Yield and quality of sweet potato forage pruned at different intervals for West African dwarf sheep

O A Olorunnisomo

Department of Animal Production and Health Sciences, University of Ado-Ekiti, PMB 5363, Ado-Ekiti, Nigeria
sholanisomo@yahoo.com

Abstract

In order to study the effect of cutting management on yield and quality of sweet potato (SP) forage grown for dry season feeding of sheep, SP vines were subjected to four cutting regimes: pruning at 4, 6, 8 weeks interval and uncut plots (control), using a randomized complete block design. Cut forages were fed to twelve mature West African dwarf (WAD) sheep confined in metabolic pens for 14 days to determine the digestibility.

Dry matter (DM) yield of forage was 8.6, 12.0, 12.2 and 8.1 t/ha while biomass yield was 12.9, 17.0, 18.1 and 17.0 t/ha for 4, 6, 8 weeks cutting interval and control plots respectively. Crude protein (CP) content of the forage was 26.7, 25.0, 21.8 and 20.1% while DM digestibility was 75.7, 68.7, 65.2 and 64.0% for the respective cutting intervals. Frequent cutting of SP vines (4 weeks interval) did not significantly (P > 0.05) improve forage yield whereas root yield and total biomass production were significantly (P < 0.05) depressed. Pruning at longer intervals (6 and 8 weeks) significantly (P < 0.05) improved forage yield at the expense of root yield. Biomass production was however, not significantly (P> 0.05) affected. CP content of forage increased as frequency of cutting increased while the fibre content reduced. DM and nutrient digestibility of the forage improved (P < 0.05) with more frequent cutting of the vine. The number of animals supported by one hectare of SP crop increased as interval between cuttings increased although quality of forage was compromised.

Pruning at 6 weeks interval optimized the yield and quality of SP forage fed to WAD sheep.

Keywords: forage, pruning, quality, sheep, sweet potato, yield

Introduction

In the southwest of Nigeria, the dry season covers a period of four months, which corresponds to a time of feed scarcity for ruminant animals. During this period, quality and yield of grasses from natural pasture is low. The poor quality of conserved grasses from natural pastures does not justify the labour and cost of preserving these forages (Crowder and Chedda 1982). Sweet potato is a high yielding and quality fodder that can be preserved for ruminant feeding during the dry season in Nigeria.

Sweet potato is traditionally a root crop (Ruiz et al 1981); the top however is also valuable forage for ruminants and other livestock species (Backer et al 1980; Figueroa and Rodriguez 1994; Gonzalez et al 2003; Giang et al 2004). Under improved cultivation, sweet potato is capable of very high dry matter yield per unit area of land (Moat and Dryden 1993; Rashid et al 2000). Sweet potato vine has a high crude protein content (18-30% in DM), which is comparable to leguminous forages (An et al 2003; Mupangwa et al 1999 1997; Farrell et al 2000).

Yield and quality of forage species vary with the age of the plant. Dry matter accumulation usually increases with increasing age while the nutritive value declines (Crowder and Chedda 1982). Moat and Dryden (1993) reported an increase in dry matter yield of sweet potato, a decrease in protein content, and a fairly constant NDF content in sweet potato forage as the age of the plant increased. Cutting of forage at regular intervals is a potent agronomic tool used in maintaining a balance between yield and quality in forage species (Crowder and Chedda 1982; Whitney 1970; Olubajo 1974; Humphreys 1987). Removal of sweet potato vines during growth however reduces the supply of photosynthates during the remainder of the plant's growth with an eventual reduction in root yield. (Nwinyi 1992).

This experiment was designed to evaluate the effects of cutting sweet potato vines at different intervals on yield and chemical composition of the forage. Since sweet potato is also a root crop, the effects of cutting on root production were also examined.

