Chemical composition and <i>in vitro</i> ruminal fermentation of selected grasses in the semiarid savannas of Swaziland

Abstract

Little is known about the grass species type, composition and nutritive value in the semiarid savannas that sustain most of Swaziland\'s cattle population through the seven-month-long dry season. This study was conducted to investigate the nutritional characteristics of grasses collected from two grazing areas (Big Bend and Simunye), which differed mainly in soil types. Mature grass species were harvested and evaluated for chemical composition (organic matter, neutral detergent fibre [NDF], acid detergent fibre [ADF], crude protein [CP] and minerals) and in vitro ruminal fermentation (in vitro gas production, in vitro organic matter degradability and partitioning factors). The most common grass species in the Big Bend grazing area were Bothriochloa insculpta, Cenchrus ciliaris and Urochloa mosambicensis. In the Simunye grazing area the most common species were B. insculpta, U. mosambicensis, Heteropogon contortus, Panicum deustum and P. maximum. For grasses harvested from Simunye, the most (p < 0.05) degradable (532 mg g^–1 dry matter) was B. insculpta, which also had the least fibre (597 g kg^–1 NDF and 351 g kg^–1 ADF) and the highest CP content (79.8 g kg^–1). The most common grass species harvested from the Big Bend area did not differ (p > 0.05) in their Mg, P, Cu, Fe, Zn, CP and NDF content. However, U. mosambicensis had the highest (p < 0.05) ADF content. The least fermentation efficiency (partitioning factor = 2.2 mg degradable organic matter [DOM] ml^–1 gas) was observed for U. mosambicensis as a result of low DOM coupled with high cumulative gas production. It was concluded that all the grasses investigated in this study show a deficit for Ca, P and protein. Therefore, supplementation is needed to ensure maximum forage utilisation and to satisfy nutrient requirements of ruminant livestock.

Keywords: cumulative gas production; organic matter degradability; ruminant livestock; tropical grass species

African Journal of Range & Forage Science 2009, 26(1): 9–17