Growth rate and nutritive value of sown tropical perennial grasses in a variable summer-dominant rainfall environment, Australia

Growth rate, proportion of leaf and stem and nutritive value of sown tropical perennial grasses have not previously been documented for the dryland, frost-prone summer-dominant rainfall region of eastern Australia. An experiment was conducted in northern inland New South Wales with Chloris gayana (Rhodes grass) cv. Katambora and Digitaria eriantha (digit grass) cv. Premier, and compared to Sorghum bicolor ssp. bicolor x S. bicolor ssp. drummondii (forage sorghum). The grasses were harvested every 2 or 6weeks and fertilized at five rates of nitrogen (N; 0-300kgNha(-1)) over two growing seasons (September-May), 2006-2007 and 2007-2008. Growing season rainfall was below median for both years of the experiment and growth rate was highly variable, reflecting variable rainfall. Sorghum generally had the highest growth rate. Digit grass generally had higher growth rates than Rhodes grass, was more responsive to growing season rainfall, commenced growth earlier and had a longer growing season. Nitrogen application extended the growing season of both perennial grasses. Growth response to N application was minimal when rainfall was low and response following significant rainfall was higher for fertilized than unfertilized grasses. In general, the proportion of green leaf was greater than green stem, although the proportion of stem increased when defoliation interval increased. Nutritive value of the perennial grasses was higher in leaf than stem and declined during the growing season. Differences between the grasses were slight, but indicated that fertilized digit grass defoliated at 2-week intervals had higher growth rate and nutritive value than Rhodes grass.