INTRODUCTION

In tropical areas, the nutritional value of fodder resources is a recurrent problem for livestock farmers (Awono et al., 2012). The production performance of livestock is strongly linked to the nutritional value of forages in the pastures (Klein et al., 2014). Lack of data on the inventory and nutritional value of forage species present in a locality is a hindrance to the planning and formulation of rangeland management plans (Suheel et al., 2015). In Cameroon, Lucha and Chuyong (2016) and Pahimi et al. (2020) carried out studies on the inventory of forage species in two agroecological zones. They identified 151 forage species in the Northwest region and 41 species in the North region. However, these studies were limited only to the inventory of fodder resources, without any attention to their nutritional value.

In recent decades, the number of livestock farmers increased in the Western Highlands of Cameroon (WHC), leading to an increase in forage demand, especially during the dry season, where forage shortage is crucial (Ministère de l'Elevage, des Pêches et des Industries Animales [MINEPIA], 2011). Improvement of the nutritional value of forage species in this area could be an asset to enhance meat and milk production in Cameroon. The general objective of the present study is to contribute to a better understanding of ruminant feeding in Cameroon through the design of a database on available forage resources with their nutritive values. More specifically, the aim was to identify and determine the nutritional value of forage species consumed by ruminants in the WHC.

MATERIALS AND METHODS

Description of the study area

The WHC (Figure 1) is characterized by a rainfall of 1500–2000 mm with a 9-month rainy season and a 3-month dry season. The average annual temperature ranges from 22°C to 25°C. The soils are ferralitic, sandy-clay, generally not deep, and rejuvenated by erosion. Rich in humus, they are suitable for food and perennial crops.

Nutritional value of forages

The nutritional value of each forage species identified (energy and protein value) was estimated from the chemical composition. This research used the French system for assessing forage nutritive value initially developed in the 1970s by the “Institut National de la Reserche Agronomique” (commonly known as INRA, and since 2020 renamed INRAE—National Research Institute for Agriculture, Food, and the Environment). The system has been refined over several decades and has been described by Colin-Schoellen et al. (2000). Effectively, the system rates the nutritive value of a feed compared to the feed value for ruminants of 1-kg barley, with separate calculations, depending on whether the feed is consumed by ruminants for milk or meat production, and considering a range of feed quality criteria including estimated dietary protein utilization.

The energy value of forages was calculated as the forage unit for lactation (UFL) and the forage unit for meat production (UFV) from the following equations (Sauvant et al., 1995):

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Estimation of digestible protein for a forage requires knowledge of the theoretical degradability of its nitrogen content in the rumen (DT) and the actual digestibility of the proteins in the intestine (dr), in addition to its CP and OM contents. The digestible proteins in the small intestine correspond to the PDIE (protein digestible in the small intestine supplied by rumen-undegraded dietary protein + protein supplied by microbial protein from rumen-fermented OM) and PDIN (protein digestible in the small intestine supplied by rumen-undegraded dietary protein + protein supplied by microbial protein from rumen-degraded dietary protein) values using the following equations (INRA, 2007):

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Knowing that (Zirmi-Zembri & Kadi, 2016)

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where for green forage DT = 0.73 and dr = 0.75, PDIA denotes digestible protein in the small intestine supplied by rumen-undegraded dietary protein, PDIMN denotes microbial protein digested in the small intestine when rumen-fermentable N is limiting, PDIME denotes microbial protein digested in the small intestine when rumen-fermentable energy is limiting, FOM denotes fermentable OM, and OMD denotes OM digestibility (% DM).

RESULTS

Classification of forage species

The dendrogram (Figure 2) grouped forage species so as to maximize dissimilarity between groups. On the horizontal axis, represented by a dotted line on the dendrogram, there are three groups: group 1 (G1), group 2 (G2), and group 3 (G3). Table 4 presents the description of the different groups of forages according to their number, species, and botanical family.

Table 4. Description of different forage groups.

Groups	Number	Species	Botanical family
G1	6	Persea americana, Pennisetum purpureum, Pennisetum clandestinum, Panicum maximum, Bracharia ruziziensis, Trypsacum laxum	Lauraceae, Poaceae, Poaceae, Poaceae, Poaceae, Poaceae
G2	9	Centrosema pubescens, Draceana fragrans, Arachis glabrata, Vernonia amygdalina, Imperata cylindrica, Ipomoea batatas, Digiatria ciliaris, Cynodon dactylon, Musa paradisiaca	Fabaceae, Asparagaceae, Fabaceae, Asteraceae, Poaceae, Convolvulaceae, Poaceae, Poaceae, Musaceae
G3	7	Sida acuta, Calliandra calothyrsus, Entandrophragma cylindricum, Manihot esculenta, Psidium guajava, Desmodium uncinatum, Desmodium intortum	Malvaceae, Fabaceae, Meliaceae, Euphorbiaceae, Myrtaceae, Fabaceae, Fabaceae

Results of hierarchical classification of forage species were obtained, identifying three groups based on a criterion of maximum dissimilarity between groups. Chemical composition and nutritional value data for the three groups are presented in Table 5.

