Effects of inclusion levels of sweet potato vine silage in the concentrate mixture on feed intake, milk yield and composition of crossbred dairy cows fed natural pasture hay

Kassu Tsegaye¹*, Yoseph Mekasha² and Merga Bayssa¹

¹ School of Animal and Range Sciences, College of Agriculture, Hawassa University, Hawassa, Ethiopia
² Agricultural Transformation Institute, Addis Ababa, Ethiopia.
* kasuk@hu.edu.et

Abstract

Sweet potato vine silage is rich in protein and is well suited for feeding ruminants; however, the effect of the inclusion of sweet potato vine silage in the concentrate mixture on the performance of dairy cows has not been well documented. This experiment was conducted to evaluate the effect of supplementation with sweet potato vine silage and concentrate mixture on the feed intake, milk yield and composition of crossbred dairy cows. An on-farm feeding trial was conducted on sixteen lactating crossbred dairy cows of uniform parity and stage of lactation using a randomized complete block design (4 treatments and 4 replications). The average body weight and initial milk yield were 425.75 ± 1.97 kg and 9.25 ± 0.31 liters/cow/day, respectively. The treatments included natural pasture hay provided ad libitum as a basal diet supplemented with a concentrate mixture alone (SPVS0), 10% sweet potato vine silage in the concentrate mixture (SPVS10), 20% sweet potato vine silage in the concentrate mixture (SPVS20), or 30% sweet potato vine silage in the concentrate mixture (SPVS30). Total DM intake (hay + supplement intake) was comparable across treatments. Hay intake was greater at SPVS0 and SPVS10 than at SPVS20 and SPVS30, while supplement intake was lower at SPVS0. The SPVS10, SPVS20, and SPVS30 treatment groups, in which sweet potato vine silage was included, had similar supplement intakes. The daily milk yield increased with increasing levels of sweet potato vine silage. Milk fat and solid-not-fat differed across the four diets. Cows on diets with 0, 10, or 20% sweet potato vine silage, including the daily ration lost weight. The inclusion of 30% of the sweet potato vine silage in the concentrate mixture resulted in the highest performance of crossbred dairy cows. Therefore, further study is required to evaluate the effect of the inclusion of a higher level of sweet potato vine silage in the concentrate mixture on the performance of lactating crossbred dairy cows.

Keywords: crossbred dairy cow, milk production, protein supplement, sweet potato vine silage

Introduction

In Ethiopia, the introduction of high-yielding dairy cattle, mainly Holstein Frisian and Jersey cattle, from abroad and crossbreeding with indigenous cattle have been pursued to improve milk production (Emebet and Zeleke 2008; Azage et al 2010). The large cattle population, the suitable climate for improved and high-yielding cattle breeds, and the relatively good environment of Ethiopia result in substantial potential for dairy development (Zelalem 2012). Despite its potential for dairy development, the productive and reproductive performance of dairy cattle is low due to inadequate and low-quality feed resources; poor nutritional, genotypic, and health care management; poor marketing systems; insufficient training; and weak extension services (Azage et al 2013).

The majority of ruminant diets consist of native pasture and crop residues. These roughages are characterized by high fiber content and low concentrations of protein. When fed alone, these roughages have poor digestibility, which results in low DM intake and milk yield (Reynal and Broderick 2003). Hence, supplementing dairy cows with protein sources is necessary for improved productivity and resource efficiency (Mutsvangwa et al 2016). However, supplementing dairy cattle with conventional protein source is difficult for smallholder dairy producers due to lack of access and costly, and thus, it is important for search unconventional feed resources where are cheap and rich in protein. Hence, before introducing unconventional feed ingredients to dairy cow rations, the effects on feed intake, milk yield, and composition need to be evaluated.