Materials and methods

The experiment was conducted on a field that had been left to fallow for a year after several years of cultivation to maize and cassava crops. The experiment spanned a period of 216 days. Total rainfall for the year was 1570mm and mean temperature and humidity were 27.4^oC and 81.4% respectively. Samples of soil in the top 30cm were taken from the experimental area before the commencement of the experiment and bulked for laboratory analysis. Available nutrients were determined using standard chemical methods (AOAC 1995). A dual-purpose variety of sweet potato (TIS-Ex-Igbariam) collected from the National Root Crops Research Institute (NRCRI), Umudike, Nigeria was used in this study. The vines were cut into 30cm pieces with a minimum of 4 nodes. The plot size in this experiment was 4 x 6m. The plots were arranged along the contour of the field in such a way that the longer part was at right angles with the slope of the land. A space of 2m was maintained between the plots and trailing vines were regularly re-directed to their respective plots. A planting distance of 50cm within the row and 80cm between rows was adopted in this study, giving rise to a population 60 plants / plot or 25000 plants / ha.

Sweet potato vines were subjected to the following cutting regimes (treatments):

• 4 weeks cutting interval

• 6 weeks cutting interval

• 8 weeks cutting interval

• Uncut plots (control)

Each treatment was replicated three times and was randomly assigned to plots within a block. The experimental design adopted for this experiment was the randomized complete block design.

The plots were ridged using a hoe. Fertilizer was applied to the plots in three splits as 300kg/ha of NPK-15-15-15 at planting and 75kg/ha of urea each at 90 and 120 days after planting (DAP). Weeding was done two to four times before final harvest using a hoe. Uncut plots were weeded only two times while the most frequently cut plots (4 weeks) were weeded up to four times. Sweet potato top was harvested from the plots one to five times depending on the frequency of the cutting. Uncut plots were harvested only once while the most frequently cut (4 weeks) was harvested five times. Forage was harvested from control plots at 150 DAP while other plots (4, 6, 8 weeks) were initially cut at 90 DAP, to allow for the tuber bulking phase of the plant, and subsequently at the specified intervals. Roots produced after the final forage removal were also harvested and yield per plot recorded. Cumulative forage yield, residual root yield and biomass production were determined for each treatment. Samples of the forage taken from the plots during each harvest were dried in the oven at 65^oC to constant weight, bulked for each plot and chemical composition determined as an average from three plots. Freshly harvested forage was chopped using a locally fabricated cutter, sun dried and stored for the digestibility trial.

Twelve male WAD sheep aged approximately one year and weighing between 14.5 and 16.8 kg were used to estimate the digestibility of sweet potato forage cut at the different intervals. The animals were placed in individual pens with floors adapted for faecal collection. Sun dried sweet potato top and fresh water were offered ad libitum for 14 days. Animals also had access to saltlick. Total faeces and feed refused were collected and weighed in the last 7 days. Ten percent of faeces collected were kept for analysis. Forage and faecal samples were dried in the oven at 65^oC to constant weight, milled, and stored in airtight containers until required for analysis.

Crude protein content of samples was determined as N x 6.25 using the Kjeldahl method (AOAC 1995) while fibre and lignin fractions was determined by methods of Van Soest and Robertson (1985). The gross energy of the forage and faeces was calculated from organic matter components of the material by the relationship described by Nehring and Haenlein (1973). Data obtained were subjected to analysis of variance and Duncan's multiple range tests using the SAS (1995) procedures.

Results and discussion

Table 1 shows the soil analysis of the experimental field before commencement of the study. Physical examination reveals that the soil texture was sandy loam.

