Livestock Research for Rural Development 15 (1) 2003

Citation of this paper

Evaluation of sweet potato tuber (Ipomea batatas l.) as a feed ingredient in broiler chicken diets 

T Maphosa, K T Gunduza, J Kusina and A Mutungamiri*

Department of Animal Science, *Development Technology Centre, University of Zimbabwe,
PO Box MP167, Mount Pleasant, Harare, Zimbabwe
mkhonto@avu.org 

Abstract 

The potential of sweet potatoes as an ingredient for broiler chicken feed is not well known in Zimbabwe. An experiment was carried out to determine the potential of raw sweet potato meal as a feed ingredient in broiler diets. Diets were formulated to contain 0, 25, 50, 75 and 100 % of raw sweet potato meal sweet potato meal as a direct substitute of maize. The proximate composition of the experimental diets and pure sweet potato was determined. Six-day old broilers were randomly allocated to one of the starter dietary treatments then on the finisher diet at 28 days of age. Live weights of birds and cumulative feed intake were recorded once a week. Birds were slaughtered at eight weeks of age. Carcasses from each experimental unit were then analysed for DM, EE, CP, ash, Ca and P.

 

Increasing the proportion of sweet potato meal in the starter diet negatively  influenced weight gain, food intake and food conversion. However, it had no effect on weight gain when included up to 50% in finisher diets.  The relative weight of the pancreas, gizzard, intestines and caeca increased with increasing sweet potato meal inclusion. There were no differences in carcass composition among treatment groups. Inclusion of sweet potato meal up to 50 % in finisher diets had no adverse effects on the performance of the broilers.

 

The recommended level of sweet potato inclusion in the present study is 50 % in finisher diets and none in starter diets.  

Key words: Broilers, carcass, chicken, conversion, growth,  sweet potato   
 

Introduction 

Poultry is an important source of protein to the ever-expanding population in rural areas. The cost of feed has been indicated by farmers in the smallholder sector as the major constraint in poultry production (Munyawu et al 1998). The poultry producers have experienced a rise in the cost of production due to the increasing cost of feed. The cost of the maize ingredient, which makes 65 % of the current poultry feeds, is very high (Scott 1995; Mutetwa 1996). Maize also happens to be the staple food in Zimbabwe. The prospects to increase the output of cereals to a magnitude, which will satisfy both human and animal needs remains unforeseen. Therefore, an alternative to cereals in animal feeds might be the only immediate solution (Scott 1995). In Zimbabwe there has been a considerable gradual increase in land used for sweet potato production from 1996 to 2000 coupled with a decrease in maize production in the communal lands (CSO 2001).  

Maize and sweet potato have comparable metabolizable values of 14.5 and 14.8, respectively (Woolfe 1992). The digestibility of sweet potato carbohydrate fraction is reported to be above 90 % (Ravindran 1995). However, the level of starch decreases with period of storage and instead the level of reducing sugars, total sugars and total dextrins increases (Woolfe 1992). Sweet potatoes have also been reported to exhibit trypsin inhibitor activity ranging from 20 to 90 % inhibition (Woolfe 1992). However, Ravindran (1995) reported that trypsin inhibitor levels present in sweet potato tubers are low and should not be a cause for concern under practical situations. A recent study carried out in Nigeria recommended 27 and 30 % levels of sweet potato in the starter and finisher diets respectively (Agwunobi 1999). However, Woolfe (1992) reported having replaced 50 to 75% of maize in poultry feed with dried sweet potato flour without adverse effects on the growth of broilers. Currently, not much work has been done if any on the use of sweet potato as poultry feed in Zimbabwe. Thus the objective of this study was to evaluate performance of broilers fed on diets with sweet potato flour as a partial and complete replacement of maize on a weight for weight basis.  

Methods and materials 

Location 

The research was conducted at the University of Zimbabwe, Department of Animal Science bioassay laboratory.

Diets  

Clean and uncontaminated raw sweet potatoes were obtained from Murewa communal areas. They were sliced into chips and dried before milling using a Hippo 1½ hammer miller with no screen. Maize was purchased from a local farm. The rest of the ingredients were purchased from a reputable milling company. Manual mixing was employed to prepare the feeds. Sweet potato flour samples were analysed to determine their proximate composition and gross energy before inclusion into diets. All the laboratory analyses were carried out in duplicate following the AOAC (1990) protocol. The five diets were formulated to have  0, 25, 50, 75 and 100 % sweet potato replacement of maize (Tables 1 and 2). Samples of each formulated diet were analysed to determine their proximate composition (Table 3).  

Broiler management 

Day-old (Cobb 500) broilers were bought from a reputable breeder. The chicks were placed in an electrically heated wire-floored battery brooder for six days. The birds received constant illumination and free access to water and feed. After the six-day brooding period the birds were put on the experimental diets. A total of 180 birds were randomly distributed among 5 treatments with 9 birds per cage and 4 cages per treatment. The birds were on the experimental starter diet for 4 weeks and another 4 weeks on the finisher diets.  .