Table 5. Average chemical composition and nutritional values for the three forage groups.

Item	Group 1	Group 2	Group 3	p Value
Chemical composition
DM	90.97 ± 0.56b	88.79 ± 1.74c	94.14 ± 1.16a	0.040
Ash	10.51 ± 1.82a	11.43 ± 2.69a	7.91 ± 2.72b	0.016
OM	80.50 ± 1.78a	77.59 ± 2.62b	83.27 ± 2.24a	0.001
CH	62.81 ± 2.71a	49.15 ± 5.00c	59.88 ± 5.97b	0.001
CP	11.91 ± 5.19c	24.28 ± 3.08a	19.75 ± 3.46b	0.001
EE	6.15 ± 3.58	3.96 ± 2.52	4.39 ± 3.47	0.445
CF	27.95 ± 3.29a	23.15 ± 6.30b	19.56 ± 5.42c	0.037
NDF	69.89 ± 12.20	59.08 ± 11.09	55.33 ± 21.33	0.236
ADF	38.92 ± 7.09	39.16 ± 10.11	34.91 ± 13.29	0.696
Nutritional value
UFL	0.75 ± 0.03b	0.71 ± 0.16c	0.97 ± 0.10a	0.001
UFV	0.68 ± 0.08c	0.80 ± 0.15b	0.93 ± 0.10a	0.008
PDIN	8.92 ± 0.75c	14.41 ± 3.01a	13.15 ± 1.32b	0.001
PDIE	51.47 ± 13.81b	45.59 ± 18.81c	86.65 ± 15.46a	0.001

• Note: Means followed by a different letter within a row are significantly different (p < 0.05).
• Abbreviations: ADF, acid detergent fiber; CF, crude fiber; CH, carbohydrates; CP, crude protein; DM, dry matter; EE, ether extract; NDF, neutral detergent fiber; OM, organic matter; PDIE, protein digestible in the small intestine supplied by rumen-undegraded dietary protein + protein supplied by microbial protein from rumen-fermented organic matter; PDIN, protein digestible in the small intestine supplied by rumen-undegraded dietary protein + protein supplied by microbial protein from rumen-degraded dietary protein; UFL, forage unit for lactation; UFV, forage unit for meat production.

DISCUSSION

A total of 22 forage species were identified by herd owners in the WHC. This number is very much lower than the 151 and 41 forage species recorded, respectively, by Lucha and Chuyong (2016) and Pahimi et al. (2020) in the Northwest and North regions of the same country. This may be because most of the land in WHC is used for agriculture and the space reserved for livestock is made up of marginal land with very low fertility, such as the sides of mountains and the edges of roads. The forages identified were classified into groups rich in CG (G1), in protein (G2), and with high UFL (G3). The highest DM content (91.93% ± 0.92%) was recorded in D. intortum, while the lowest content was recorded in I. cylindrica (84.61% ± 1.12%). In the Doucen region of Algeria, Djennane (2016) reported DM content ranging between a maximum value of 97.89% in Plantago notata and a minimum value of 18.82% in Maresia nana. According to Vélez-Terranova et al. (2022), the DM content of forages is higher in the dry season due to the increase in the rate of evapotranspiration in the plants, thus reducing the availability of water in the leaves. In the present study, the highest content of ash (17.49%) was recorded in D. ciliaris. This result is close to the 19.86% of ash recorded in Astragalus armatus by Zirmi-Zembri and Kadi (2016) in Algeria. Since the plant can take up minerals from the soil, its ash content depends strongly on the mineralogical composition of the soil (Vélez-Terranova et al., 2022). The highest protein content (29.43% ± 0.45% DM) was recorded in V. amygdalina. Oboh (2006) stated that the leaves of V. amygdalina are rich in CP (33.3%); however, these authors warned that tannin (0.6%) and cyanide (1.1 mg kg⁻¹) contents of this species are likely to cause toxicity symptoms, including hemolysis of erythrocytes in mammals. The maximum and minimum CF contents were recorded, respectively, in I. cylindrica (33.41% ± 0.25% DM) and M. esculenta (12.50% ± 0.47% DM). In Algeria, Djennane (2016) recorded the highest content of fiber in Launanen residifolia (47.09% DM) and the lowest in Centaurea dimerphategia (7.27% DM). The NDF values ranged from a maximum of 87.21 ± 1.33% DM in P. clandestinum to a minimum of 23.37% ± 1.31% DM in C. calothyrsus. Elsewhere, Shah et al. (2019) recorded the maximum and minimum NDF contents, respectively, in Thysanolaena maxima (79.22% DM) and Ficus semicordata (53.21% DM) in India. In the present study, the fiber content in P. clandestinum was observed to be 87.21% ± 1.33% DM, compared with 60.86% reported by Camacho-Ospina et al. (2021) for the same species in the municipality of San Miguel de Sema, Columbia. This difference may be explained by the variation in the soil composition and the physiological stage of the plants sampled. HAC revealed three main groups of forages, essentially rich in CG (G1), protein (G2), and energy (G3). G1 consisted mainly of plants belonging to the Poaceae family that are known to be generally rich in fiber (Klein et al., 2014). The CP content of studied forages in the WHC varied from 12.31% to 23.49% DM. The present result differs from the findings of Zirmi-Zembri and Kadi (2016) and Badourou et al. (2021), who, respectively, recorded values ranging from 0.70% to 28.60% DM and 5.64% to 13.13% DM for the forage species in Algeria and Benin. The protein content varied from one species to another and this can depend on the nitrogen content of the soil. Indeed, nitrogen fertilization increases the forage protein content and the quantity of biomass produced by forage plants (Tendonkeng et al., 2011).

