Effect of irrigation, nitrogen and defoliation on pangola grass in the dry season at the Ord Valley, north-western Australia

Abstract

The effect of irrigation, cutting and nitrogen treatments on dry matter and nitrogen yield of pangola grass (Digitaria decumbens) growing in a calcareous clay soil in the Ord River Valley of northwestern Australia, was studied. Irrigation treatments ranged from 4-13 irrigations and nitrogen treatments from zero to 630 kg N ha-1 over the 5 months in each of two dry seasons. Cutting treatments to 15 cm at intervals of 2, 3, 6 and 12 weeks were applied only in the first year. In the first year dry matter yield (DM) increased from 3100 to 5200 kg ha-1 with an increase from 5 to 7 irrigations, a change of average daily available moisture from 78 to 107 mm, but there was no further increase from 13 irrigations (1 48 mm). DM increased linearly in the second year as average daily available moisture increased from 61 to 141 mm (4-12 irrigations). DM increased and N per cent decreased as cutting intervals increased. When cut at 2, 3 and 6-week intervals, pangola response in DM to nitrogen applied was linear, but was curvilinear when cut each 12 weeks. Average nitrogen recovery rates improved from 16 per cent to 30 per cent with an increase in nitrogen applied from 123 to 203 kg N ha-1 and remained at a similar rate to 630 kg N ha-1. Increased moisture improved apparent nitrogen recovery linearly to a maximum of 40 per cent at an average daily available moisture of 141 mm and 203-630 kg N ha-1 applied. For both DM and nitrogen yield the response to nitrogen and cutting frequency was proportional to the amount of moisture available. It was concluded that when cutting for hay at the Ord River, efficient use of applied nitrogen would be achieved by irrigating at least every two weeks, cutting about every 6 weeks and applying 250 kg N ha-1 during the dry season.