 

Measurements

Records on feed intake and live weight were taken weekly. Relative weight of the gizzard, pancreas and gastrointestinal tract segments (intestines and caecum) to mass of broilers were measured and calculated at slaughter.  At the termination of the experiment a bird from each cage was sacrificed by cervical dislocation. The birds were cut along the midline and half of the sample from each bird was ground in a mincer (Kusina 1988) to determine crude protein, ether extract, ash, calcium and phosphorous following AOAC protocol (1990). 

Statistical analysis  

All data were subject to analysis of variance and means were separated using Dunnett’s test (SAS 2000). The effects of sweet potato meal sweet potato meal on the variables measured were analysed using the General Linear Models of SAS (2000). Data were analysed as a completely randomized design and were presented as the means of each group and pooled standard error.  

Results 

Feed intake, live weight gains, mortality and feed conversion ratio are summarized in Table 4. The inclusion of sweet potato had a negative effect (P<0.05) on performance of birds. There was a significant decline in weight gain of birds with increase in inclusion rate of sweet potato meal during the starter phase. There was a numerical decline in feed intake although no significant difference up to 75% maize replacement rate. There was no difference (P<0.05) in feed conversion of birds up to 50% maize replacement but it continued to  deteriorate with increase in inclusion of sweet potato meal.  

Inclusion of sweet potato meal negatively affected growth rate and feed conversion of birds during the finishing phase. Feed intake of birds on the finisher diet was lower (P<0.05) than the control at maize replacement rates at and above 75%.  Mortality increased with increase in sweet potato maize replacement level in the diets beyond the 25% level. Inclusion of sweet potato had an effect (P<0.05) on the size of digestive organs (Table 5). The relative weights of the pancreas, intestines and caeca were significantly higher (P<0.05) for birds on and above 50% maize replacement rate. There was an increase in size of the pancreas with increase in sweet potato concentration of the diet. There was no difference (P>0.05) in relative weight of the gizzard across diets. The length of the intestines was significantly shorter among birds on diets with 75% and 100% maize replacement by sweet potato. Birds on diets containing 50% maize replacement by sweet potato passed watery droplets. Inclusion of sweet potato in the diets had no effect  (P>0.05) on carcass quality of broilers at eight weeks of age. 

Discussion 

Proximate composition of diets and  sweet potato meal

The CP, EE and CF content of sweet potato meal were slightly lower in this study compared to the values reported by Ravindran and Blair (1991). The variation can be attributed to differences in varieties, geographical areas and the conditions under which the plant was grown. The ash content of the sweet potato meal used in this study was similar to that reported by Ravindran and Blair (1991). The starter and finisher diets had crude protein contents, which were within the recommended range (NRC 1984). The energy content was similar across treatment diets. This means that maize and sweet potato are similar in energy content, an observation also made by Ravindran and Blair (1991).  

Broiler performance (Starter phase) 

Live weight gain and feed conversion were negatively associated with replacement rate of maize by sweet potato meal (Figures 1 and 2).

Figure 1: Effect on weight gain of broilers of replacing maize by sweet potato root meal

 

Figure 2: Effect on conversion of broilers of replacing maize by sweet potato root meal

 

The decrease in weight gain with an increase in level of sweet potato in the starter diet could have been a result of decrease in feed intake of birds on 100% sweet potato meal and poor nutrient utilisation by birds on 75% and 100% maize replacement by sweet potato meal. A decrease in feed intake of birds with increasing levels of sweet potato concurred with observations made by Tewe (1991). Feed intakes of birds on 25%, 50% and 75% maize replacement levels were lower than intake of birds on the control diet, which could have been a reflection of poor palatability and acceptability of sweet potato to broilers according to Banser et al (2000).  Feed conversion ratio deteriorated with increasing level of sweet potato flour in the diet which agrees with the report of Tewe (1991). The decrease in the efficiency of utilization of feed was attributed by Agwunobi (1999) and Tewe (1991) to the increased rate of passage. Tewe (1991) reported that sweet potato tuber was not efficiently utilised by young chicks of less than two weeks of age. Ravindran (1995) attributed this to the presence of anti-nutritional factors like trypsin inhibitors in sweet potatoes.   

