Variations in quality of whole-plant foliage components from sweet potato harvested at 12 and 20 weeks after planting

I Etela* and U I Oji

*Department of Animal Science and Fisheries, University of Port Harcourt, Choba, Nigeria
ibetela@yahoo.com
Department of Animal Science, Rivers State University of Science and Technology, Port Harcourt, Nigeria
uchennaoji2001@yahoo.com

Abstract

The study compared crude protein (CP) and neutral detergent fibre (NDF) contents, rumen dry matter (DM) degradation characteristics (soluble fraction, a; degradable fraction, b; potential degradability, PD) of sweet potato foliage components (whole-plant tops, WPT; leaf-blade, LB; leaf-petiole, LP; stem, ST) at 12 and 20 weeks after planting (WAP).  

Mean CP content was least (P < 0.05) in ST and highest in LB, at both harvesting dates.  Mean NDF was highest in ST at both harvesting stages while, LB recorded higher NDF content with maturity.  Mean a-value was lowest (P < 0.05) in LB and highest in LP, at 12 and 20 WAP.  Mean b-value was lower at 20 WAP except in the petiole where it was similar.  The PD-value was lowest (P < 0.05) in ST and highest in LP, at 12 and 20 WAP.  The CP content showed negative correlation with the a-value (r² = -0.42; P < 0.01) and positive correlation with the b-value (r² = 0.43; P < 0.01) at 12 WAP while, it had no relationship at 20 WAP.  Similarly, NDF contents did not record any relationship with the a-value while, it gave negative correlations with the b-value (r² = -0.66; P < 0.01) and PD-value (r² = -0.70; P < 0.01) at 12 WAP, and (r² = -0.78; P < 0.01) and (r² = -0.89; P < 0.01), respectively, at 20 WAP.  The results from the study could be useful for managing sweet potato foliage production for smallholder crop and livestock production systems.

Keywords: Animal performance, crude protein, foliage component, maturity date, neutral detergent fibre, rumen degradation

Introduction

Sweet potato [Ipomoea batatas (L.) Lam.] is a widely cultivated crop in Nigeria, from southern part through the northern part (Tewe et al 2003).  The fodder has long been identified and used as supplement to low plane feeding in smallholder livestock producing (but extensive crop producing) areas of Asia, sub-Saharan Africa and Latin America (Devendra 1989).  The nutritive value of forage depends on several factors such as variety, foliage components, voluntary intake, digestibility and production of meat or milk per unit of the forage consumed either as sole or supplemented diet (Coleman and Moore 2003).  Thus, assessing the quality of sweet potato foliage components (whole-plant tops, leaf-blade, leaf-petiole and stem) would enable the selection of desirable varieties that exhibit high proportions of the more digestible foliage components with higher foliage and root yields.  Although some work have been done on the nutritional evaluation of sweet potato fodder, only little has been reported on the rumen degradation characteristics of the foliage components at different stages of growth (Ffoulkes et al 1978; Orodho et al 1996).

The nylon bag (in situ) degradability of forage has been described as a very useful means for assessing differences in nutritive value between crop cultivars (Verbic et al 1995; von Keyserlingk et al 1996).  Similarly, crude protein (CP) content and rumen degradation characteristics have been identified as important determinants of forage quality and are useful parameters for estimating both feed intake and performance of ruminants that are also influenced by stage of maturity of forage (Ørskov et al 1988; Blümmel and Ørskov 1993; Ingvartsen 1994).  In tropical roughages, it has been demonstrated that in sacco degradation gave better predictions of voluntary intake than in vivo digestibility (Ibrahim et al 1995).  An understanding of the differences in degradation pattern of the botanical fractions in forages or crop residues would enhance the development of varieties/cultivars with more digestible forage and crop residues through breeding and selection.  Therefore, the paper reports the findings from a study aimed at determining CP, NDF and rumen DM degradation characteristics of sweet potato foliage components as influenced by stage of growth.

Materials and methods 

Fodder harvesting and processing

Fodder used for the study were gotten from twenty (20) sweet potato varieties harvested at 12 and 20 weeks after planting (WAP) obtained from the National Root Crops Research Institute (NRCRI), Umudike, Nigeria (05°29′ N; 07°33′ E; 122 m a.s.l.).  The harvested foliage (whole-plant tops; WPT) were then hand-separated into the foliage components (leaf-blade, LB; leaf-petiole, LP; stem, ST) and later dried at 60 °C for 48 h in a Gallenkamp forced-air oven.  Two-thirds of the dried samples were then ground in a Wiley mill fitted with 2.5-mm screen, while the remainder was ground through 1-mm screen using a RetchMuhle laboratory mill.  Both sub-samples were then stored in airtight whirlpacks for subsequent analyses.  The 2.5-mm samples were used for in sacco degradation studies, while the 1-mm samples were analysed for crude protein (CP) by the macro-Kjeldahl method (AOAC 1990) and neutral detergent fibre (NDF) analysed following the procedures outlined by Van Soest et al (1991).

