THE EFFECTIVENESS OF KIKUYU PASTURES IN THE CAPTURE OF NITROGEN FROM DAIRY EFFLUENT ON THE ATHERTON TABLELANDS

Pressure on dairy farmers to remain viable since the industry was deregulated in July 2000 has accelerated the rate of production intensification. Milk is still produced on a year round basis in Queensland but now decreased milk price and increasing production costs are requiring farmers to intensify to a greater extent. The industry is still primarily pasture based but there is an increasing trend to feed larger numbers of cows in some form of purpose built intensive feeding system. Increased intensification results in an increased reliance on supplementary feeding, a greater need to ensure peak pasture production with fertiliser and more pressure on the waste management system. A common waste management system on the Atherton Tablelands of northern Queensland requires storage of all effluent in a single long term storage pond. This waste is then applied to pasture in the dry season through a sprinkler system. Alternatively effluent may be applied on a twice daily basis as yards are cleaned after milking. Although tropical grass can use large amounts of nutrient, potential loss of nutrients by run-off and leaching can occur in the wet season of tropical north Queensland when effluent is applied daily. This study was designed to test the management strategy of applying dairy effluent to a tropical pasture in summer when the pasture growth and therefore nutrient requirement is at its highest. The objective was to determine nutrient use efficiency of kikuyu and to determine leaching losses of nitrogen and therefore the potential environmental hazard. Stored dairy effluent was applied through the irrigation system at 0, 18.5, 42.4, 56.0 and 74.6 kg TKN ha‾¹ over the wet season to a kikuyu (Pennisetum clandestenin) dominant pasture which had been sown to annual ryegrass (Lolium multiflorum) the previous autumn. Irrigation water was applied at rates which were in inverse proportions to dairy effluent. Pasture yield was measured every three weeks over a 107 day period. No significant difference in total pasture dry matter yield, pasture growth rate, plant nitrogen percent or total plant nitrogen yield was measured. Total pasture yields were 5243, 5808, 6213, 6035 and 5836 kg DM ha‾¹ over 5 harvests. Pasture growth rate ranged from 33.4 to 78.9 kg DM day‾¹. Mean plant nitrogen yield was 161.8, 178.2, 194.2, 197.5 and 196.9 kg ha‾¹. There was no significant difference in soil N and soil cation concentration (except sodium) to a depth of 80 cm at the completion of the trial period. At 50 to 80 cm soil depths treatments 1, 4 and 5 were significantly different to treatments 2 and 3 in sodium concentration. There was no difference in soil N levels to a depth of 20 cm prior to or after the treatment application. Following extensive measurement and sampling of leachate it was calculated that 29.1 and 86.8 kg ha‾¹ of total N was leached to 50 cm from treatments 1 and 5 respectively. Measurements of soil N, plant N and leachate N indicate that there was a gain of 20 kg N ha‾¹ in the N budget for treatment 1 and a gain of 55 kg N ha‾¹ from the system during the trial period for treatment 5. Possible reasons for these results are discussed. The results indicate that it is difficult to apply dairy effluent to pastures during the summer wet season on the Atherton Tablelands without nitrogen losses through leaching. However given the large amounts of organic N and the potential for high rates of mineralisation, substantial N losses can occur without any additional N application. The results also suggest that the yield of a tropical pasture, which follows a well fertilised annual temperate pasture such as ryegrass, is not enhanced by the application of dairy effluent during this time. However, pasture quality may be improved by the application of dairy effluent