Effect of high dietary levels of sweet potato leaves on the reproductive performance of pure and crossbred Mong Cai sows

 

Hoang Nghia Duyet, Nguyen Dinh Son, Nguyen Van An and Truong Thi Thuan,

Hue University of Agriculture and Forestry

 

Abstract

Twenty four Mong Cai gilts of 8 to 10 kg live weight, after 1 week for adaptation and vaccination, were divided into two groups, each group consisting of 12 gilts. Six Mong Cai gilts in each group were mated with the same Mong Cai boar and six mated with the same Large White boar. The effects on the reproductive performance of two levels of sweet potato leaves (SPL) in gilt (Stage 1), gestation (Stage 2) and lactation (Stage 3) diets were evaluated. Diets LSP and HSP consisted of 20 and 50% SPL in DM, respectively. Diets in the gilt and pregnancy period contained 12% and in lactation 14% crude protein. The sweet potato leaves were fed in the fresh state.

 

Daily intakes of fresh sweet potato leaves (kg) ranged from 2.0 to 5.0 in the growing phase, 5.5 to 6.0 in gestation and 6.0 to 7.0 in lactation in sows on the HSP diet. In the growing period the low SPL diet resulted in significantly higher mean live weight gain compared to the high SP diet. Mean feed conversion ratios were not significantly different between treatments but the costs of live weight gain were significantly higher for the LSP treatment. The HSP diet resulted in significantly lower live weight gain during gestation, and higher weight losses and longer time for return to heat in lactation. However, litter size at birth and at weaning, and mean birth and weaning weights were not significantly different between treatment groups. The negative effects of HSP on lactation parameters were more marked when Mong Cai sows were mated with the Large white boar.

 

It is concluded that the optimum levels of SPL in the diet are 50% in the gilt growing and pregnancy periods and 20% during lactation. 

 

Key words: Gilts, Mong Cai, reproduction, sows,  sweet potato leaves

 

Introduction

Vietnam has about 1 million Mong Cai sows (50% of the total sow population), concentrated in North and Central Vietnam. Traditional feeds of Mong Cai sows in central Vietnam include large amounts of sweet potato leaves in the gilt and pregnancy periods. Farmers only use concentrates, based on for example rice bran and cassava meal, in lactation, in order to give higher milk yields. Pig producers thus make extensive use of locally available products and by-products from agriculture, such as rice bran, cassava, cassava pomace and sweet potato leaves. The pigs produce manure for crops, food and income for small scale farmers.

 

Thua Thien Hue is a province in the coastal area of Central Vietnam, and around a third of the total area consists of sandy soils, with very poor fertility, but which can be used for growing sweet potato. According to FAO (1997) the productive potential of the sweet potato varies from 24 to 36 tonnes/ha of fresh roots  and from 4.3 to 6.0 tonnes of dry matter/ha of foliage (Morales 1980). It is also possible to obtain up to three harvests yearly (Ruiz et al 1980). The main nutritional importance of sweet potato is in the starch content of the root, but it is also a source of important vitamins, such as vitamin A, ascorbic acid, thiamine, riboflavin and niacin. Recently, it has been shown that the fresh vines can provide up to 27% of the dry matter and 40% of total dietary protein for growing/ finishing pigs. However, so far there has been little research carried out on the most suitable levels of sweet potato leaves for Mong Cai sows, so this study was planned with the following objective: 

• To determine the effects on weight changes and reproductive performance i of ncluding low (20%) and high (50%) levels of sweet potato leaves (SPL) in diets for Mong Cai gilts and pregnant and lactating sows.

 

Materials and methods

Experimental design

The experiment was carried out at Tien Phong Farm, Dien Tho commune, Quang Nam Province in 2001 and 2002.  There were four treatments arranged as a 2*2 factorial, applied to 24 Mong Cai gilts, after weaning (8 to 10 kg live weight).  The main effects were:

• Level of sweet potato leaves: Low (LSP, 20% of dietary DM) and High (HSP, 50% of dietary DM)

• Breed of boar: Mong Cai and Large White

 

After 1 week for adaptation and vaccination the gilts were randomly divided into 2 treatment groups, each group consisting of 12 gilts. Of these six were mated by AI with the same Mong Cai boar and six with the same Large White boar. The experimental design and feed allowances are shown in Table 1.

 

Diets and feeding

The chemical composition of the ingredients and diets are shown in Tables 2, 3 and 4. The sweet potato leaves were fed in the fresh state, immediately after they were harvested. The dry components of the diets were mixed together and fed separately.

 

 

Measurements

 Growth rate, back fat thickness, feed intake, feed conversion ratio (FCR), and live weight gain were recorded during the growing and gestation periods. Litter size at birth and at weaning, birth weight and weaning weight, mortality of the piglets at birth, total litter weight at 21 days, live weight changes of the sows during lactation, interval from weaning to oestrus, feed intake, and  FCR for producing 1 kg of piglets at weaning were also calculated. Feed ingredients and diets were analysed for proximate constituents by AOAC (1998) procedures.

 

 Results and discussion

The intakes of fresh sweet potato leaves reached from 6 to 7 kg/day in the sows on the HSP treatment during lactation (Table 4), indicating that the palatability or bulkiness of this component of the diet did not appear to be a limiting factor.  The growth rates of the gilts were higher than reported previously for this breed (Hoang Nghia Duyet and Nguyen Kim Duong  1995), and did not differ between dietary treatments.  The cost per 1 kg live weight gain was lower for the HSP treatment, due to the lower cost of the HSP diet. This means that at current feed prices there are economic advantages from using a high level (50%) of sweet potato leaves in diets for growing gilts of the Mong Cai breed. 