On the other hand, the efficient utilization of crop residues by ruminants is also difficult because these residues are high in fiber and low in protein. Thus, effective and economical sources of energy and nitrogen (N) are needed to supplement low-quality roughage diets for ruminants. Oil seed meals and cereal grains are effective supplements (Khan and Kedir 2008). The common protein source for dairy cattle is agro-industrial byproducts (AGIBPs), such as oilseed cakes; however, there is a shortage of such concentrates in rural areas of Ethiopia (Tolera et al 2012) and other developing regions. In Ethiopia, most agro-industries are located around large towns and work less than 50% of their capacity (Tolera et al 2012), and AGIBP contributes only 2.03% of the total livestock feed (CSA 2021). Therefore, the development of agro-industries and improving their efficiency could reduce the gap between the demand and supply of protein supplements. In addition to promoting agro-industries, the introduction and development of forage legumes in rural areas, without competing with food crops, could provide alternative protein supplements. The use of forage legumes, such as cowpea, in dairy cow rations can reduce the cost of feed without affecting milk yield or composition (Corea et al 2017). Supplementation of dairy cows with Calliandra or Sesbania is associated with an increase in milk production of at least 1 L/kg of feed (Makau et al 2020). Hence, improved forage legume production could provide useful nutrients in areas where agro-industrial byproducts are unavailable (Tolera et al 2012).

The sweet potato is important due to its potential as livestock feed. To date, research on sweet potato plants has focused mainly on tuber production. There are very few reports on the production of sweet potato vines and their nutritive value for livestock feed. The vines comprise both leaves and stems. These sweet potato vines can be effectively utilized as dairy cattle feed. The crude protein content of the leaf part is between 26 and 33% of the DM, while the CP content of the stem is between 10 and 14% (Ishida et al 2000). The inherent characteristics of sweet potato vines include high levels of water-soluble carbohydrates, palatability, crude protein, and high digestibility (> 62%) (Ali et al 2019). The crude protein level of sweet potato forage can range from 180 to 300 g/kg dry matter among different varieties; whereas the crude fiber content is approximately 180 g/kg DM. Phesatcha and Wanapat (2012) reported improved milk yield in Holstein crossbred cows fed Pelleted sweet potato vines. Additionally, Murugan and Nedunchezhiyan (2012) indicated that ensiling sweet potato vines eliminated trypsin inhibitors, leading to improved protein digestibility. In the rainy season, vines are also available in many peri-urban and urban food markets (Katongole 2008; Katongole 2011; Kabirizi 2017). However, due to their high moisture content and bulkiness, many sweet potato vines cannot be preserved for a very long period because they rot and deteriorate in nutritional value. To facilitate the use of sweet potato vines beyond the rainy season and to bridge the feed gap in the dry season, there is a need to conserve sweet potato vines in the form of silage. Traditionally, sweet potatoes can be grown two to three times per year, producing large quantities of sweet potato vines that can be ensiled. Therefore, no known studies in the Sidama Region of southern Ethiopia have assessed the performance of crossbred dairy cattle fed sweet potato vine silage under smallholder dairy production systems. Therefore, the purpose of this study was to evaluate the effects of different inclusion levels of sweet potato vine silage in the concentrate mixture on the feed intake, milk yield and composition of lactating crossbred dairy cows fed natural pasture hay.

Materials and methods

Study site

The study was conducted at a dairy farm located in Bensa District, which is located in the Sidama National Regional State of Ethiopia. The site is located at 32098' E and 34029'E latitude and 8008'N and 809'N longitude. The district is located 420 km southeast of Addis Ababa and 135 km northeast of Hawassa city, the capital of the Sidama Region. The study area is situated at altitudes ranging from 1452 to 3129 meters above sea level and receives a mean annual rainfall of approximately 1208.5 mm. The average annual temperature in the district is 19°C (Bensa Woreda Pilot Learning Site Diagnosis and Program Design, LIVES (2012)).

Experimental animals, design and treatment

A total of sixteen lactating crossbred dairy cows of uniform parity and stage of lactation with an average milk yield of 9.25 ± 0.31 liters and average body weight of 425.75 ± 1.97 kg were selected from dairy farms located in the Bensa district. The experimental animals were randomly assigned to the treatments in a randomized complete block design (RCBD). Cows were blocked based on parity and randomly allotted to one of the four dietary treatments, with four animals in each treatment. The proportions of supplement and feed ingredients are presented in Table 1. On average, 6.65 kg of natural pasture hay was offered ad libitum (at 20% refusal), supplemented with a concentrate mixture alone (SPVS0), 10% sweet potato vine silage in the concentrate mixture (SPVS10), 20% sweet potato vine silage in the concentrate mixture (SPVS20), or 30% sweet potato vine silage in the concentrate mixture (SPVS30). The treatments (Table 1) were iso-caloric and iso-nitrogenous and were formulated to satisfy the energy and crude protein requirements of a dairy cow weighing 454 kg and producing 10 liters per day of milk with 5% fat (NRC 2001). Before the commencement of the experiment, to ensure that the nutrient composition of the experimental diets satisfied the nutrient requirements, the total dry matter intake was estimated (NRC 2001), and the experimental diet was formulated by fixing at a 52:48 hay-to-supplement ratio. The amount of supplement used varied between 6.2 and 6.9 kg among the treatments. The proportions of feed ingredients and the chemical compositions of the experimental diets and feed ingredients are presented in Table 1.