The cumulative forage and residual root yield from sweet potato plots harvested at different intervals are presented in Table 2

Dry matter yield

Frequent cutting at 4 weeks interval did not significantly (P>0.05) improve forage yield of sweet potato when compared to control, however, the root and total biomass yield were significantly (P < 0.05) depressed. Cutting sweet potato vines at 4, 6, 8 weeks interval reduced the root yield by 50, 41, and 31% respectively. Frequent defoliation of sweet potato plant disrupted the photosynthetic process, leading to a reduced leaf, root and biomass production (Dahniya 1979). This result agrees with other reports that defoliation had a negative influence on root production in sweet potato (An et al 2003; Kiozya et al 2001 and Ruiz et al 1980). At longer cutting intervals (6-8 weeks) yield of sweet potato forage increased significantly (P < 0.05) when compared to control while the root yield was significantly (P < 0.05) depressed. This agrees with the findings of Uddin et al (1994) who reported that forage yield increased with delayed cutting while root yield was depressed. Biomass production of sweet potato cut at 6 and 8 weeks interval were however not significantly (P > 0.05) different from control. Increasing the interval between cuttings gave the plant sufficient time to recover from the previous cutting. Since biomass production among these treatments was not significantly different, it may be inferred that cutting did not alter biomass production in sweet potato but re-partitioned dry matter accumulation to favour leaf production at the expense of the root. This is in agreement with the findings of Dahniya (1979) and Mannan et al (1992).

Chemical composition

The chemical composition and gross energy content of sweet potato top cut at different intervals is given in Table 3.

Results show that there were no significant differences (P > 0.05) among the treatments for dry matter, ether extract, crude fibre, nitrogen free extract, ash and gross energy content of sweet potato forage. However, the crude protein, neutral detergent fibre, acid detergent fibre and lignin components of the forage differed significantly (P > 0.05) among the various treatments.

Crude protein content of sweet potato forage increased as cutting interval became shorter while the fibre components reduced. This is in agreement with the findings of Ruiz et al (1980) who also reported an increase in CP content of sweet potato forage when the vine was cut more frequently. Oyenuga (1968) and Moat and Dryden (1993) also reported high protein content in young sweet potato vines. The gross energy content of the forage remained constant across the cutting frequencies. This suggests that cutting did not alter gross energy content of sweet potato forage but stimulated dry matter partitioning to favor protein accumulation at the expense of the fibre component in the forage.

Digestibility

The whole tract digestibility of sweet potato forage cut at different intervals using sheep is presented in Table 4.

Dry matter digestibility improved significantly (P < 0.05) when forage was cut at 4 and 6 weeks interval, however there was no significant (P > 0.05) difference in DM digestibility of forage cut at 8 weeks interval and control plots.

The digestibility of CP, NDF, and gross energy followed the same trend as DM digestibility. The DM digestibility of sweet potato forage ranged from 64.01 - 75.72 %. The forage cut at 4 weeks showed the highest digestibility while the control, showed the least digestibility. The digestibility of ADF, lignin, and gross energy followed a general trend in which digestibility increased as the cutting interval became shorter. Improved digestibility of the forage is associated with higher protein and reduced fibre in the forage as frequency of cutting increased (An et al 2003; Meyreles and Preston 1978).

Carrying capacity

The carrying capacity for one hectare of sweet potato crop fed to sheep is presented in Table 5.

The number of animals supported by one hectare of sweet potato crop was highest at 8 weeks cutting interval, and least at 4 weeks cutting interval. The quality of the forage was however, highest at 4 weeks cutting interval. The carrying capacity of sweet potato crop increased when the total biomass production was used as the basis of feeding rather than forage alone.

When quantity of forage available for feeding is judged to be more important than quality of the forage, an interval of 8 weeks may be adopted for cutting management of sweet potato, however where quality of the forage is also important, an interval of 6 weeks may be adopted to optimize yield and quality of the forage.
 

Conclusions

• Cutting sweet potato vines at regular intervals improved forage production at the expense of root yield.

• Forage yield and biomass production from sweet potato were highest at 8 weeks cutting interval while the quality of the forage was highest at 4 weeks interval.

• When sweet potato is cultivated for forage production, cutting at 6 weeks interval is recommended to optimize the yield and quality of the forage.
   

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