 

Litter size at birth and weaning, time for farrowing, birth weight and weaning weight were not significantly different between dietary treatments. Weight gain of the sows during pregnancy was lower for the LSP sows (P=0.03), and weight loss during lactation  higher (P=0.007) than for the LSP sows (Table 5). The explanation for this is the approximately 11% lower ME content of the HSP diets compared to the LSP diets, the probably lower crude protein digestibility as a result of the higher crude fibre content and the poorer amino acid balance (SPL replaced fish meal in the HSP diet). Similarly, back fat  thickness was lower, and number of days for return to heat higher for the sows on the HSP diet (P<0.05).  This means that as much as 50% of SPL can be included in gestation diets for Mong Cai sows, but that the level of inclusion in lactation diets should be lower.

 

 

 

Live weight gain during pregnancy tended to be higher (P=0.08) in  sows fed the LSP diet and mated with the MC than the LW boar, but there was no effect of sire breed on this trait for sows fed the HSP diet (Table 6). Weight loss in lactation was greater in sows fed the HSP diet and mated with the LW compared with the MC boar but this trait was not affected by sire breed in sows fed the LSP diet. Litter size at birth and at weaning, and farrowing time were not affected by sire breed in sows fed LSP or HSP diets. Piglet birth and  weaning weights were higher from sows mated with the LW compared with MC  boar on both LSP and HSP diets. In sows fed the LSP diet, back fat thickness at farrowing and days for return to oestrus were not affected by sire breed, but the former trait was reduced and the latter increased in sows mated with LW on the HSP diet.

 

Effects of sire breed on piglet weights almost certainly reflected the greater size of the LW compared with the MC boar. Similarly, loss of weight and body condition and delayed return to oestrous in LW*MC sows are likely to be due to the greater milk demand of the heavier LW cross piglets.

 

Feed conversion, expressed as amount of feed required to produce 1 kg of weaned piglets, was lower for MC*LW crossbred piglets than for purebred MC piglets, and tended to be lower for piglets from sows fed LSP rather than HSP (Table 7). However, total feed costs were reduced as the content of SP leaves was increased (HSP vs LSP), thus the gross margin of sale value of piglets - feed cost was in favour of the HSP diets. Gross margins did not differ between sire groups of piglets from sows fed the LSP diets but favored the purebred MC piglets from sows fed the HSP diets.

 

Managing sweet potatoes as a forage crop

 When sweet potatoes are managed as a semi-perennial forage crop, by harvesting the leaves at regular intervals of from 30 to 40 days, the yields can be very high (Table 8).  This degree of productivity is some 5 times higher than when the leaves are harvested at the same time as the roots, for which foliage dry matter yields are reported of  5.36 tonnes / ha (Nguyen Thi Cach 1997),  5.9 to 6.3 tonnes / ha  (Nguyen Tan Yen et al 1996) and 4.3 to 6.0 tonnes / ha (FAO 1997).

 

 

Conclusions

• Sweet potato leaves are a very popular vegetable for pigs in Vietnam throughout the year

• When the sweet potato is grown for its leaves, that are harvested regularly (10-12 times per year), very high yields of up to 30 tonnes DM / ha can be achieved.

• Including sweet potato leaves at high levels in sow diets saves concentrate feed, reduces the risk of over-fatness of Mong Cai sows, maintains a high level of reproductive performance, and produces large quantities of high quality manure.

• In the gilt rearing and pregnancy periods high levels of sweet potato leaves (50% of the DM in the diet) can be used, but the level should be reduced to 20% of diet DM in lactation.

• Negative effects of the higher level of sweet potato leaves on lactation performance tended to be more marked in Mong Cai sows mated to a Large White compared with a Mong Cai boar.

 

Acknowledgements 

This research was partially financed by the bilateral SAREC project 2000-2002.

References

AOAC  1988  Official methods of analysis, 14th Edition, Association of Official Analytical Chemists Washington, DC.

 

FAO  1997  Sweet potato. Feeding pigs in the tropics, p. 130. FAO Animal Production and Health Paper 132.

 

Hoang Nghia Duyet and Nguyen Kim Duong  1995  Pig breeding in central Vietnam and its improvement. Exploring appoaches to research in the animal sciences in  Vietnam. ACIAN proceedings, No. 68. pp. 123.

 

Morales  A  1980  El clon del boniato CEMSA 74 - 228. Centro de Mejoramiento de Semillas Aglamicas (SEMSA). Villa Clara, Cuba. pp 26.

 

Nguyen Thi Cach  1997  Yields of a good sweet potato line. p. 17.  Proceedings of a  Workshop, Hue University, 1997.

 

Nguyen The Yen,  Vu Tuyen Hoang and Pham Xuan Liem  1996  Preliminary results on breeding sweet potato as animal feed, p 92. National workshop on Developing  Vietnamese  Animal Husbandry to year 2000.

 

Ruiz  M  E, Pezo  D  and Martinez  L  1980  El uso del camote (Ipomoea batatas (L) Lam) en la alimentacion animal. I. Aspectors agronomicos. Prod. Anim. Trop 5: 157 - 165.

 

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