Herbage production and persistence of two tropical perennial grasses and forage sorghum under different nitrogen fertilization and defoliation regimes in a summer‐dominant rainfall environment, Australia

Abstract

An experiment was conducted in inland northern New South Wales (NSW) to assess the response of tropical perennial grasses Chloris gayana (Rhodes grass) cv. Katambora and Digitaria eriantha (digit grass) cv. Premier and annual forage sorghum (Sorghum bicolor ssp. bicolor × S. bicolor ssp. drummondii hybrid) cv. Sweet Jumbo fertilized with five rates of nitrogen (N; 0, 50, 100, 150 and 300 kg N ha−1) and defoliated every 2 or 6 weeks over two growing seasons. Tropical perennial grasses were highly responsive to N fertilizer, while there was no significant response by forage sorghum. Herbage production of Rhodes grass increased linearly whereas digit grass had a high response at 50–100 kg N ha−1. Nitrogen‐use efficiency was highest during the growing season when rainfall was higher. During this season, digit grass had the highest N efficiency (148 kg DM kg−1 N applied) at 50 kg N ha−1, and Rhodes grass (66 kg DM kg−1 N applied) at 100 kg N ha−1. Plant frequency of both perennial species increased and then stabilized at high levels (>84%, cell size 0·1 by 0·1 m) during the two growing seasons. Plant frequency of Rhodes grass declined over the winter period, but recovered within 6 weeks of commencement of the growing season. Soil nitrate levels indicated that unused nitrate moved down the soil profile during wet winters. Implications of leaching below the rooting zone are discussed.

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

“…Tropical perennial grasses grown in inland northern NSW and southern Queensland have a distinct window for growth, with frosts and low temperatures limiting growth at other times. Herbage production of these grasses and forage sorghum in this summer‐dominant rainfall region has been described as variable (Boschma et al ., ), and in this study, fortnightly fluctuations in growth rate data were even more pronounced. In this study, the importance of N‐fertilization for responsiveness and resilience of tropical grass species was also evident, particularly when plant growth resumed following substantial rainfall events (c. 80–90 mm).…”

“…Rhodes grass also had a shorter overall growing season (from end October to early March), was slower than digit grass to recommence growth in spring, and slower to respond to substantial rainfall events during the growing season. Slow spring growth in this environment has been reported previously (Murphy et al ., ; Boschma et al ., ) which was particularly interesting as Rhodes grass commonly had the greatest replenishment of stored soil water during the winter period (Murphy et al ., ). In an adjacent experiment with similar soils located 50 m from this experiment, Rhodes grass showed a rainfall capture efficiency (change in stored soil water divided by total rainfall for the autumn–winter period) of 70% during the 2007 autumn–winter period (equating to 197 mm stored soil water), compared with 49% (or 138 mm stored soil water) for digit grass (Murphy et al ., ).…”

“…Full site details are provided in Boschma et al . (). In summary, the site was located mid‐slope on a Red Chromosol soil (Isbell, ) with average and median annual rainfall values of 734 and 751 mm respectively (data for Duri Post Office, station 055311, located 8·6 km from the experimental site; Bureau of Meteorology, ).…”

“…Sweet Jumbo was reported in Boschma et al . (). This study reports results that quantify (i) the growth rate of the three forages; (ii) the proportion of leaf and stem of the perennial grasses; and (iii) the nutritive value of the perennial grasses under different defoliation and N application rates, in a region that is frost prone and has a highly variable summer‐dominant rainfall pattern.…”

“…Sown tropical grasses are widely adapted, persistent and productive in the northern New South Wales (NSW) and southern Queensland (Qld) region of Australia (McCormick et al ., ; Lloyd et al ., ,b; Boschma et al ., , ) and have a role in both grazing and mixed farming systems. The region has a summer‐dominant rainfall pattern with about 60% of annual rainfall occurring in the October–March period.…”