Production and forage quality of prairie grass (Bromus willdenowii) in comparison to perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) in subtropical dairy pastures

Abstract

A grazing study was conducted, over a 3-year period (1997–99), on the subtropical north coast of New South Wales, Australia, to compare the yield of prairie grass (Bromus willdenowii cv. Matua), tall fescue (Festuca arundinacea cv. Vulcan) and perennial ryegrass (Lolium perenne cv. Yatsyn), on a well-drained red krasnozem soil at Wollongbar Agricultural Research Institute (WAI) and on a heavy clay soil at Casino. The effect of grazing interval (equivalent to the time taken to regrow 1.5, 2.5 or 4 leaves/tiller) in spring, and forage quality of prairie grass in winter and spring was also assessed. At both sites, the dry matter (DM) yields of prairie grass over the establishment year and in year 2 were significantly (P<0.001) higher than for the other 2 grass species (mean for 2 years over the 2 sites was 23.8, 8.9 and 7.7 t DM/ha for prairie grass, ryegrass and tall fescue, respectively). In year 3, there was no production of tall fescue or ryegrass at the WAI site while prairie grass produced 11.3 t DM/ha although this was obtained from natural seedling recruitment after the sward was sprayed with a herbicide in February of that year. At the Casino site, ryegrass and tall fescue still made substantial growth in year 3 (3.1 and 2.1 t DM/ha for ryegrass and tall fescue, respectively) but this was significantly below the yields of prairie grass (5.5 t DM/ha). More frequent grazing of prairie grass in spring (equivalent to 1.5 leaves/tiller of regrowth) led to significantly (P<0.05) less plants surviving summer and less seedling recruitment in the following autumn. The annual yield of the 1.5 leaf treatment was significantly (P<0.05) lower than the remaining treatments but only in the third year of the study. Analysis of prairie grass forage samples, taken in June (vegetative sward) and November (reproductive sward), gave magnesium values of less than 0.2% DM which is below the concentration found in ryegrass and that recommended for dairy cattle. The Ca : P and K : (Ca + Mg) ratios in prairie grass improved, as a forage for dairy cows, with regrowth time up to 5 leaves/tiller. Metabolisable energy remained constant with regrowth time in June at 10.8 MJ/kg DM but fell significantly in November from 10.7 MJ/kg DM, immediately post-grazing, to 9.2 MJ/kg DM at the 4.5 leaves/tiller stage of regrowth.

In contrast to observations in ryegrass, the water-soluble carbohydrate content of forage samples of prairie grass taken in November showed a substantial increase with regrowth time to over 12% DM at the 3 leaves/tiller stage of regrowth.

The high productivity and forage quality of prairie grass obtained over a 3-year period suggests this grass species could be a suitable temperate perennial grass for subtropical dairy pastures. An appropriately long grazing interval in spring seems critical to optimise plant survival over summer and for adequate seed set for seedling recruitment the following autumn. If summer weeds and/or grasses invade to a significant extent, the large seedbank of prairie grass provides the opportunity to spray out the pasture in summer and rely on seedling recruitment to establish a new sward in autumn. The forage quality of prairie grass in winter and spring is similar to perennial ryegrass but the magnesium levels are substantially lower and stock grazing this type of pasture for extended periods would need to be supplemented with this mineral.