Introduction

Livestock play a significant role in rural livelihoods and the economies of developing countries (Enahoro et al., 2019), and these livestock depend on rangelands mostly in semi-arid areas. Browse species as forage are used in the feeding of domestic (e.g., goats) and wilderness ruminants and are generally regarded as the cheapest source of nutrients for these ruminants.

The relationship between season and plants and livestock has been observed around the world especially in semi-arid areas (Hassen et al., 2017; Mudzengi et al., 2020; Castro et al., 2021). This is due to the fact that climatic conditions in semi-arid regions are likely to increase the competition of resources among livestock (Mudzengi et al., 2020). Due to seasonal variation, plant growth and accumulation of leaves can be affected negatively or postively which will either lead to a shortage or an abundance of available forage for livestock (Hassen et al., 2017). For example, during the drought season herbaceous layer tends to deteriorate its nutritional content and livestock happen to rely on browse species for nutritional needs. This will normally have an influence on ruminal fermentation of substrates, and the ruminants exposed on these low-quality forages will have difficulties meeting their maintenance requirements on certain seasons leading to low feed intake and ruminal fermentation on ruminants and this may lead to poor growth rate (Ravhuhali et al., 2011). The browse species are useful and inexpensive source of nutrients for livestock especially during drought seasons and they can be the alternative to commercial feed resources in most communal areas.

The nutritional value of forage depends on the amount of proteins, the concentration of acid detergent lignin and digestible carbohydrates, and these nutrients normally vary with seasons. Ravhuhali et al. (2020) stressed that leaves of many browse species have crude protein concentration of above 10%. With the comparison to conventional diets, browse leaves are known to have the potential to be used as protein supplements to diets that have low protein and fermentable energy values (El Hassan et al., 2000). Apart from the crude protein content of plants species, the chemical estimates such as dry matter digestibility (DMDigest), total digestible nutrients (TDN), relative feeding values (RFV), digestible energy and metabolisable energy vary with the kind of ruminant livestock and are of great importance for the animal’s nutritional demands (NRC, 1985). Forages with high concentration of lignin are also known to have a low TDN than other fodder crops. The chemical estimates which are related to chemical composition varies widely among the woody species. Dry matter digestibility of browse species ranges from 58 to 75% (Boufennara et al., 2012; Mokoboki et al., 2019), total digestible nutrients of 50–65% and relative feeding value of up to 95% (Mokoboki et al. 2019).

However, in spite of their abundance, many wild browse species have been generally undervalued mainly because of insufficient knowledge about their potential feeding value in different seasons. Exploring the nutritional value of the resources available in different seasons for browsing livestock is considered as one of the keys to the efficient and sustainable production of animals (Minson, 1990). Some of the ligneous species studied here are consumed to a certain degree by small ruminants (Kababya et al., 1998). There is a need for a laboratory analysis for better understanding nutritional composition and utilization of these indigenous browse woody species. Therefore, the objective of the study was to assess the effect of season and species on nutritive value and in vitro ruminal dry matter degradability of browse species found in the selected rangelands areas in semi-arid South Africa, while hypothesizing that there would be a variation in the parameters.

