Seasonal variation in chemical composition and in-vitro gas production of woody plant species of semi-arid condition of Namibia

Woody plants are essential in ruminant nutrition in semi-arid regions particularly during the long dry season, however, their nutritional value varies considerably among species, seasons and geographical zones. A study was conducted to evaluate the seasonal variation in chemical composition and in-vitro gas production of woody plant leaves and their relationship. Leaves from sixteen (16) major plant species utilized by livestock were collected from six constituencies of Namibia in the wet season (January: summer), early-dry (May: winter) and late-dry (September: spring) season. There were significant influences of seasonal variations on dry matter (DM), ash, neutral detergent fibre (NDF) and soluble tannin (ST) concentrations of the plant species ( p < 0.05). However, crude protein (CP) and acid detergent fibre (ADF) were not affected by season ( p > 0.05). Rhigozum trichotomum (900 ± 13.9 g/kg), Vachellia hereroensis (902 ± 13.8 g/kg) and Baphia massaiensis (910 ± 13.2 g/kg) had lower DM content during the late-dry season as compared to wet and early-dry season. The NDF content was highest in Grewia bicolor (610 ± 48.1 g/kg DM) in the wet season and lowest in Vachellia mellifera (232.0 ± 41.5 g/kg DM) in the early-dry season. Seasonal variation influenced the rate of degradation but not the potential gas production, however, the species were not significantly influenced by seasonal variation for the parameters measured. Species with higher gas potential showed lower rates of degradation (r = − 0.71, p < 0.01). Though the chemical composition of the species was poorly correlated ( p > 0.1) to the in-vitro gas production parameters, a positive correlation existed between ST and degradation rate (r = 0.52; p < 0.01). Seasonal variation influences the quality of woody plants and therefore, their leaves should be harvested during their optimal nutritious stage to improve livestock productivity.