V. amygdalina and M. esculenta were found to be very rich in PDIN and PDIE. This finding could be explained by their high levels of protein and energy, respectively. According to Baumont et al. (2009), there is a positive correlation between protein content and PDIN, and energy content and PDIE. Zirmi-Zembri and Kadi (2016) identified Calycotum spinosa as forage providing the most digestible protein (PDIN and PDIE > 200 g kg⁻¹ DM) to ruminants. M. esculenta was a forage with very high energy content as measured by UFV (1.12 ± 0.01 UF kg⁻¹ DM) and UFL (1.08 ± 0.01 UF kg⁻¹ DM) in the WHC, while Zirmi-Zembri and Kadi (2016) reported Haloxylon scopariumas the forage with the highest UFL (1.5 UF kg⁻¹ DM) and UFV (1.6 UF kg⁻¹ DM) in Algeria. This difference can be explained by the fact that the nutritive value of forage varies from one species to another and soil factors vary from one area to another (Klein et al., 2014). According to Bouazza et al. (2012), forage is of good quality if it has CP > 10% DM and NDF < 30% DM, and of poor quality if it contains CP < 10% and NDF > 50%. A forage is considered a good energy source if it has UFL ≥ 0.7 and UFV ≥ 0.6. C. calothyrsus (CP = 22.10% ± 0.29% DM; NDF = 23.37% ± 0.12% DM) could then be considered as good-quality forage because of its high protein and low fiber contents. Forages with a high NDF (NDF > 50%) content are less digestible by the animals. In the present study, the majority of forages had more than 50% NDF. Values for NDF in this study could be expected to be comparatively high because samples were collected at the flowering stage and the fiber content increases with the physiological progression of the plant into reproductive development (Klein et al., 2014).

To improve the production performance of ruminants, it is essential to provide forages rich in nutrients necessary for the proper functioning of the rumen microorganisms. Forages from G1, G2, and G3 can also be assessed according to their potential to provide rumen microorganisms with the nutrients that they need. The energy and protein values of forages from G2 and G3 indicated that they are suitable to stimulate the proper functioning of rumen microorganisms. G1 forages, which have high fiber content, could limit ruminal digestion. In similar research, Sidi et al. (2016) recorded three groups of woody plants rich in fiber, energy, and protein in the community rangelands of Northeast Benin.

CONCLUSIONS

Twenty-two forage species palatable to ruminants in the WHC were characterized through their chemical composition and nutritional value. They were classified into groups of forages rich in CF (G1), protein (G2), and energy (G3). Further studies should be carried out to determine the antinutritional factors present and the in vitro or in situ digestibility of these forages.

AUTHOR CONTRIBUTIONS

Leslie Tieubou Tsopgni: Formal analysis; investigation; methodology; writing—original draft. Jules Lemoufouet: Methodology; supervision. Felix Meutchieye: Supervision. Langston Wilfried Edie Nounamo: Formal analysis; investigation. Camile Nyembo Kondo: Investigation. Jean Raphaël Kana: Conceptualization; methodology; supervision; validation. Mama Mouchili: Formal analysis; supervision. Back Armel Feudjio: Investigation.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.