Nutritive value of subtropical grasses invading North Island pastures

Abstract

A study was undertaken to ascertain the possible nutritional impact of subtropical grass invasion into pastures, by chemically analysing five common subtropical grasses. Leaves taken from kikuyu (Pennisetum clandestinum), paspalum (Paspalum dilatatum), smooth witchgrass (Panicurn dichotomiforum), crowfoot (Eleusine indica), and summer grass (Digitaria sanguinalis) were collected from grassland in the North Auckland, Waikato and Manawatu regions. Leaves of perennial ryegrass (Lolium perenne) were also taken from each pasture as a reference temperate grass. Relative to perennial ryegrass the subtropical species clearly showed increased levels of neutral detergent fibre (NDF; 38.4% vs 57.5%) but reduced levels of total protein (23.0% vs 13.2%), soluble sugar (11.7% vs 5.9%) and in vitro organic matter digestibility (OMD; 84.0% vs 66.6%). Similar results were obtained for grasses grown in each of the three areas. Of the subtropical grasses, summer grass was of the highest nutritive value and crowfoot was of the lowest. The results of an in vitro rumen protein degradation experiment showed net ammonia produdtion from pasture fermentation was significantly lower for the subtropical species than for perennial ryegrass. The present study indicates that the subtropical grasses are of considerably lower nutritive value than perennial ryegrass. Their cont.inued spreading into grazing pastures will probably substantially decrease the nutritive value of the pastures and animal production. Keywords: nutritive value, subtropical grasses