Materials and methods

Harvesting site and leaves

The study was carried out at Thulamela Local Municipality (Limpopo) with a very high number of indigenous ruminants (goats and cattle) that rely entirely on rangelands. The tree species were harvested in communal areas of Thulamela Municipality around Vhembe District in Limpopo province. The communal areas (Makuya and Lamvi S 22°39′49.85′′ and E 30°45′21.88) are located in the 40 to 50 km from Kruger National Park. In summer time annual temperatures is 34 °C and the winter time is 13 °C (SAWS, 2020). In December and early June 2019 leaves (ten tree per species) from randomly selected sixteen trees species (Vachellia karroo; Senegalia nigrescens; Vachellia nilotica; Balanites maughamii; Berchemia discolor; Berchemia zeyheri; Bridelia mollis; Combretum collinum; Combretum imberbe; Dalbergia melanoxylon; Dichrostachys cinerea subsp. africana; Grewia monticola; Grewia occidentalis; Melia azedarach; Ziziphus mucronata and Ormocarpum kirkii) were harvested before defoliation from the site by hand at a height of not more than 1.5 m. The samples were harvested from the same trees in the same area for summer and winter. Each of the samples was stored separately in labelled brown paper bag. Samples were dried at room temperature and transferred to the oven set at a temperature of 60 °C overnight, after which they were ground to pass a 1 mm sieve and kept in a tight plastic container for chemical analyses. Harvested species were also grouped according to growth form and herbivory (Table 1). Van Wyk et al. (2012) book was used to identify the species. Information on the woody species utilization was obtained from local farmers and herders from the respective grazing communal areas.

Table 1 Scientific and common names, growth form, herbivory and browse form of browse species identified in semi-arid areas of Limpopo province
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Chemical analyses

Organic matter (OM), ash and nitrogen content (Micro-Kjedahl method) were determined (AOAC, 1999). Crude protein was calculated by multiplying nitrogen value with a factor 6.25. Fibre fraction (neutral detergent fibre and acid detergent fibre) was determined using ANKOM2000 according to Van Soest et al. (1991) using ANKOM F57 bags. Acid detergent fibre (ADF) bags were soaked on 72% of sulphuric acid for ADL determination. The formula used to predict total digestible nutrients (TDN) was \(82.38-(0.7515 \times ADF)\) as described by Bath & Marble (1989). The formula for dry matter digestibility was DMDigest% = \(88.9-(0.779 \times \% ADF)\). Dry matter intake (DMI) was calculated using the NDF concentration and 2.5% of animal body weight as described by Mertens (2002). Relative feed value (RFV) was calculated using the following method: RFV = \((\%DMDigest \times \% DMI)/1.29\) (Jeranyama & Garcia, 2004). The equation for estimated digestible energy (DE Mcal/kg) = \(0.27 + 0.0428 (DMDigest\%)\) as reported by Fonnesbeck et al. (1984). DE values were converted to ME using the formula reported by Khalil et al. (1986) ME (Mcal/kg) = \(0.821 \times DE (Mcal/kg)\).

In vitro ruminal degradation

The in vitro ruminal DM degradability of leaf samples was determined using the DaisyII incubator consisting of a thermostatic chamber (39 °C) with four rotating jars according to ANKOM Technology for in vitro true digestibility. Ground samples were weighed into ANKOM F57 bags (0.45–0.5 g), heat sealed and placed in the digestion jars. Two buffer solutions were prepared in advance and combined at a ratio of 1:5, according to ANKOM Technology, and 1600 mL of the combined buffer was transferred to each of the four jars and warmed. Rumen inoculum was collected in the morning before feeding from a ruminally cannulated Bonsmara cow (metabolic weight = 550 kg). The cannulated cow was kept in a stall and had access to Buffalo grass and Lucerne. Strained rumen fluid was held at 39 °C under a stream of carbon dioxide gas. ANKOM F57 bags containing samples, were inoculated by adding 400 mL of rumen inoculum to each digestion jar containing buffer. Each jar was purged with CO2 before being covered and placed in the incubation chamber. Filter bags were withdrawn at 24, 36 and 48 h after inoculation and were washed with cold water for 20 min and dried for 12 h at 105 °C. The ethical approval certificate was acquired from the North West University Animal Research Ethic Committee (NWU-00126-13-A9). The animal was cared for according to the guidelines of the institution and Federation of Animal Science Societies (FASS, 2010) for the use of agricultural livestock in teaching and research activities.