Experimental design and in sacco study

The study was conducted at the International Livestock Research Institute (ILRI), Ibadan, Nigeria research farm (07°30′ N; 03°54′ E).  The experimental design was split plot design with three rumen-fistulated N’Dama (Bos taurus) steers weighing on average 310 kg and aged between two and half years used as replicates where nylon bags plus feed samples were incubated.  The animals used were fed twice daily with 2 kg of wheat bran between 08:00 h and 14:00 h and later allowed free access to 4-wk re-growth Guinea grass (Panicum maximum) pasture between 14:00 h and 08:00 h the next morning.  The feeding regimen was maintained for 10 d prior to and throughout the incubation periods.  Incubation was done in duplicates at 6, 12, 24, 48, and 96 h with pre-weighed nylon bags (Polyester, Switzerland) of pore size 41 μm and each containing about 3 g of samples.  At the end of each incubation period, samples with bags were withdrawn, rinsed under cool and clean tap water and dried to constant weights at 60 °C for 48 h.  The difference between initial sample plus bag weight (SBWt) and residue plus bag weight (RBWt) at each incubation period (t) was considered as DM disappearance percent (p).

Statistical analysis and prediction equation

The degradation constants a (water soluble fraction) and b (slowly degradable fraction) were estimated from the model: p = a + b(1 – e^-ct) using PROC NLIN (Ørskov and McDonald 1979) with the aid of the Statistical Analysis Systems Institute Inc. software package (SAS 1999).  Potential degradability (PD) was then estimated as a + b.  The data were analysed as split plot using analysis of variance procedures (PROC ANOVA) with harvesting dates (12 and 20 WAP) as two main-plots, the foliage components (whole-plant tops; leaf-blade; leaf-petiole; stem) as four sub-plots and three rumen-fistulated N’Dama steers as three replicates.  The replicate by harvesting date by foliage component interaction was used to test differences between the sweet potato foliage components.  And where significant differences were recorded, mean separation was done using the least significant difference (LSD) option at 5 % level of probability.

Results 

Chemical composition

The mean crude protein (CP) and neutral detergent fibre (NDF) of sweet potato foliage components (whole-plant tops; leaf-blade; leaf-petiole; stem) from 20 varieties at 12 and 20 WAP are depicted in Table 1. 

At 12 and 20 WAP, the stem recorded the least (P < 0.05) CP values while, the highest values were recorded by the leaf-blade.  The CP contents decreased in whole-plant tops and leaf-blade while, it appeared to increase in the leaf-petiole and stem fractions from 12 to 20 WAP.  The NDF values at 12 and 20 WAP were lowest (P < 0.05) in leaf-blade and leaf-petiole while, the corresponding highest values were recorded for the stem fraction at both stages of growth (Table 1).  The results also indicate that the NDF in leaf-blade increased faster than in the other foliage components with maturity from 12 to 20 WAP.

Rumen dry matter degradation characteristics

Figure 1 indicates that potential degradability (PD-value), estimated as a-value plus b-value, for the whole-plant tops and stem were similar at 12 and 20 WAP. 

The leaf-blade recorded a reduction (P < 0.05) in PD-value as the crops matured from 12 to 20 WAP while, the leaf-petiole recorded a slight increase (P < 0.05) in PD-value with maturity.  There were declines (P < 0.05) in the b-values for the whole-plant tops, leaf-blade and stem while, the b-value in the leaf-petiole remained the same with maturity from 12 to 20 WAP.  Generally, only PD-values for whole-plant tops and leaf-blade at 12 WAP, and leaf-petiole at both 12 and 20 WAP were equal to or greater than 500 g kg^-1 DM while, the stem recorded PD-values lower than that at both stages of maturity.  There were increases (P < 0.05) in a-value with maturity from 12 to 20 WAP in all four foliage components.

Correlation between nutrient contents and rumen degradation

At 12 WAP, the CP content had negative correlation with the a-value (r = -0.42; P < 0.01), a positive correlation with the b-value (r = 0.43; P < 0.01), but recorded no correlation with the PD-value (Table 2). 

Also, the NDF did not show any relationship with the a-value while, it recorded negative correlations with the b-value (r = -0.66; P < 0.01) and the PD-value (r = -0.70; P < 0.01).  At 20 WAP, there was no correlation between the CP content and rumen DM degradation characteristics while, the NDF content has negative correlation with the b-value (r = -0.78; P < 0.01) and PD-value (r = -0.89; P < 0.01) only.