Animal management, feeds and feeding

Before the inception of the experiment, the experimental animals were treated for internal and external parasites with tetraclozan and ivermectin, respectively. The experimental animals were housed in individual pens and had free access to clean drinking water. The experimental feeds were selected based on their availability in the study area. Natural pasture hay was received from the farm at the experimental site. Secondary data were used to formulate the concentrate mixtures (Seyoum et al 2007; NRC 2001). The ingredients used to formulate the concentrate mixture were purchased from the Hawassa Elito Farming Union. The sweet potato variety Hawassa-83 was selected over the other varieties because it is available in the study district and because of its early maturation and nutritional and silage qualities. Sweet potato vine cuttings were obtained from the southern agricultural research farm and planted on 1 ha of land from April 1, 2021, to June 30, 2021, for the experimental site farm. The spacing between rows was 60 cm, and that between plants was 30 cm. Planting, weeding and harvesting were uniformly performed by the follow-up of the researcher. The vine was cut manually at 3 cm above ground 120 days after planting using a knife. After one day, the tuber was removed by a fork, and dirty materials were removed manually by hand. The sweet potato vines were chopped to a size of approximately 2-5 cm according to the suggestion of Giang et al (2004), wilted under the sun for 12 hours and turned 10 times per day. The dry matter at ensiling was approximately 30%-35%. The process of ensiling was carried out according to the method suggested by Kung and Shaver (2001). The sweet potato tubers used were washed with tap water to remove sand and sliced manually into small pieces (approximately 1 cm) using a knife, and the chopped vine and tuber were weighed and mixed on plastic sheets at a ratio of 70%:30% vine to tuber on a fresh basis, according to the recommendation of Stathers et al. (2005). Finally, the 525 kg mixture of sweet potato vine silage per cow was placed inside an impermeable sack lined with plastic, and the mixture was compacted with a wooden stick. The sack was tightened with rope fibers, covered with strong plastic to ensure anaerobic fermentation and placed above ground to reduce fermentation during heat development. The material was considered ready for use after 90 days of ensiling. The animals were then supplemented with sweet potato vine silage at levels of 0, 10%, 20% and 30% of the total daily ration to constitute the four treatment diets (Table 1). The supplemental feed was offered first after the morning milking before providing natural pasture hay. Sweet potato vine silage and each feed ingredient in the concentrate were measured daily, thoroughly mixed for each treatment and fed to the cows in two equal rations at 0700 and 1500 h daily for an adaptation period of 15 days, followed by an actual 90-day feeding experiment. The quantity of supplements (both silage and concentrate) offered and refused per day should be recorded properly for both the adaptation period and actual experimental period. The daily feed intake of supplemental feed on a DM basis was determined by the difference between the feed offered and that of the refusals.

Feed and milk sample analysis

The feed samples were dried in a forced-air oven at 60°C for 48 h and ground to pass through a 1 mm sieve using a laboratory Wiley mill (Thomas Scientific). The ground samples were kept in sealed plastic bags at room temperature until analysis. DM and ash were analyzed according to AOAC (2005). In vitro dry matter digestibility (IVDMD, NDF and ADF) was determined according to the methods of Van Soest and Robertson (1985) using an Ankom 220 fiber analyzer (Ankom Technology, Macedon, NY). The acid detergent lignin (offered feed samples only) concentration was determined according to Van Soest and Robertson (1985). The nitrogen (N) content was measured by the Kjeldahl method (AOAC 2005), and the CP concentration was calculated as N×6.25. The ME of a particular feed was estimated from the IVDMD based on the following equation: ME (MJ/kg DM) =0.15*IVDMD (g/kg) (MAFF 1984). The milk yield was measured using a graduated cylinder, and samples were collected twice daily at 0600 and 1500 h during a fortnightly period of 90 days. Milk samples (50 mL) were kept in a water bath at 28°C in the laboratory and blended using a magnetic stirrer; then, duplicate samples from each milking were analyzed using a near-infrared instrument (Milkotronic-ultrasonic milk analyzer, Milkotronic Ltd.), and weekly average values for milk composition were calculated for each cow. The 4% fat-corrected milk yield (FCM) was calculated according to Eastridge (2022): FCM (kgday^-1) = (0.4 milk yield) + (15 fat yield). Feed conversion efficiency was calculated by dividing FCM by DM intake.