Statistical analysis

Browse species data were analysed using a two-way factorial treatment design in a completely randomised design SAS (2010). The general linear model used was as follows:

$$Yijk=u+Bi+Sj+(BxS)ij+ \varepsilon ijk$$

where \(u\) was the overall mean, S was the effect of variation between species; \(Sj\) was effect of season; \(BxS\) was the interaction effect, and εijk is random error. The probability of difference (PDIFF) option in the LS means statement of SAS was used to compare the least square means.

Results

The differences (p < 0.0001) were observed on the effect of species, season and the interaction between species and season on ash, OM, CP, ADF, ADL, DMDigest, TDN, DE, ME and DMD36 (Tables 2, 3, 4 and 5). The significance differences were observed on the effect of species and season on NDF, DMI and RFV. The crude protein content ranged from 149 g/kg DM in B. mollis to 343.7 g/kg DM in M. azedarach in the summer season (Table 2), and ranges from 133 g/kg DM in Z. mucronata to 210.3 g/kg DM in B. maughamii in the winter season. Dichrostachy cinerea, B. discolor, C. collinum, D. melanoxylon, G. monticola, C. imberbe, V. karroo, M. azedarach, V. nigrescens and B. zeyheri had higher (p < 0.0001; F = 9187.6) CP content in summer than the same species in winter. In Table 3 NDF content ranged from 392 g/kg DM in M. azedarach to 572 g/kg DM in the summer, and in winter it ranged from 283 g/kg DM in B. zeyheri to 576 g/kg DM in V. karroo. With an exception to D. cinerea, G. occidentalis, V. karroo and M. azedarach; all other browse species had higher (p < 0.0001; F = 8.6) NDF in summer when compared to the same species in winter. Summer season greatly influenced (p < 0.0001; F = 45.2) ADL content of B. mollis, (502.3 g/kg DM); S. nigrescens (321.0 g/kg DM), V. nilotica (325.2 g/kg DM) while in winter D. cinerea (355.8 g/kg DM) had the highest (p < 0.0001; F = 502.7) ADL content. Bridelia mollis, B. maughamii, D. melanoxylon; C. imberbe, V. karroo, O. kirkii, M. azedarach, S. nigrescens; V. nilotica, G. occidentalis and B. zeyheri had higher (p < 0.0001; F = 502.7) ADL content in summer than the same species in the winter season.

Table 2 Effect of species and season on ash (g/kg DM), organic matter (g/kg DM) and crude protein (g/kg DM) on browse species found in semi-arid rangelands of South Africa
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Table 3 Effect of species and season on neutral and acid detergent fibres (NDF, ADF, g/kg DM) and acid detergent lignin (ADL, g/kg DM) of browse species found in semi-arid rangelands of South Africa
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Table 4 Effect of species and season on dry matter intake (kg/day), dry matter digestibility (%) and total digestible nutrients (%) of browse species found in semi-arid rangelands of South Africa
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Table 5 Effect of species and season on relative feeding value (g/kg DM), digestible energy (Mcal/kg) and metabolizable energy (Mcal/kg) of browse species found in semi-arid rangelands of South Africa
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For chemical estimates predictions in Table 4, C. collinum had the highest (p < 0.0001; F = 227.8) DMDigest % in summer. Within each species, in summer D. cinerea, B. discolor, D. melanoxylon, C. collinum; C. imberbe, G. monticola, V. karroo; O. kirkii, M. azedarach, V. nilotica; and Z. mucronata had the highest (p < 0.0001; F = 227.8) dry matter digestibility as compared to winter on the same species.

In summer, C. collinum had the highest (p < 0.0001; F = 11.4) TDN value and D. melanoxylon had higher (p < 0.0001; F = 11.4) TDN in winter.

