest CP content (18.00%), significantly different to the other treatments. Sweet potato vines fertilised with goat manure had significantly higher ash content (10.57%) compared to the other two treatments. There were no significant differences in NDF content between the treatments, whereas the ADF content of sweet potato fertilised by goat manure was highest and significantly different to the other treatments (P<0.001). There were three main factors affecting the yield of sweet potato vines in the experiment: variety, soil, and fertiliser. The soil was as discussed earlier homogenous, and there was no interaction between variety and fertiliser and variety had no effect on yields and chemical composition of sweet potato vines, so the differences between the treatments were an effect of type of fertiliser. Apparently, the DM, CP, NDF, ADF yields of sweet potato vines fertilised by chemical fertiliser and goat manure were highest, but they were not significantly different to each other. This can be due to appropriate amounts of soil moisture being present and that the goat manure could have decomposed under the activities of bacteria to release adequate nutrients for plant growth even a short time after application (Martin 2003).

Ruiz (1981) showed that some varieties of sweet potato can be grown two or three times per year and can produce up to 125 tonnes of fresh biomass, of which 64% are vines, which contain 12% to 17% of CP. During the 125 days of the present experiment, there was a small variation in yields, but after the sixth harvest the yields started to decline. Purseglove (1982) confirmed that sweet potato is a short­ day plant and photosynthesis of the leaves affected the overall productivity, even the flowering. The observations on the germination of young shoots after the fifth harvest showed that only a few young shoots were visible on the sweet potato stems along with the second and/or the third shoot, which looked old. A level of fertilisation from 0 to 180 kg of N increased the forage production from 6.7 tonnes DM/ha to 9.1 tonnes DM/ha (six cuts at 45-day intervals), and from 5.4 tonnes DM/ha to 6.2 tonnes DM/ha at a cutting interval of 135 days (Quispe 1997 in Leon-Valarde 2000). There were only a few tubers at the last harvest for all three varieties. This means that with regularly cutting of the vines at short intervals the sweet potato cannot produce tubers as well. Ruiz et al (1980) and Leon-Valarde (2000) confirmed that root production of sweet potato was markedly reduced when defoliated regularly.

The DM, CP, NDF, ADF content in sweet potato vines did not show any variation due to treatment (Table 3).

 An (2003) reported similar results for DM, CP, NDF and ADF, but when separately analysing CP of leaves found a higher value by the effect of fertiliser. Orodho (2003) studied two different varieties of sweet potato and showed that sweet potato vines contained 17.5% CP, 28.0% NDF, and 22.0% ADF in DM, and that these values were not different between the two varieties. Dominguez (2000) reported similar results on nutrient composition of sweet potato vines of 14.2% DM, 18.5% CP, 12.5% ash, 26.2% NDF and 22.3% ADF (percent of DM) and concluded that the vines had a lower carbohydrate content, but were higher in fibre and protein in comparison to roots, and could be used as a source of vitamins and protein. In this study the ash content of sweet potato fertilised with goat manure was higher compared with the other treatments. This can be due to the fact that the goat manure was rich in minerals in a form that the plants can easily take up.

In general, the phenotypes of plants will partly influence the yield and nutrient value. In this case the three varieties of sweet potato are different in phenotype and production type. Trang giay and Duyen ngoc are typical for tuber production, and are short cycle cultivars with lobe leaves (small and medium area leaves) and high tuber yield, whereas Hshinchu is typical for forage production (entire and big leaf), can be grown all year round in the farmers' backyard, and high yields of vines have been recorded. The results of the present experiment, however, showed no influence of variety on yields and chemical composition. The recordings of pest incidences showed very little pest damage on the sweet potato vines. The few insects occurring during the experiment could have been from earlier harvests of vines. Leon-Valarde (2000) confirmed that sweet potato has few crop pests and diseases, and provides good ground cover for soil conservation.

Conclusions

• Sweet potato produced a considerable amount of vines within a four month cropping cycle with sequenced harvests of 15 to 20 days intervals. The nutrient value of the vines was stable both in regard to time and variety.
  
  
• Goat manure has an effect on soil characteristics, especially organic matter and bulk density, that is advantageous for the growth of the sweet potato plant.

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Received 2 March 2016; Accepted 10 April 2016; Published 2 June 2016

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