Body weight changes and body condition scoring

The live body weights of the animals in each treatment were determined using heart girth tape, and body weights were taken at the beginning of data collection and subtracted from the final weights at the end of each treatment to compute the live weight changes of the animals on the different diets. Body condition scores were taken at the beginning and end of each data collection period following the 15-d adaptation period. Body condition scores were taken according to a 5-point scale, with a 0.25 increment. A body condition score of 1 denotes a very thin cow, 5 a severely over conditioned cow, and 3 a medium cow (Heinrichs et al 2021).

Statistical analysis

The data on supplemental feed intake, milk yield and composition, live weight change and body condition score were subjected to analysis of variance (ANOVA) for RCBD (SAS 2008). Means were separated using the least significant difference (LSD) test. The model used for analysis was Yijk = µ + ai + ßj + eijk, where µ = overall mean; Yijk= observation of the jth block and the ith treatment; ai = effect of treatment i; ßj = effect of block j; and eijk = experimental error.

Results

Dry matter and nutrient intake

The dry matter and nutrient intake of the lactating crossbred dairy cows fed the experimental diets are presented in Table 2. There was a general increase in total dry matter and organic matter intake with increasing levels of sweet potato vine silage supplementation in the concentrate. The mean dry matter and organic matter intake of cows supplemented with 30% sweet potato vine silage were greater than those of the other treatment groups. Total dry matter and organic matter intake were lowest for the cows that received no sweet potato vine silage and highest for the cows that received the highest level of sweet potato vine silage in the concentrate. Crude protein intake increased with increasing sweet potato vine silage supplementation. There was a significant difference in neutral detergent fiber and acid detergent fiber intake among the treatments. The effects on NDF and ADF intake did not follow a consistent trend.

Milk yield, milk composition and feed efficiency of the experimental cows

The daily milk yield was highest for cows fed the highest level of sweet potato vine silage in the concentrate and lowest for those cows fed no sweet potato vine silage. The daily milk yield increased with increasing sweet potato vine silage supplementation within the concentrate (Table 3). The fat-corrected milk yield varied in a similar way as the daily milk yield, being lowest in cows that did not receive sweet potato vine silage supplementation in the concentrate. The percentage of milk protein was similar among the treatments. The milk protein content was lower for those cows fed no sweet potato vine silage and highest for those cows fed the highest level of sweet potato vine silage. Considering the milk components, milk fat and nonfat solids were affected by the different diets, although the results did not show any consistent trend across the varying levels of sweet potato vine silage supplementation in the concentrate.

Figure 1. Milk yield trends of the lactating dairy cows fed increased level of sweet potato vine silages in the concentrate

Body weight changes and body condition score

A reduction in the body weight of the experimental cows was recorded at SPVS0, SPVS10 and SPVS20. The reduction in body weight in this group of cows might be attributed to the fact that nutrients are utilized for milk production rather than for tissue accretion. This might be because the highest portion of nutrient intake might be converted to milk rather than body tissue. Improved body weight change was recorded in 30% of the sweet potato vine silage supplemented with the daily rations of the cows (SPVS30).

Discussion

Dry matter and nutrient intake

The increase in dry matter intake with increasing levels of sweet potato vine silage supplementation observed in this study is in agreement with the findings of an earlier study (Megersa 2016) in goats. Dry matter intake and digestibility depend on the fermentability of the energy source. When all the necessary nutrients are supplied to microbes, especially nitrogen, which is usually the most limiting nutrient, the microbial population will increase and ferment feed at a faster rate, thereby creating space for more feed intake. The lower level of neutral detergent fiber in sweet potato vine silage than in natural pasture grass hay also led to effective degradation and hence better feed intake for cows that received sweet potato vine silage supplementation. High levels of neutral detergent fiber in feeds limit their intake. Although diets based on byproducts usually contain lower amounts of degradable starch, they might be rich in other nutrients, such as cell wall soluble components, rumen undegradable protein and potentially digestible fiber (Hall MB and Herejk C 2001). The improved dry matter intake of the cows that received sweet potato vine silage subsequently led to the observed increase in milk yield. In general, feed intake is one of the most important parameters used to evaluate the quality of diets (Coutinho et al 2014). Hence, the increasing trend of feed intake with increasing levels of sweet potato vine silage supplementation observed in the present study suggested that 30% of sweet potato vine silage can be included in the concentrate mixture of lactating dairy cows.

