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Early maturing sweet potato varieties for high altitude highlands of Papua New Guinea

Kud Sitango and Nancy Dopo

National Agricultural Research Institute, P.O. Box 120, Mt. Hagen, Western Highlands Province, Papua New Guinea
Email: hahp@global.net.pg

Abstract

Sweet potato (Ipomea batatas) is the main staple of people in the highlands of Papua New Guinea. The common varieties take nine to twelve months to mature in high altitude areas. The crop is frost sensitive and in high altitude areas, crop-destroying frosts occur periodically, resulting in prolonged food shortages while crops recover. Early maturing varieties of sweet potato offer the opportunity for quicker recovery of food production following severe frost occurrences. Of thirty early maturing varieties collected from various high altitude areas and other selection programmes, ten were selected for further evaluation based on early yielding ability; namely: Lian morea, WHCK 005, PRAP 219, PRAP 469, WBS 010, Munibmam, SSYK 019, Lipulipu, PRAP 546 and WHCK 007

Media summary

Early maturing sweet potato varieties will reduce the period of severe food shortage following frost damage in the high altitude areas of Papua New Guinea

Key words

Early maturity sweet potato, edible tubers, frost recovery, Papua New Guinea, subsistence farming,

Introduction

The high altitude highlands is a distinct agro-ecological zone in Papua New Guinea that ranges from 1,800 m above sea level to the upper limit of cultivation at about 2,700 m. Sweet potato is the staple crop for most of the population of the highland’s region (Lutulele, 2001), with approximately 800,000 people living in the high altitude highlands zone.

The areas above 1800m have mean monthly maximum temperatures of 20-23oC and mean monthly minimum temperatures of 8-12oC. Average annual rainfall is above 2500mm. Frost becomes a major problem at altitudes of above 2200 m (Bourke, 1988). While frosts occur most years, they generally cause little damage. Severe frosts, often associated with El nino associated drought, are experienced at irregular intervals and have caused major food shortages. Increases in death rates in the population associated with some of these events have been recorded in 1941 and more recently in 1997 (Bourke, 2001). Other years in which severe frost have been recorded at high altitude locations are 1953, 1958, 1960, 1961, 1972, 1974, 1980 and 1982 (Bourke, 2001).

The unpredictability of these events has meant that people in frost susceptible areas have traditionally resorted to non-agricultural means to find solutions to the resultant food shortages. These include, killing domestic pigs, hunting of wild animals, gathering of nuts from the high altitude pandanus (Pandanus julanettii and P. brosimos) and migration to lower altitude areas to live with relatives, allies or traditional trading partners until after the frost period finishes and food production returns to a degree of normalcy (Wohlt, 1978 and Bourke, 1988). Increasing population, changing of society values and a break-down of traditional trading relationships have meant that the traditional systems cannot cope as well as they once did.

The common sweet potato varieties grown in the high altitude zone take 9 to 12 months from planting to first harvest. This means that following destruction of sweet potato gardens by frost, the period of food shortage extends for a long period following the replanting of new gardens. In Papua New Guinea there is wide genetic diversity in the varieties of sweet potato grown. Yen (1974) has suggested that there are possibly 5,000 varieties of sweet potato in PNG. The highlands collection located at Aiyura in the Eastern Highlands Province contains 1200 accessions (Van Wijmeersch, 2001). As part of a drought recovery project, Bang and Gende (2001) evaluated 30 varieties at Aiyura (altitude 1600 m) including some selected high yielding varieties and others selected from farmers’ fields on the basis of early maturity. Normal maturity at Aiyura is approximately six months. Bang identified nine varieties that produced acceptable yield of between four and seven tonnes per hectare at four months after planting. This work indicated that there was some possibility in selecting varieties that could produce useful tubers somewhat earlier than the commonly grown varieties. Following this, some preliminary evaluation was conducted of 65 varieties at Tambul in the Western Highlands Province (altitude 2300 m), including the above nine varieties, and 56 from various high altitude areas identified by farmers as early maturing. Some of these varieties produced yields of between 10 and 20 tonnes per hectare at five to seven months after planting. Results from these studies suggested that further work to identify suitable varieties for early maturity was warranted.

Method

The trial was conducted at the National Agricultural Research Institute, High Altitude Experiment Station, Tambul, at an altitude of 2300 masl. It is situated at latitude 5o 53’ south and longitude 143o 57’ east. The mean monthly maximum temperature is 20oC and the mean monthly minimum temperatures range from 10-12oC and average annual rainfall is 2617 mm.

Thirty sweet potato varieties were planted in a randomised block design with nine replicates. Plots consisted of a traditional compost mound (Waddell, 1972). These mounds are about 2.5m to 3m across, and about 0.6 to 0.8m high and made by covering mixed dry grasses and other plant matter with soil from the inter mound area. For this trial, mounds were made by placing 10kg of dry grass at a spacing of 3.0m by 3.3m and covered by soil from the inter mound area. Completed mounds were approximately 2.5m in diameter and approximately 0.75m high. This gave an effective plot size of 9.9m2.

There were 10 planting points per mound with one vine planted per planting point. Vines were approximately 40cm long and buried 5-10cm deep for about two thirds of the length. This left approximately 10-15cm of the vine exposed. Nine planting points were equally spaced around the mound at about half the height of the mound. One planting point was placed in the top-centre of the mound. The trial was planted in July 2003 and harvesting was done in January 2004.

At harvest, the plants were destructively harvested with tubers divided into marketable size tubers (> 100g), small tubers (developed tubers < 100g) and pencil tubers (thin undeveloped tubers). Both tuber numbers and weights were recorded. For this paper only the weights of marketable tubers have been presented.