The ME content ranged from 2.1 Mcal/kg in G. monticola and S. nigrescens to 2.9 in C. collinum in the summer season and 1.9 Mcal/kg in D. cinerea to 2.8 Mcal/kg in D. melanoxylon in the winter season (Table 5). In Table 6B. discolor; B. zeyheri and O. kirkii had better (p < 0.0001; F = 11.3) DMD values after 36 h withdrawal period in winter. Dichrostachys cinerea and G. monticola had the least (p < 0.0001; F = 11.3) DMD36 values post 36-h withdrawal period in winter samples. Bridelia mollis, B. discolor, C. collinum, C. imberbe, O. kirkii, S. nigrescens, V. nilotica, G. occidentalis and B. zeyheri had the same (p > 0.0001; F = 5.9) DMD48 values across two seasons.

Table 6 Effect of species and season on the in vitro dry matter degradability (DMD g/kg) (DMD24, DMD36, DMD48) of browse species found in semi-arid rangelands of South Africa
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Discussion

Chemical composition of browse species

Browse plants remain the major source of feed during the period of scarcity because they are capable of surviving and retaining most of their nutritional value during the dry season in most arid and semi-arid regions (Aruwayo & Adeleke, 2019). This qualifies them to be an alternative source of feed for ruminants as they are also easily accessible to farmers in communal areas who are financially constrained to buy commercial feeds. The browse species are diverse and have numerous fodder components which include leaves (green/brown), twigs, flowers and fruits of which have a much longer period of availability throughout all seasons. Chemical composition differences have been noted among different African vegetation types in semi-arid areas (Nsubuga et al., 2020). In this study a wide variation was noted among browse species in all parameters measured which is in line with Abebe et al. (2012). The current study revealed that, the ash concentration was better in winter than summer, with a range between 48 and 163 g/kg DM. These results are in line with those reported by Aganga et al. (2005) (32.5–95.8 g/kg DM). The summer result on Z. mucronata is higher than the one reported by Ravhuhali et al. (2020) (39.4 g/kg DM). Therefore, the high ash content found in these species gives an overall representation of the mineral content (organic) present in the feed material that is essential for improving growth in animals (Msiza et al., 2021). Seasonal change is vital in the overall availability of forage for animal consumption, especially during the months when plant growth is less active. Plant growth tends to be slower in the winter season because of the negative effect of low ambient temperatures on growth (Hassen et al., 2007), thus low rainfall (a limiting factor) and other climatic conditions impose additional constrains for the process of photosynthesis to take place, therefore less forage yield is produced and CP content is likely to be lower. From this study M. azedarach in the summer (343.7 g/kg DM) season was higher than that of Mokoboki et al. (2019) (206.7 g/kg DM) in the same season. Berchemia discolor species recorded a CP content of 231.8 g/kg DM (summer) and 195.5 g/kg DM (winter). These results were comparable to the one reported by Osuga et al. (2012) (195 g/kg DM). In this study all the browse species had CP concentrations higher than the minimum level in both seasons (above 85 g/kg DM/day in goats during maintenance stage) (NRC, 2000) and they can have the potential to be used as a protein supplement to grass-based diets. Neutral detergent fibre (NDF) content for the browse leaves ranged between 283.0 and 575.9 g/kg DM, with summer tending to be generally much higher than winter except for D. cinerea. This finding was in line with Fentahun et al. (2020) who also reported a higher NDF concentration for the browse in dry seasons than in rainy seasons. The NDF concentration in winter is higher than summer because of the high intensity of solar radiation and the less amount of rainfall caused faster maturation during the winter season, of which it resulted in higher cell wall contents than those in the summer season (Aruwayo & Adeleke, 2019). Low NDF concentration on M. azedarach and B. zeyheri (392.6 g/kg DM and 283 g/kg DM, respectively) in both seasons is within the recommended ones for browsers and grazers (30–40% DM) and this can have a direct relationship with DMI (Van Saun, 2016). The significance of DM intake lies in the utilization of forage by the animal and it is a determining factor of energy and performance in ruminants. The ADF results in this study were within a range of 202.4–502.3 g/kg DM and this was within the range of that reported by Gemeda & Hassen (2015) (273.3–495.1 g/kg DM) and lower than that reported by Anele et al. (2008) (604–607 g/kg DM). Saleem et al. (2012) agrees that the ADF concentration level in browse species can positively or negatively affect the digestibility. The lignin content in most of the browse species in our study were ranging between 89 to 355.8 g/kg DM for both seasons and was much higher in summer than in the winter season. Tjelele (2007) reported similar findings highlighting that lignin content in browse leaves varies based on the season, such that in warmer temperatures (summer) the lignin content increases and in lower temperatures (winter) lignin content decreases because of limited light for plant development. Mokoboki et al. (2019) stated that lignin has an ability to reduce the nitrogen balance of animals by increasing endogenous and microbial nitrogen loss in faeces. Moreover, when plants have lower ADL level, the microflora is able to properly break down the low lignified leaves in the digestive tract of the ruminant because it is not resistant to chemical and enzymatic degradation (Ramantsi et al., 2020).