Milk yield and Milk composition of lactating dairy cows supplemented with different levels of sweet potato vine silage in the concentrate

All the cows used for this experiment were healthy during the entire experimental period. The results of the effect of dietary treatments on the average daily milk yield and composition are presented in Table 3. Sweet potato vine silage-supplemented cows produced significantly more milk (p<0.05) than cows fed concentrate alone. Similar results were also reported by Galla et al (2020), who reported that crossbred cows fed different levels of sweet potato vine silage had significantly greater milk yields than no supplemented crossbred cows. The milk yield was greater and similar in cows that were fed different sweet potato vine silage supplementation levels. These findings are in agreement with those of Khalid et al (2013) and Okalebo et al (2002). This was due to increased dry matter intake and hence increased nutrient ingestion and nitrogen utilization with sweet potato vine silage supplementation (Ali et al 2019). Higher milk production requires greater feed intake (Kaufman et al 2018); hence, the increased milk yield observed in the present study could be related to increased dry matter intake with increased inclusion of improved forage in the diet (Mekuriaw et al 2020). The fat-corrected milk content improved with increasing sweet potato vine silage supplementation. The increase in fat-corrected milk is most likely a result of increasing levels of fibrous material with increased dry matter intake in diets known to balance the acetate-to-propionate ratio in the rumen liquor, as acetate is a major milk fat precursor (Lu et al 2005). There was no difference in the fat content of the milk of cows among treatments, which might be related to the lack of differences in the neutral detergent intake of cows in all treatment groups. It has been noted that cows with low roughage rations yield milk with lower fat content than cows fed a higher proportion of roughage diet (Moran 2005). There was also no difference in the lactose content of the milk among the treatment groups. This is expected, as the lactose content of milk remains unaffected by dietary changes (Moran 2005).

Average body weight changes and body condition score of the lactating dairy cows fed the experimental diets

There was a noticeable loss in body weight for cows in the daily ration at 0, 10% and 20% sweet potato vine silage supplementation. Animals usually lose weight as they mobilize body reserves, possibly due to a negative energy balance to produce milk and/or due to increasing levels of milk yield (Poncheki et al 2015). The body condition score is strongly related to milk yield (Souissi and Bouraoui 2020); similar to the milk yield, the body condition score was not affected by the inclusion of sweet potato vine silage in the concentrate mixture.

Conclusion

The inclusion of different levels of sweet potato vine silage as a substitute for a concentrate mixture increased the dry matter intake, milk yield and composition of crossbred dairy cows. Sweet potato vines can be preserved in the form of silage to yield silages with high dry matter content, increasing dry matter intake by livestock and increasing crude protein content for microbial protein synthesis. Sweet potato vine silage is therefore a valuable feed resource that can lead to improved animal performance. On-farm evaluation of feed supplements for lactating crossbred dairy cows in the present study revealed a significant difference (p<0.05) between the treatment groups in terms of milk yield and the composition of milk fat, lactose and solid-not-fat. In this regard, 30% sweet potato vine silage was found to have the greatest effect on milk yield, milk composition and weight gain. All the parameters studied significantly increased with increasing levels of sweet potato vine silage supplementation. The effects were more pronounced at the highest level of supplementation. This is because sweet potato silage has high in vitro dry matter digestibility. The findings of the present study support the use of sweet potato vine silage as an alternative protein supplement to agro-industrial byproducts in rural settings, where sweet potato vines are widely produced.

Acknowledgments

The authors greatly acknowledge the partial financial support of the NORAD - ICP (Norwegian Agency for Development - Institutional Collaboration project), which is working together with Hawassa University.

Conflicts of interest

The authors declare no conflicts of interest regarding the publication of this paper.

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