Results

The yield of marketable tubers is presented in Table 1.

Varieties Lian morea, WHCK 005, PRAP 219, PRAP 469, WBS 010 Munibmam, SSYK 019,Lipulipu, PRAP 546 and WHCK 007 produced high marketable yields (> 6.33 t/ha) (Table 1).

Table 1. Yield of 30 sweet potato varieties at six months

No.

Variety name

Yield marketable tuber weight (t/ha)

1.

Lian morea

9.00

a

2

WHCK 005

8.37

ab

3

PRAP 219

7.30

abc

4

PRAP 469

7.14

abc

5

WBS 010

6.90

abcd

6

Munibmam

6.88

abcd

7

SSYK 019

6.77

abcd

8

Lipulipu

6.70

abcd

9

PRAP 546

6.42

abcd

10

WHCK 007

6.33

abcd

11

Baru

5.60

bcde

12

Pasindia

5.34

cdef

13

Goife

5.29

cdef

14

Argo

5.25

cdef

15

PRAP 91

5.24

cdef

16

PRAP 506

5.13

cdef

17

Simb

5.07

cdef

18

Karamui Ambu

4.89

cdefg

19

Baim

4.83

cdefg

20

Unska

4.83

cdefg

21

Siname

4.08

defgh

22

Karuku

3.96

defgh

23

Pepol

3.76

efgh

24

Bomai ambu

3.35

efgh

25

WJW 002

3.20

efgh

26

PRAP 559

2.45

fgh

27

NGI 24

2.09

gh

28

WMK 008

2.04

gh

29

Minbe wam

1.76

h

30

Sugar

1.70

h

 

Average yield

5.06

 

Note: Means separated by Duncan’s Multiple Range Test at P<0.0. Values in a column followed by different letters differ significantly.

Discussion

The marketable yield of the 30 varieties ranged from 1.70 t/ha for variety Sugar to 9.00 t/ha for variety Lian morea with a trial average of 5.06 t/ha (Table 1). Tubers of marketable size are considered edible, although in times of severe food shortage, small tubers can also be consumed.

The highest yielding varieties gave a reasonable yield of edible tubers in six months which is about three months earlier than traditional varieties are normally harvested. This indicates that early maturing varieties are available and that continued evaluation of the many available varieties in the highlands, could result in identifying a wider range of material.

At this point, these varieties have not been assessed for general farmer acceptance, this will be carried out at a later stage. However, in times of severe food shortage, taste and other consumer preferences become less significant. Further evaluation of these varieties and the possibility of wider screening of more varieties should enable better selection for farmer acceptance.

The varieties with high edible tuber yield of between six and nine tonnes per hectare at six months after planting would indicate that there would also have been significant yield somewhat earlier than this.

The ten varieties identified in this trial, namely Lian morea, WHCK 005, PRAP 219, PRAP 469, WBS 010 Munibmam, SSYK 019, Lipulipu, PRAP 546 and WHCK 007 could be released for farmer evaluation and also assessed further for earlier maturity and also for yield ability at the normal harvest period to identify which varieties may fit better into the normal garden practice, but at the same time offer the ability to produce an edible crop early in times of hardship.

Given the number of varieties which have produced a significant early yield out of such a small percentage of the available germplasm, there is a high probability that much better material may be identified with wider screening.

References

Bang. S and Gende B.2001. Early Maturing Sweet Potato First Trial Report. National Agricultural Research Institute, Aiyura, Eastern Highlands Province, Papua New Guinea.

Bourke, R.M .1988. Taim Hangre: variation in subsistence food supply in the Papua New Guinea Highlands. PhD Thesis, Canberra, Australian National University.

Bourke, R.M.2001. An overview of Food Security in Papua New Guinea. (In:) Bourke, R.M., Allen, M.G. and Salisbury, J.G. (ed). Food Security for Papua New Guinea. Proceedings of Papua New Guinea Food and Nutrition 2000 Conference, PNG University of Technology, Lae, 26-30 June 2000. ACIAR Proceedings N0 99.

Lutulele, R .2001. Sweet potato development in the PNG Highlands: Implications for future research and extension focus. (In:) Bourke, R.M., Allen, M.G. and Salisbury, J.G. (ed). Food Security for Papua New Guinea. Proceedings of Papua New Guinea Food and Nutrition 2000 Conference, PNG University of Technology, Lae, 26-30 June 2000. ACIAR Proceedings N0 99.

Van Wijmeersch, P. 2001. The status of sweet potato variety evaluation in PNG and recommendations for further research. (In:) Bourke, R.M., Allen, M.G. and Salisbury, J.G. (ed). Food Security for Papua New Guinea. Proceedings of Papua New Guinea Food and Nutrition 2000 Conference, PNG University of Technology, Lae, 26-30 June 2000. ACIAR Proceedings N0 99.

Waddell, E.W. (1972). The mound builders: agricultural practices, environment, and society in the Central Highlands of New Guinea. University of Washington Press, Seattle, Washington. American Ethnological Society Monograph, No. 53. 253 p. ISBN 0-295-95169-9.

Wohlt, P.B. 1978. Ecology, agriculture and social organization; dynamics of groups composition in the Highlands .; variation in subsistence food supply of Papua New Guinea. PhD Thesis. Minneapolis, University of Minnesota.

Yen D. E. 1974. The sweet potato and Oceania: An essay in ethnology. Bernice P Bishop Museum Bulletin 236. Bisop Museum Press, Honolulu, Hawaii.

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