Discussion 

Chemical composition

The observed crude protein (CP) and neutral detergent fibre (NDF) contents in the foliage components are within expected range, with the leaf parts (leaf-blade; leaf-petiole) containing more CP and less NDF contents than the stem.  The CP and NDF contents for leaf-blade in the study were in agreement with previous reports (Dominguez 1992).  However, the reported CP contents for sweet potato leaf-blade, leaf-petiole and stem were lower than previously reported by others (Orodho et al 1996).  Differences in maturity stages at harvesting and other management practices (such as fertilizer application, methods of harvesting and laboratory analysis procedures, and so on) have been identified as being responsible for such differences in forage quality (von Keyserlingk et al 1996).  The decrease in CP contents at 20 weeks after planting (WAP) for leaf-blade was probably due to the increase in concentration of cell wall component (NDF) with maturity as recorded in the study.  However, leaf-petiole and stem recorded increased CP and decreased NDF contents at 20 WAP due partly to some re-growths during the period.  In a related study with sweet potato forage, similar observations were made for CP content which increased with maturity in sweet potato stem (Done et al 1978).  The trend in the leaf-blade was contrary to this proposition thus, a low leaf-to-stem ratio would restrict the intake and digestibility of the forage at 12 WAP than at 20 WAP since the stem contains CP content below the recommended 70 g kg^-1 DM for cattle and the reported high NDF (Khazaal et al 1995).

Rumen dry matter degradation characteristics

There is limited information in the literature on rumen DM degradation characteristics of sweet potato leaf-blade, leaf-petiole and stem fractions.  But the observed results from the study provide an insight as to how the various foliage components of sweet potato could affect the overall nutritive value of the whole-plant tops.  The relatively higher a-values in the leaf-petiole suggest that starch and other readily soluble components in sweet potato plants were, primarily, stored in the leaf-petiole before translocation to the roots and less so in the leaf-blade where photosynthesis occurs and the stem which serves as the medium of translocation.  All four foliage components exhibited higher a-values at 20 WAP, which suggests more active photosynthetic activities (source) at that stage of growth, coupled with reduced rate of nutrient translocation to the roots (sink).

The relatively higher b-values recorded for both the leaf-blade and leaf-petiole, compared to the stem, showed that they were of better quality than the latter.  The decrease in b-values is expected with maturity, but the similarity in b-values for leaf-petiole could not be fully explained either although, the high a-values at 12 WAP recorded for leaf-petiole could be responsible for this observation.  The implication is that, varieties with very high proportions of leaf would likely maintain similar levels of b-values at both 12 and 20 WAP.  The low degradability for stem and high degradability for leaf-petiole reported are reflections of the proportions of lignin in their cell walls.

The observed decrease in PD-values for leaf-blade at 20 WAP suggests that the pattern of lignification in the leaf-blade was highest, while it was least in the leaf-petiole.  Earlier works with sweet potato foliage, based on in vivo and in vitro DM digestibility, gave values between 640 g kg^-1 DM and 740 g kg^-1 DM (Ffoulkes et al 1978; Woolfe 1992; Orodho et al 1996).  These studies, however, reported only values for the whole-plant tops without details on the values for leaf-blade, leaf-petiole and stem, as separately report in the present study.  Verbic et al 1995 observed from their study that the leaf components of maize hybrids were better utilized than the stem components.  Although leaf is, generally, considered more digestible than stem of similar age in most forages, stems in rice straws have been found more digestible than the leaves (Ørskov 1991; Vadiveloo 1995).  This confirms the existence of both species and varietal differences in the utilization of the various foliage components.  The PD-value reported from the study agrees, in part, with these findings.  This is because the quality of the foliage is believed to be an aggregate of leaf-blade, leaf-petiole and stem qualities that occur at different ratios for different varieties.  The higher DM degradation characteristics at 20 WAP for leaf-petiole and stem could be linked with their recorded higher CP but lower NDF at 20 WAP (Table 3).

Correlation between nutrient contents and rumen degradation

Crude protein contents had a greater influence on the a-value and b-value at 12 WAP than at 20 WAP where no relationship was observed.  The changes in a-value as depicted in Figure 1 are in agreement with the above observation.  On the other hand, the NDF contents at 12 and 20 WAP had negative correlations with the b-value and PD-value although; there was a greater impact at 20 WAP.

Conclusion 

• The study has shown the interaction between the crude protein and neutral detergent fibre contents and the rumen dry matter degradation characteristics (soluble fraction and degradable fraction) and the observed correlations are informative for sweet potato production and the management of the crop for incorporation into crop and livestock systems for increased productivity under small-scale farming.

Acknowledgement 

The grants and facilities for the study were provided by the International Livestock Research Institute, Ibadan (ILRI-Nigeria).  Also, the assistance of Messrs T E Adeniya, S Ayodabo, M Olayiwola and other ILRI-West Africa staff in data collection are all appreciated.

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Received 9 September 2008; Accepted 16 June 2009; Published 1 September 2009

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