The digestibility of the substrates is normally influenced by the amount of ADF and most of the species had higher DM digestibility in summer when compared to winter. The DMDigest results in this study were ranging from 52.5 to 76.3 g/kg DM in summer and 43.1–67.2 g/kg DM in winter. Factors that may influence the variation of digestibility among woody species include nitrogen, cell wall content, lignin and bioactive compounds (Mlambo et al., 2015). It is essential to understand the manner in which ruminants use ME for their metabolizable functions due to their different efficiencies that are based on their nutritional requirements including their physiological stage (pregnancy, gestation, growth) and general maintenance (Filho et al., 2011). The ME of the studied species ranged between 1.91 and 2.79 Mcal/kg. Most browse species in this study had lower ME values in summer than in winter, this result is lower than that reported by Nsubuga et al. (2020) (above 3 Mcal/kg). The ME of these species shows that they can be able to support the maintenance and high activity of goats (0.57–2.78 Mcal/kg) and other livestock such as dairy and beef cattle (NRC, 1985).

In vitro dry matter degradability

A lot of research has given a detailed account on the nutritive value and degradability of browse leaves (Ramantsi et al., 2020; Ravhuhali et al. 2020). The latter authors added that rumen degradability is capable of reaching up to > 600 g/kg when there is a higher concentration of CP than fibres in the feed diet. According to Fentahun et al. (2020) a high to medium DM degradability suggests that there is great value in feeding these browse species to livestock mostly in the winter season. The degradability of browse species increased with an increase in the incubation time meaning that it needs more time to be broken down by microbes which maybe a result of its high fibre contents of the species. Indeed, various extent and rate at which tropical browse species respond to rumen degradability is expected because of their different chemical composition profile and the season (Ma et al., 2019). The degradability was ranging from 315.1 to 605.7 g/kg DM (summer) and 223.1 to 611.6 g/kg DM (winter). In vitro DM degradability of this study was within Belachew et al. (2013) for tropical species leaves (264.6–824.9 g/kg DM), of which it is sufficient enough to satisfy the energy demands of less productive animals with lower production levels (Mokoboki et al., 2005). Even though in this study, tannins and phenolics were not studied, we cannot shy away from the association of lignin and tannin concentrations and their influence on effectively reducing the fermentation and digestion of the browse leaves (Gemeda & Hassen, 2015).

Conclusions

Results from the current study, indicate that seasonal dynamics on browse species have an effect on the nutritive value, chemical estimates and in vitro dry matter degradability of indigenous woody species. In both seasons, most of these browse species have the potential to supplement low quality natural grasses because they go beyond the minimum requirement of protein and have reasonable amounts of fibre concentration. The overall assessment of these species indicates their outstanding nutritive traits which make them suitable for their use in ruminant diets and can be given first preference for ruminants to browse them. Further studies need to be taken to investigate the level of bioactive compounds found in these woody species for amelioration and maximizing browsing of these species.