Broiler performance (Finisher phase) 

A similar trend for reduced live weight gain with increasing level of sweet potato meal was observed during the finisher phase (Figure 1).   The degree of recovery of birds on the 50% maize replacement level  suggests that the birds had undergone a period of compensatory growth after digestive tract adaptations had occurred (Banser et al 2000). The lower feed intake at 75% maize replacement, could be due to smaller gut size at the start of the finisher phase since there is a physical limit to the gastro intestinal tract capacity resulting in reduced feed intake.  The feed conversion ratio at 100% maize replacement was pooer than the control, which is consistent with other reports (Tewe 1991; Agwunobi 1999). The feed conversion was generally poorer for birds on the finisher diets compared to those on the starter diets, which is a normal phenomenon according to Mutetwa (1996). From Figure 2 it can be deduced that broiler chickens fed on sweet potato meal grew less rapidly than those on a diet containing maize. Birds fed a diet containing 50% of maize replaced by sweet potato showed a tendency to select against the sweet potato component, and this was associated with a much higher feed wastage due to the jerky movement of the head when selecting the feed. This resulted in an over estimate of the amount of feed consumed. Selection against sweet potato was made easier due to the differences in particle size of the sweet potato and maize meals. The lower resistance of sweet potato to grinding, as compared to maize, resulted in a smaller particle size of the sweet potato meal. 

Internal organs 

The depression in weight gain of birds given sweet potato meal was associated with changes in the relative weight of the organs, which implies an involvement of anti-nutritional factors. The results from this study clearly showed that there was pancreatic hypertrophy, which was observed to increase with increasing levels of sweet potato in the diets. Pancreatic hypertrophy is caused by the presence of trypsin inhibitors in the diet (Nishino et al 2001). This has frequently been observed in rats (Nishino et al 2001) and chicks (Viveros et al 2001). The increase in size of the pancreas can be used as a crude indicator of trypsin inhibitor levels in a feed. Suppression of trypsin in the intestine increases pancreatic secretion of the enzymes through a feedback mechanism, which is mediated by cholecystokinin (CCK) and, consequently induces pancreatic hypertrophy and hyperplasia (Nishino et al 2001).  

CCK induces gut motility. The plasma levels of CCK are ultimately affected and there is an increase in gut motility. Increased motility is likely to increase the muscle mass of the intestines, caeca and rates of passage. Gut motility affects rate of passage, which ultimately affects utilization of nutrients. Therefore a sequence of events that are triggered by trypsin inhibitor results in poor nutrient utilization through increase in passage rates and loss of endogenous and exogenous nitrogen. Trypsin inhibitors increase the loss of endogenous proteins such as digestive enzymes, which can be rich in essential amino acids (Viveros et al 2001).  

The increase in the weight and length of the lower parts of the guts can be attributed to the bulkiness of the diets with sweet potato as a direct substitute of maize. Bulkiness of a diet results in the bird taking in a large volume of the feed in order to satisfy its nutrient requirements ultimately exerting a pressure, which stretches the intestines (Hetland and Svihus 2001). The increase in the weight of digestive organs of birds can be attributed, to some extent, to the presence of a high concentration of indigestible materials in the intestine of the animal (Viveros et al 2001). 

Mortality 

Mortality was high among birds on a diet with 100% replacement of maize by sweet potato . This could have been due to a sudden change of diet, which stressed the birds. Of the total mortalities, 84.7%  occurred during the first week of feeding the experimental diets. Birds that died had watery and whitish diarrhoea, The high soluble sugar content of sweet potato might have been the cause of the gastrointestinal disorders in the poultry.  However, Tewe (1991) and Ravindran and Sivakanesan (1996) showed that the level of sweet potato meal in broiler diets had no effect on mortality rate in their studies. Post-mortem results revealed that the birds in the present experiment had an erosion of the gizzard, this is when the cuticle of the gizzard separates from the mucosa.  

Carcass composition 

The results showed that the level of sweet potato meal inclusion into broiler diets had no effect on the chemical composition of the carcass at eight weeks of age. However, Woolfe (1992) observed that the fat content of broilers fed with sweet potato meal was significantly lower than that of those fed with maize meal at 10 weeks of age. The difference in these findings may be due to differences in age since older animals deposit more fat compared to young animals (Kusina 1988). However, in this study there was a tendency for a decline in fat content as the level of sweet potato meal in the diet increased. 

Conclusion

The addition of sweet potato meal in broiler starter diets had a negative effect on the performance of the birds. However, sweet potato could be included in broiler finisher diets at 50 % maize replacement without adversely affecting productivity. The above recommendation has been made, assuming that the farmer has the sweet potato at their disposal.

Future studies should also explore the possibility of increasing the maize replacement level beyond 50 % through pelleting the sweet potato diets to reduce selection against the sweet potato ingredient.

There is also a need to investigate the optimum storage conditions that will reduce the level of soluble sugars in sweet potato containing diets since these have been reported to cause gastrointestinal disorders.

Additional studies are necessary in order to determine the role of trypsin inhibitors in sweet potatoes on the bioavailability of nutrients and also the best processing methods to deactivate these. Crop breeders are encouraged to start a sweet potato variety selection programme for varieties with low trypsin inhibitory activity, which could be used for animal feed.   

Acknowledgement

Mr T Rukuni, the director of Development Technology Centre (DTC), is gratefully acknowledged  for facilitating the financing of this study. The technical assistance of Mr Karosi and Mr Sena is gratefully acknowledged.

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